Any amateur mirror makers still alive in Oz/Nz?

[Rac]

Remember a couder mask only samples from from a few points, for it to be of good use it would be expected that the Ronchi lines bend smoothly from zone to zone as they DO NOT in the before image and that is why I "fixed" it.
Yes I posted a Figue xp profile that may not be a close match to the real mirror but only because the mirror looked like a dogs beakfast.

[dave brock]

Hi Mark.

Any particular reason you are complaining about this here and not on the forum that it was posted to?


Have you ever seen a mirror this rough from this maker?

Dave

[dave brock]

Hi Raymond.
Time to update your signature, you no longer have a Royce conical.
Good work :thumbsup:

Dave

[Rac]

Just to update.

I Emailed Royce and he takes full responsibility for the state of it. He never questioned my test images. It was made about the time he was very ill but he didn't stop working(maybe should have).

I'm sure he is currently making great mirrors.

He has said good work on the fixing of it.

[sopticals]

Amen:thumbsup:

[Satchmo]

Hi Dave

I am not a cloudy nights member - and the discussion here was prompted by the posting here of some comparitive foucaultgrams which is what I was asking for clarification - I dont have any problem in discussing what has been presented here with the author.

[Rac]

I think the confusion has come from that the knife edge images were only ever posted to show the quality of the surface and not of the figure as that is what the Ronchi image was for.

So for surface smoothness IMO then they are very comparative.

[DaveGee]

I thought I'd jump in here with a notice of my Mirror Blank Give Away. See thread...
http://www.iceinspace.com.au/forum/showthread.php?t=141560

[sopticals]

Hasn't been a posting for awhile. So thought to show there are some of us "still alive".

Picture of my current 18" diameter by 1 1/2" (edge thickness) plate glass project. Currently just finished the #400 SiC. Will finish the fine grind with #800 for approx 90 mins.

Focal length 72" (f4). Should make for a compact ladderless scope when done.

Eyepiece would be just a little easier to reach than that of my 33"f4.1, with focuser height at 12' when pointed at zenith.

Stephen.

[Rac]

Is this 18 for yourself or someone else?

[croweater]

Good stuff Stephen. There's something about a round slab of glass! Also love your obviously well used grinding bench :thumbsup:

[sopticals]

Was supposed to be one of six similar 18"ers for someone else local, but, there has been a "hiccup", so now maybe its headed somewhere else,(USA)?

[sopticals]

Sure is. Getting more difficult:( to obtain good quality glass at a reasonable price.

Stephen.

[JuniorDave]

Thought others may be interested.
Thanks to Stephen I have started working on a 365mm F3.something mirror.
Last nights rough grinding brought it below f4. With a 6mm sagitta.
I'm completely new to all of this. We got our first telescope a month and a half ago. A Nova 4inch reflector. Which I have now made a table top mount for.
To me I enjoy the challenge of being able to make something so perfect as a parabolic mirror only using the most basic of tools and tests.
Once I get to the end of fine grinding I plan to build the scope, or at least most of it so that I can test the mirror during polishing.
The glass isn't perfect, the back has two large chunks out of it, so no idea what the end quality will be. But it's all part of the learning process. Next piece of glass is also on its way from Stephen.

[sopticals]

Hi David,:welcome:

welcome "on board the mirror grinders merry-go-round". Watching :eyepop:with interest.:thumbsup:

Stephen.

[JuniorDave]

Rough grinding complete.
12hours, about 140 wets. about 20 wets per mm of sagitta.
at 365mm F3.3 shape. 1200mm focal length.
It's been a good learning experience. Although it has taken longer than it could have with other methods, I now have a very smooth curve that is looking really good with fine grinding. Past 120 and finishing up 220 now. Discovering how hard it is to gauge what tiny pits are from what grit.
Sure I will learn.

Thanks for the 3rd piece of glass Stephen. Will have to work out what I do with it.
I have decided the next project will be grinding the other side of my tool into a constant thickness 10 or 12 inch mirror. It will be quite thin. Maybe 20mm.

[sopticals]

David,:hi:

The last rough cut blank, (one you have just received) you could tidy up by getting it water jet cut to 13",or thereabouts. Was 14 1/4" rough cut? So then it could be made into another mirror. Just my thoughts. :thumbsup:

Stephen.

[sopticals]

Needed to re-tile my 14" glass tile tool after working 3 by 18" to 24" mirrors.
Old tile depth down from 7-8mm original thickness to about 1.5-2mm, so obviously wouldn't have had a "dogs show" of lasting the distance grinding out my current 22"f3.5 mirror project.

As the curvature on the worn tool was near enough from previous project, an 18"f4, it was a simple matter to apply a commercial tile adhesive over the old surface, and lay on the new sheet of glass tiles.

The adhesive used normally dries in 24hr,(at 23degC), but because of the low winter temperature, took 3 days to cure. Just finished cleaning out the grooves, (between tiles), used tools seen, (old haksaw blade and paint brush).

See pics: including the 14" tool "astride" my 22" tool (used for 30"and 33" mirrors)

Stephen.

[Rac]

22" f3.5 sounds fun!

Your tile tools looks just like mine.

[sopticals]

I must be doing something right.:thumbsup:

[sopticals]

Finished the grind of 22"f3.5 (10mm sagitta). Silicon Carbide grits used #36 (Hog:with 7 3/4" 5kg cast iron barbell weight), #60, #120, #220, #400, #800 (using the 14" glass tile tool).

Stephen.

[nobby2]

So much discussion in this thread it's difficult to find a starting point.
I have to convince myself I can test a mirror before buying a blank etc.

Just looking for confirmation that I am doing things correctly.

I have built a foucault tester based on the Stellafane model and have a silvered completed 8" GSO mirror to test.

From the attached images can anyone tell me if:
1. The position of the LED looks OK. Just above the slide hole for full passage of light to the mirror.
2. I have aligned the returned mirror reflection (small circle) back to the knife edge.
3. I have moved the knife edge to the radius of curvature (twice the focal length). On my mirror focal length 1200mm/47.244", f/5.9 making ROC at 94.49 inches.
4. I have observed the returned beam at its smallest and most focused to confirm its at ROC. I have moved the tester closer and further away from the ROC.

Problem is, I am not getting the expected shadow either side of ROC.
At exact ROC the light is very bright.
Is this to be expected when testing on a completed silvered mirror?
:)

[nobby2]

I have the same question as I just read on a previous post. Just completed a faucault tester and the exact position of the light source is not clear on many examples. Some use a torch light some distance to the side, some use a light on a fixed post separate to the knife edge. I have used a LED light as per the Stellafane models shining through the bottom, middle of the 2" slide hole close to the knife edge.

[sopticals]

Hi nobby2 (Neil),:welcome:to exploring the prospect of mirror making ATMing.

No body has chimed in at this point so thought it might be of interest to you to see a very simple rig made from scraps of ply, a 133LPI Ronchi screen, a cheap LED torch (has multiple LED's, covered all except one), a small piece of green cellophane wrapper paper, (through which I pass the LED beam), a few screws etc.

Pics: side view, front view, and a shot of Ronchi taken of my 33"f4.1 mirror,(still a work in progress). Photo of R-gram taken by free hand held Canon Power Shot A1200 digital camera.

I will let others show their test rigs for Foucault,(will be much more sophisticated than my simplistic Ronchi rig).

Stephen.

[nobby2]

Thanks sopticals for your reply, much appreciated. Mirror making is obviously not a big thing in Australia. I think its time I moved on to the Ronchi Screen.
Did you make your Ronchi screen or order it. I notice the US publisher Willmann-Bell sells screens at various lines per inch. I read that 100 is a good starting point.
I Think I have the structure of the tester working OK, just looking for some confirmation that I have the basics sorted.

[dave brock]

Hi Neil.
First off, don't get the willmann-Bell screens, they count the lines wrong and are 1/2 what they state. Get them from http://ronchiscreens.com/

With your tester, it's a bit hard to tell from your pics but the light seems in the correct position with the knife edge cutting 1/2 way over it. The return beam also needs to be cut into with the knife to get the shadow (above the light). Start say 1/2" inside where you think the RoC is and there should be a dark, sharp shadow coming from the same direction as the knife. Move the knife back until the shadow forms the doughnut pattern. Continue moving the knife back and the shadow will come in from opposite the knife.
A possible problem I can see is you need to have your eye right in close behind the knife edge which might be difficult with your set up.

Dave

[dave brock]

One more thing.
It is brighter with a coated mirror but the shadows are just as visible and it can be troublesome if the coating is badly deteriorating or very dirty/dusty.

Dave

[dave brock]

The focal length of your mirror may not be exactly 1200mm. That is often a rounded off figure.

Dave

[nobby2]

Thanks Dave. Just the information I was looking for. I can now move on and order the screens.
I was wondering if my silvered completed mirror was too reflective to get the shadow. It looks like I was doing all the correct things. Yes, every grain of dust makes viewing more difficult. Will have to try again with a closer eye to the knife edge. Much appreciated.

[OzEclipse]

Hi Neil,

Foucault testers are generally illuminated to be bright enough for diffraction patterns to be visible on an un-silvered polished mirror - reflectivity 1-4%. A 96% mirror will therefore be very bright.

What you are seeing could be correct. I just can't tell from that photo. You could be seeing some minor un-eveness in the figure and a Foucault shadow. It's hard to tell from that photo.

My tester is a dual ronchi / Foucault tester based on Jean Texereau's original model. It uses a razor and a very fine brass mesh at each end of a rotating arm so you can position the ronchi grating or Foucault blade at the optical axis. Keep the light source and viewing point as close together and on the optical axis as possible. The horizontal strands have no optical effect because of the vertical slit over the light source only diffracts with the vertical strands of the mesh.

I'll post some pics of my tester when I get a chance.

There are many ways to determine the exact ROC. You can use the sun to determine the focal length with a piece of card attached to the end of a long stick. Point the stick toward the sun, slide the mirror along and mark the position on the stick where the suns image comes to a focus on the card. With your tester and a a silvered mirror, place a piece of tracing paper at the viewing point. Adjust the position of the tester until the slit comes into sharp focus.

Joe

[noodles468]

Hi Neil,

Nice work on the tester :thumbsup:

I'm in the process of building my first foucault tester too, so it's good to see the various approaches in this thread.

I noticed you had no setup yet for measurement, were you planning on using a dial gauge? I purchased one but am having a great difficulty working out how to mount the thing solidly. It only has a flat base so nothing to screw into, I think I will need some kind of clamp setup. The stellafane plans are quite vague about this.

If anyone has any suggestions/examples that would be excellent :)

[sopticals]

Decided to start work on another 33" mirror blank. Be a spare/backup for my biggest dob. The first mirror is f4.1 so will be aiming for same with this one also. Applied the 3mm edge bevels today using shown belt sanders using #120 grade belts. In second picture (background) my other current project, a 22"f4 (hogging just completed [#36 grit]).

Stephen.

[hamishbarker]

Newby Hamish here, I guess this fits in DIY mirror to a certain extent:

I've bought a cheap 8" F4 mirror on aliexpress for $94 and am planning to do 3 things:
1. set it up for a star test to see if it looks ok; and
2. build a mirror test rig to learn ronchi testing and attempt to diagnose any shortcomings.
3. if defects, strip the aluminium and try refiguring it (have bought pitch and Cerium oxide).
4. retest, rinse and repeat steps 2-4.
5. get it realuminized.

I'm in Nelson, NZ. Is anyone nearby who would be willing to assist or have a look to critique mirror/test results?

Any tips on figuring and for where and how expensive aluminizing an 8" mirror is likely to be? (Stephen, where did you get your big mirrors aluminized, and how many $?)

my aims are twofold:
1. get a decent 8" mirror for first scope (planning old school dobsonian tube/mount for quick and easy construction. Maybe in future build a truss for compact/lightweight but we have a big van.).
2. get some experience with figuring, with a view to taking on a future bigger diameter mirror project (16" or maybe bigger - I see Stephen still has 22" for sale but this seems like a lot of glass to take on!).

wishing dark skies to all!

Hamish

[Merlin66]

Just an aside....
I use the Surplus Shed adjustable slit assemble in the Foucault tester and various different bulbs - neon/ fluoro etc.
Easy to use and adapt.
Currently out of stock at Surplus Shed, but I have a few available.

[pixelsaurus]

Good luck with that, Hamish. There are some guys your way have resurrected Clive Rowes aluminizing plant and expect to be able to do mirrors up to 16" in diameter.

[hamishbarker]

aluminizing up to 16"? Awesome! Near Nelson??

I live on the sea side of the port hils, far enough around that there are no port lights, and only one house light so the stars are great even from our deck. I saw the orion nebula (well, the hint of it) the other night. I didn't even know it was there, saw "Hmm, that looks funny and cloudy there" through binoculars, then googled and realised I had seen my first nebula.

[sopticals]

Hi Hamish, most probably the mirror will be spherical. To perform satisfactory it would require parabolizing. However a simple Ronchi test would confirm that. I have a spare 85 lpi grating which I could let you have,(PM me). The test is easy to set up and gives an immediate visual profile of the mirrors figure. Pic of my Ronchi tester made of scrape materials.

Regarding aluminizing: I have had a few small mirrors aluminized by Andrew Leary at 265 Waipara Road, Kerikeri, 0295.
Email: scopes@scopesnz.com (Can do to 14").

Another option is Kiwistar Optics in Lower Hutt.
Daves email: d.cochrane@irl.cri.nz (Told can do to 1m).

Regarding coating my bigger mirrors I have been silvering them myself. Been having an issue with importing fresh chems, but have cleared that with NZ Customs so looks like I will be able to continue to do my own.
If anyone out their wants some silvering done, let me know.

Best regards
Stephen

[sopticals]

Silvering attempted on 33" mirror. Afraid its not perfect, (and this my second attempt with chems just in from Angel Gilding [USA], first attempt produced an unacceptable "white bloom", so had to remove the defective coating using a silver dip cleaner.)

This current coating is reasonable for try in the scope, (again its not perfect, having a small area [see photo at 10.30] where the silver did not adhere,other than that the surface has good reflectivity).

Next time I attempt the coating will have to be more fastidious with surface preparation (cleaning), and do a partial mirror rotate during the actual "spray silver" process. Also I feel temperature and humidity play a part in how well the silver "takes". I think the high humidity of 97% when I made the first attempt may have contributed to the formation of the "white bloom".

I suppose the good news is that silvering ones own optic is an affordable option with each attempt costing around US$15 of chems. If I was to get a mirror of this size aluminized I hear numbers in excess of AU$1650 for a coating. Built my 33" dob for about what it would cost to get mirror alone aluminized. Big aperture astronomy is not just for "the rich of this world". After all isn't ATMing about "getting what you can't afford"?

Photos: before and after.

Stephen

[hamishbarker]

Well my ronchi test stand for checking what this mirror is was even more scrappy. cardboard box, duct tape, books for adjusting height and shuffle it back and forth, using a duct-tape covered red bike led with the lens removed for light.

The bands are dead straight - it's spherical. diameter 200mm, RoC 1500, so f=3.75. Although terrible in terms of a practical sort of mirror (since an f6 or so would be much easier to figure and still be a very easy handling scope, I think it is actually good since it means I have some challenging figuring work to do but on a small scale. Since I have bought a big 550mm diameter (22") piece of glass from Stephen, I am going to need the practice. :help::)

[hamishbarker]

Not too shabby, classic Kiwi DIY spirit! Please do be careful not to get poisoned by whatever chemicals are used (particularly for the tinning, as that sounds potentially nasty)!

I guess the air down your neck of the woods is pretty clean so silver may last better than in city air, autumn farm burnoffs and diesel tractor emissions notwithstanding.

[nobby2]

I am using a second hand 8" mirror as a test. It came with some small chips in the back only and was silver coated.
I spent some time before starting, building my Foucault and Ronchi tester and have 3 gratings to use. I built a sagitta measuring tool out of a Dial Indicator and piece of bar stock.

The mirror started out as F/6.7 and Sagitta of 1.869mm. My aim is to get it to a F/4.5 requiring a Sagitta of 2.822mm.
Here are the lessons I have learned so far and the questions I still have.

I didn't have to use chemical removal of the coating as the first grind achieved that.
The Ronchi works fine while I had the high reflection of the silver coating but provides no reflection for a Ronchi test at least at the stage of using Carbon grit #220.
Question 1- Is this to be expected?
I was expecting to start using #80 or #120 however was sent a set of grits starting at #220.
I am currently achieving approx. 0.2mm glass removal per hour using the #220.
After three hours of grinding with #220 Grit I have increased the sagitta from 1.869mm to 2.33mm leaving 0.492mm to go. Possibly 3 hrs to go before progressing to the next grind. Question 2- Is this reasonable progress?

[sopticals]

Hi Neil,:hi:

#120 would have been the ideal grit to change ROC in a reasonable time. If your stuck with #220 then just "put in the yards" and get it done. Good things take time.

Don't expect Ronchi to work until your a good way into polishing.

In the meantime, "grind on, don't worry". Be patient.:thumbsup:

Stephen.

[nobby2]

Thanks for the encouragement from Sopticals (Stephen).
Progress report:
After ordering more #220 grit I was able to get close to the target Sagitta of 2.822. I actually only got to to 2.78 and was concerned that I wouldn't make it.
I pushed on with #320. I was able to pass required depth. Up to this time I was using mirror on top so I now had to try for the first time to use the Tool on top. Much easier to use this way. After moving the tool off centre, to one side, I was able to bring the depth back to the required figure.
By now the mirror is becoming smooth and transparent.

My Big doubt and first question is: My required Sagitta is only in a narrow circle in the middle of the mirror. Tutorials I have watched on line maintain the depth out toward the edge. What technique do you use when using a tool the same size as the mirror to widen this inner deep section.
Including pictures for the record.

[sopticals]

Hi Neil, I don't see anyone else jumping in here. Been a long time since I've used full sized tools on small mirrors.

Looking at the wear on your tile tool, it appears that overall contact between mirror and tool isn't that good. Probably because you started out with such a fine grit (#220). Normally one would "break in" such a tool with #80 (or coarser for bigger mirror) to quickly grind off "the high ground" and "raise the valleys".

IME with larger mirrors and small diameter hogging tool and sub diameter tile tools (which have 100% contact), I find the center depth target does arrive more quickly, and outer regions lag behind somewhat, (particularly the 70% zone). To deal with this when target sagitta for center is close/there I concentrate strokes on the 70% zone (which also helps the edge "to catch up"as well).

Stephen.

[Rod]

Hi Neil

Welcome to the world of mirror making!

I was looking at your photos and if the tool is dry and clean, as Stephen says it is not making full contact with the mirror. It looks like you have ground off the tiles' glazed surface in parts but not everywhere. The tiles should look the same colour all over.

Some people make a cardboard or thin plywood template to monitor the curve. More commonly you use the pencil (or 'sharpie') test to tell if the tool and mirror are properly mated:

http://gotgrit.com/fsharpie1.php

https://stellafane.org/tm/atm/grind/fine.html#Pencil-Test scroll down the page to see the description and what to do to bring tool and mirror into contact.

Hope that helps

Rod.

[nobby2]

Thanks for the reply. I am going away for 7 weeks and may drop off the forum with any progress report. Will return with much interest I had noticed two of my central tiles were sitting lower.

[dave brock]

Not entirely sure what you mean here. Maximum sagitta should only be in the middle and reduce as you go to the edge. I don't see any evidence of a hole in the pic of the mirror surface. If the curve is spherical then your strokes will be smooth and without any sticking/slipping. Just a nice even drag over the whole stroke. The surface of the mirror should also look nice and even with no areas that have obviously coarser pits.

Dave

[nobby2]

Great to get Some reassurance. I have returned from my 2 1/2 month away and just completed the tool test with a texta grid on the mirror. Produced an even wearing from centre to edge after one rotation of #320 grit, so cross fingers. Hope to include some images. Can only use the iPad at the moment.

[sopticals]

Hi nobby,:) any updates?

Stephen

[nobby2]

Thanks Sopticals. Will post pictures of my next stage, Polishing. What a black Art! I was using Tropical Resin Pitch from my Australian supplier. The first attempts had many bubbles. The second attempt with a higher mix of Castor Oil produced a much improved pitch lap which I used for an hour before it started breaking up and breaking off its plaster base. It was obviously still to hard and brittle.
I have since ordered some Gugolz Pitch #55 from GotGrit.com suitable to my current working temperature of 16C.

[sopticals]

Good move.:thumbsup: Gugolz #55 is the one I use at working temps around 10C, (I add a small amount of Stockholm tar to it, to modify it to about equivalent to #50 [or slightly lower] for the lower temps I work at). Gugolz is nice to work with. I used resin (rosin?) moderated with pure turps and beeswax way back in the 1960's (loved the smell), it worked ok if one got the balance right with the ingredients for working temperatures.

Stephen.

[nobby2]

Image record as promised from pervious post. I am attempting to polish an 8 inch mirror. I was using Tropical Resin Lap. The first attempts had way to many bubbles. The second attempt with the addition of more Castor Oil was much improved however only lasted one hour before it broke up. Obviously too hard and brittle. I have since ordered some Gugolz Pitch #55 from GotGrit.com suitable for my current working temperature of 16C.

[noodles468]

I recently had a go at the 'tropical' lap myself. My problems were different to yours though Neil, went overboard with the castor oil and now have a lap which is easy to cut channels into, but which I fear is much too soft. Think I will give it a few more goes with the resin to see if I can get it right before I give in and go with pitch.

My lap pictured here, freshly poured (but prior to trimming).

[nobby2]

Measuring progress polishing an 8 inch mirror. First picture has curved rough lines with scratches. The second picture is straightening up and looking even with some fine scratches. The target is to create a spherical mirror befor fine turning it into a parabolic mirror.

[sopticals]

Looks like your moving in the right direction:thumbsup: Neil. How many LPI is the Ronchi grating? You might want to consider reducing pic size a little. I use "paint" function to size my pics. My camera is a basic Canon A1200 (12.1 Mega Pixel). I do the 20% fix to give nice size pics for the forums.

Stephen.

[nobby2]

Thanks Sopticals for the hint on pic sizes. I am in the figuring stage using 65 lines per inch. I also have 85 and 133 that I may get to. I am working on offsets in front of and behind ROC and comparing with computerised models. Attempting to asses under or over correction. The learning curve is using the correct strokes to improve the shape. Found even 5 or 10 minutes can make a difference at this stage. Have to be very patient.

[nobby2]

Getting closer to a Parabolic 8" Mirror with the following comparison of the Ronchi calculator 85 grates per inch and a picture taken just using my iPhone. The first two compares inside ROC at -0.2 and the second two at Outside of ROC at 4.0. Pictures low quality
217121

217122
I set my offsets on the Ronchi calculator at -0.3, -0.2, -0.1, 0.0, 0.1, 0.2, 0.3, 0.4, 0.5. The minus offsets return results from zones closer to the centre of the mirror and the positive numbers results from the outside zones of the mirror.

[nobby2]

Can anyone recommend a mirror coating service in NSW or Australia. Closest to Central Coast NSW would be convenient ?

[Saturnine]

Hi Neil
I believe Wayne Sainty of Saintech in Taree does most of the mirror coatings for this area. There was / is a price list in the archives here somewhere along with ph.number and address . Several forum members have used his coating services . Someone else may jump in with a quick link to the post.

[nobby2]

Results of 133 grating on Ronchi. Testing using a pdf file.

[nobby2]

Before and After pictures. It was purchased as an old mirror with chips in the rear structure. Once coated (silver) should the rear (back of mirror) chips effect the final performance?

[Saturnine]

hi Neil
Have been following your adventures as I have a few blanks sitting around that I would like to turn into mirrors when I get the time. As for chips on the rear edges, you wont even know they are there when the front surface is coated, they will have no effect on the image as you are only reflecting off the front surface. Suppose that goes without saying really.

[Baza]

Gumtree https://www.gumtree.com.au/s-ad/mont-albert/miscellaneous-goods/telescope-mirror-blanks/1163433720

I tried to buy them but he won't post the blanks. Hopefully somebody in the Mont Albert VIC area will be able to use them.

[sopticals]

Hi, I have a few blanks of various sizes lying around, if you want. Only I'm in NZ, but willing to post:thumbsup: if you want to buy. Send me a PM.

Stephen.

[Baza]

I have a 10" blank after being pointed to Andrews.
I've started grinding but seem to be stuck at 3.18mm, I am looking to go to about 6mm which should be around F3. Full size glass tile tool in use which has gone from 8 mm to about 5.5 mm thick. Just pushing across the blank with about 60% cover.
Any suggestion about getting to 6mm appreciated.
Barry

[OzEclipse]

Hi Barry,

I assume the depths you're quoting is where you are measuring the total depth of the curve, edge to edge of the blank?

If so, you are currently at f5 and need 5.2mm to get to f3. 6mm will give you f2.5.

Now that you have that curve, you can glue a full set of 8mm tiles to the first batch. You need long strokes to hog out the centre.

f3 is a deep soup bowl to parabolize on your first mirror.

Good luck

Joe

[dave brock]

If by pushing across the blank you mean tool on top then you should be mirror on top to deepen.

Dave

[Baza]

Thankyou

[OzEclipse]

Barry,

Are you measuring the depth of the curve on the mirror or by measuring the edge tile thickness on the tool? If the latter, this is not a reliable method.

If you are using the mirror curve depth across the whole disk, I have written you an excel spreadsheet to do the mirror curve depth to focal length calculation. You have to input the diameter of the mirror in mm and the curve depth in mm, it will calculate the rest for you.

Personally, I prefer the sun method. As long as it is a sunny day, tack a piece of white card on the end of a long dowel rod. Longer than the focal length.

Slosh water on the ground mirror surface, point it and the dowel at the sun and mark where the sun comes to focus. Measure the distance to the white card and that is the focal length. You can also put distance marks on the dowel to simplify measurements. It's simple and bulletproof.


Joe

[Baza]

No just commenting on how quickly the tool is being changed, it could be pyrex v ordinary glass.
Calculator indicates about f4.6 atm.
Barry

[OzEclipse]

Ok so your edge tiles are down to a bit more than half thickness. I would keep grinding for now. It gets tricky. You don't want to run out of glass near the end of the 80 grit because then if you have to retile, you will be grinding just to grind the resurfaced tool to match the mirror. You definitely don't want to run out of glass on the edge during the fine grind or you'll have to go back to coarse grind to resurface the tool. Just monitor the situation. If it looks like you won't make it, resurface the tool with fresh tiles when there is maybe 3mm left on the edge. Then you'll have an even 8mm all over to finish the job.

Joe

[Baza]

Thanks, might do a tool refurb. soon.

[sopticals]

Hi Barry, I assume your using #80 grit. I would most definitely suggest "re-tiling" at this point in your grind to ensure you "break in" the new tiles as getting them to conform to the mirror curvature would take too much time once finer grades are employed, in fact almost impossible at say #220 and beyond.

Stephen.

[Baza]

Thanks, ordered dental plaster to make a new one & I'll wait until it arrives. Will also build a bit of weight into the cast as I found the current one worked much better with an additional 3kg.

[Baza]

Progress is slow, most days I get about an hours work. Currently fine grinding and was amazed at how quickly the rough grind marks were removed from the centre of the mirror. The edge however is a different storey, it is obviously turned down to about 1.5mm in from the outer edge but is slowly coming out. Probably 8-10 hours more fine grinding which should clean up the edge.
Sagitta is at 5.5mm and seems stable, can't see any pits at 10x over the majority of the surface.
Guess it is time to make some test gear.

Pitting at edge.

[sopticals]

Good things take time.:thumbsup:

Stephen.

[Stefan Buda]

To fix up the edge, it would be much quicker to just go back to coarse grit.

[Baza]

Still polishing & beginning to test but would like clarification.

I have measured the sagitta at 6.02mm which gives an F of 2.6 and ROC of 1298mm.
When the mirror is in the test stand the reflected light comes to a tiny circle at approx 1254mm giving a differential of 44mm. Yellow paper not sitting flat but was necessary to get an image.

My conclusion is that the reflected light measurement is correct and that my sagitta measurement is wrong.

Has anyone had experience with this?

[Baza]

The light distance measurement was correct. More polishing, ROC is 1290 mm, Ronchi is showing straight lines across the centre but a TDE. A bit over a month before I get back to polishing.

[Lognic04]

I'm mirror making! :D i have a build thread here :)

[sopticals]

Hi:hi:, Watching with interest.

Stephen.

[sopticals]

Coating had pretty much expired on my 25" mirror (silver coating approx 13 months old). So using my collection of "garden sprayers" etc and "spray silver chemicals", did the re-coat. Actual chemicals used cost less than US$20.

Surprisingly previous DIY coating lasted quite well. As a comparison have attached a couple of pics. First is of the 25" mirror freshly coated. Second pic of my 33" mirror and 12+ months old silver coat (rough [after a clean or two] but still usable [just]).

Stephen.

[DavidU]

Steve, I would be interested in where to get the sivering spray.
cheers

[sopticals]

Hi Dave,:)

AngelGilding: see pics of what you need. Enough chemicals to do approximately 6-8 25" mirrors.

Stephen

[frankly1]

Hi all, been a bit busy for a few years so I haven't recoated my 19" mirror for awhile. It will be the third Qld Astrofest with same silver coating. I will admit it is a little "dusty" now.

[pixelsaurus]

What is the best way to strip the coating of an overcoated mirror?

Cheers.

[sopticals]

Ferric Chloride will strip aluminium coat.

Stephen.

[sopticals]

Couple more mirrors ready to polish.Will begin work on the 33"f4 first. Its the second 33 I have attempted. The first is "in service" but needs more work also. Its ok for low power work (115x-200x). Goes deeper than my 25" with over 70% more light gathering power.

Stephen.

[Baza]

There inspiring and look great. Be worth a trip to NZ just to see them.

[pixelsaurus]

Cheers. Sorry for the delay. Will it also remove any overcoating as well?

[sopticals]

Yes.:thumbsup:

Stephen

[sopticals]

Hi guys,:) you got any new mirror grinds on "the make"?

I've been so enjoying the full 2 degree wide views of Milky Way with my 10" Zhumell (one of my two "grab and go" scopes) using a Nagler 31T5 that for a "more intense" experience at the 1200-1250mm focal length I have decided to grind out a 12 1/2" blank I have sitting spare, to approx f4. Exit pupil at the 40.3x is close to 8mm, but my dark adapted pupil opens pretty wide at 7mm plus so won't lose much light.

Have played with 6"f8, 8"f6, 10"f5 (all around 1200-1250mm fl) so why not try 12.5" f4 for the same size FOV?

Anyway started the project today. See picture.

[pixelsaurus]

No grinding at the moment. A friend has given me a complete 225mm diameter Dall Kirkham mirror set ,which need to be coated, plus a transfer lens. I need to find a tube or summit. This after flocking my 12" newt and building an EQ platform.

[Stefan Buda]

Another one alive in Melbourne:
A couple of 250mm, f2.7 CDK primaries in the making.

[Lognic04]

Bump! Is anyone still out there making mirrors? I'm working on a 10 inch f3 then next a 16 f3 :)

[DaveGee]

I finished a 10"F3.something a few years ago but never tubed it. However I've started on a scope to house it. No secondary, video camera at prime focus.

Here are some photos. The filenames say 10f3 but I don't remember the context of what I was doing at the time (~2006)

I observe occultations and this is primarily to observe asteroid occultations, pre-point and drift while I sleep.

[Lognic04]

Hi Dave, good to see someone working on a mirror! Just a fair warning, your mirror shows quite bad Stig (see the tilt of the Ronchi bands changes.

But, I'm excited to see the finished scope!

[sopticals]

Just finished a 12 1/2" f4.14 mirror (well ready for final star testing). The second of my 33"ers, this one a f3.95 is going through corrective figuring to
correct a "rolled edge" defect. Did manage to get the inside 30" to an acceptable level of performance, but need to do better.All my work is done by hand, no machinery of any kind, (except did do the edge bevel with a belt sander).

[sopticals]

Recently threw together a cheapy Dob assembly to star test mirrors from 6"f8, 8"f6, 10"f5, 12 1/2" f4 (currently "housed") to 14 1/2" f3.6.

Construction cost was minimal as only used "stuff" I had lying around from
previous projects. Didn't have enough of same type of wheels for cart, so use a pair from a discarded child's toy for rear. Will get a matching
pair in due course.

Stephen.

[cpoc]

Does anyone know if there are any groups still doing mirror making/grinding in the Ballarat area? I think the local observatory used to but I don't think they do now?

I'd love to give it a go but would rather do so under supervision.


Thanks for any help.

[sopticals]

Seemingly "no response" to your request. Maybe they have "all died".:shrug:

Mainly just a few of us "old timers" left out there. Gone are the days when one "had" to fabricate ones own optics. All to easy now to buy affordable Chinese scopes.

Stephen.

[Rod]

Hi

I was active in Ballarat way back in the 90s when a few of us were making mirrors. I've lost touch since moving from there so not sure what's happening there now.

I notice you are in Melbourne. There is an active group of mirror makers now within the ASV who are part of the Instrument Making section.

Stefan and Logan are two whose posts you may have seen.

Regards

Rod

[Sparksinspace]

Is there anyone in Perth that still does mirror making and want's to be annoyed by a 37 year old noob that has no idea but is keen?
I work week on week off so I can spend a good solid amount of time learning.

Even if there is some sort of formal training courses I could do with testing and making optics would be great.

I've talked to Logan on CN and he might be refiguring my mirror for me but if there was someone closer that I could lean from and do it myself then all the better.

[Startrek]

Try Palmway Optical
Located mid north coast NSW
See attached
Cheers

[sopticals]

Hi, if you are looking for someone to aluminize/re-aluminize your mirror at a very reasonable price, contact Andrew at scopes@scopesnz.com

I have had a number of mirrors,(6"-12.5") coated by Andrew. Didn't cost "an arm and a leg". Most recent was the 12 1/2" at a cost of NZ$75 plus GST.
At present his chamber can handle up to 18", but he is thinking of building a larger vacuum tank able to manage possibly up to 700mm (27.5").

Doesn't offer an overcoat at this point in time,but is looking to include this option.

Oh! thought I might take the opportunity to drop a couple of Ronchi pics of where I'm up to with my 33"f4 mirror.

Stephen

[hamishbarker]

Hi Stephan,

are you only using the 12" subdia lap on the 33", or do you use a bigger one also?

My in-house polishing area (front half of living room) has been "annexed" (we replaced carpet on concrete floor with floorboards, so I polishing is banished from there), so I am setting up in the basement. I am considering making a subdia lap as I found progress slow and difficult with the full diameter 22" lap. Hopefully I will get restarted in the next few days (coming back to parabola from sphere(ish!). But mainly I have been busy with the renovation, work and family life (and made a safe, unobstructed solar dobsonian in time for the mercury transit. No sunspots now so it's a bit useless for the moment. :-(

Cheers,
hamish

[sopticals]

Hi Hamish,

Although I have previously used 22",17",11" down to 7" "subby" laps on each of the two 33" mirrors, I decided to just use the 12 1/2" lap used for polishing my recent 12 1/2" mirror (first time I used a full sized lap since early 1960's).
With the full sized lap on the smaller mirror I found I had an almost perfect sphere on coming out of the polish. From there figured if I dug out my old 7" lap it would be "easy peasy" to parabolize. Well couldn't have been easier, as it took only 3 1/2" hours to get a good parabola without any edge defects. My testing was simple, just 133 lpi Ronchi grating (bench testing) at ROC and Ronchi 133 lpi eyepiece star test to get the "jail bar" straight line (null) going through focus.

So on this second attempt at figuring this 33 I decided to "put in the yards" to remove the wide rolled edge using just the one lap (12 1/2") for the whole job. Had to go back to a sphere (or pretty close), which just seemed to happen as I removed the "shoulder" where rolled edge started. Once the edge was corrected I used the same strategy as for parabolizing the smaller 12 1/2".

Stephen

[hamishbarker]

As Stephan has already seen on "the other forum" and by email ( and who deserves big thanks for selling me the fine ground blank and giving me the (rather out of date now but they worked!) silvering chemicals.) I decided that my 550mmf4.6 was done. i read in a book that opticians often don't declare a mirror done, it is just that the mirror gets taken away from them. It was the second time (well, third if I count the time I was close except for a terribly turned outer 25mm.) that it was ready to go. I measured it at about 1/8 wavefront error a few weeks back and hubristically decided I could better that, and ended up overcorrected so had to go backwards a bit. Then I proceeded slowly, just 80 strokes and then testing, pretty much every day for a week or two.



Now it's nominally 1/5 wavelength, but a lot of that is a bit of turned down edge. maybe when I have more experience (and hubris) I'll attempt to fix that at some future recoating occasion.


yesterday was home silvering in the garden. Lots and lots of scrubbing with cotton balls, precipitated chalk and demineralised water. and then another scrub. Finally the tin chloride spray, a thorough rinse, then the magic couple of minutes with the silver solution and reducer in hand spray bottles.



There's a patch of frosty coating, so I need to redo the coating, but we had a little first light celebration with a friend and couple of beers and managed to see alpha crux through a tiny cloud hole, as well as thoroughly inspect the red anticollision lights on the nearby radio tower a couple of km away while collimating and star testing.


what a journey. I can't wait for some proper observing!


To anyone who is contemplating attempting their first mirror, go for it. one key thing I learned is the importance of having somewhere to be able to work on the mirror without needing to set up each time. I progressed a lot faster then. Also, procrastination is easy.



Hamish

[Sunfish]

Thanks for that encouragement. I have three Pyrex mirror blanks given to me to work on. A rough ground 100mm f5 for practice . An f7 200 mm which I would like to regrind to F4 . And finally , once I can work out what I am doing , a blank 12 inch which may be sometime before I can get to.

I have just received my grinding materials, 80 , 220 , 600 silicon carbide , 25 micron and 14micron aluminium oxide as that was what I could get locally.

I have made a mirror cell for an old finished 8 inch f7 so I can the tube assembly and secondary assembly process sorted out.

Any tips starting out or advice about my mirror cell?

[Sunfish]

It would be interesting to see how your silvering turned out. Any images yet?

[Spacecat]

Fantastic effort ! Was that your first mirror ?

[hamishbarker]

Yes, first mirror. pretty crazy to take on such a monster. Originally I was looking for a 16 inch blank but saw stephan's trademe post. I have a spare 22 inch blank completely unground. I liked to think it was my insurance against accidentally dropping the one I was working on, although not sure my wife would let me start again. :-) I don't think I will use it to make a matching mirror for a 22" bino (although never say never).

She is happy that the test rig has left the living room at last, and the polishing bench will soon leave the spare room.

I'll take some pictures of the coating and post soon.

[hamishbarker]

The 200f7 will probably be fine with simple three point support and two lower edge supports at 90 degrees, such as in dobson's original instructions. For dob mount this is ok, as the gravity load is always in the same direction. If it may be used on an equatorial mount, then this doesn't work. I had to redo my cell for my 200f6 dob as I also use the tube on my CGEM equatorial mount. I used a design by Gary Seronik , which is a backing disk of wood, three small 90 degree brackets around the mirror circumference and the mirror fixed with nice thick but not to large in area blobs of silicone to the disk, plus small blobs between the brackets and mirror edge.

https://garyseronik.com/a-simple-double-plate-mirror-cell-for-your-reflector/

Ensuring 2-3mm of clearance (which is filled by the silicone) means dimensional changes of the wood don't result in stress/deformation of the mirror. It seems to work ok, star test is still good and symmetric. If it was stressing, the star test would probably appear 3-lobed.

The self-weight of a 200mm mirror seems to be not too critical in terms of support. Main think is to be able to collimate it.

An added benefit of the double-plate cell is that mirror distance along the tube (for example to get a bit more back focus at the focuser) can be tweaked a little by having the acorn nut stick up higher and using longer collimation bolts.

[Sunfish]

Thanks for that.
Much appreciated. I look forward to seeing more of your mirror. A lot of interesting mirror making and finishing going one there.

[hamishbarker]

I have recoated the mirror. the frosty patch is gone, but I'm still not 100% pleased. There is a faint haze, like looking though cirrus, when looking directly at the mirror under strong lighting. Looking down the tube it isn't visible and through the eyepiece (at the local radio tower red beacon, was so far too cloudy for stars) the performance looked no different to the previous coating.


I suspect the haze may be costing a few % in reflection and extra scattered light.



disturbingly, more tiny scratches are visible under strong lighting. I'm worried that perhaps my chalk supply is also containing some silica. It is labeled pharma grade so should be ok I guess. I will check this by dissolving a sample with dilute hydrochloric acid, which should dissolve calcium carbonate but not silica (or alumina if it was contaminated with clay).


If so, bugger. At this stage I will not regrind and refigure!

[Sunfish]

That does sound disturbing. Perhaps , like my SCT, a few scratches and defects in the finish are nothing one can see . The Mel Bartels site suggests that you can live with final scratches.

There is an Indian scientist and mirror maker who suggests that every grade of grinding material he gets requires testing with a method he describes.


Maybe I should try that if there is a risk of problems.

https://sites.google.com/site/diy100atm/getting-started

[hamishbarker]

some sensational views of jupiter and saturn and the moon this evening, after a bit of unexpected recollimation. normally this telescope holds collimation even if the mirror has been removed and replaced. Eventually I realised that the need for collimation had been caused by me not screwing the mirror box tailgate completely shut. Thankfully the safety latch was closed and one screw just hand tightened, but that could have been a disaster of a dumped mirror onto the concrete!



the scratches might be making a little scattered light across the field, but overall I am very happy with the performance. No deep sky tonight as scudding clouds had me rapidly switching between jupiter, saturn the moon, alpacrucis as holes in the clouds presented themselves.

[sopticals]

Nice report Hamish.:thumbsup: Being your own optician is a plus. Wouldn't worry too much about the fine scratches, as you will no doubt at some point in the future do a re figure (too remove the residual TDE) at which time the offending blemishes may disappear (if not too deep).

Stephen.

[Sunfish]

Excellent news. Good to hear the mirror performs. Always a chance of some problem.

Any idea what to do if the the glass tool sticks to the mirror on 600 grit?

[Sunfish]

Put it in the freezer . That worked.

[Sunfish]

I have just fine ground my first mirror. An F4 4.25 inch. Seemed to work reasonably well except for one very faint wiggly scratch. The earlier worse scratch ground out with aluminium oxide so I suppose I need to try another round.

I am wondering how much mineral oil or other material to temper my black pitch with for the polish lap. It has been used once but is too hard. I tried a few drops of mineral oil, less brittle but the penetrometer test only gave me a mm with 500g after 30 minutes at 19 degrees. I think I need 2.5mm.

Any advice welcome.

[sopticals]

Stockholm Tar is excellent for use as a Gugloz softener. Use sparingly as one can go "to far". Available from animal vet shops.

Stephen.

[Sunfish]

Thanks for that advice. I have read about Stockholm tar as and ingredient in DIY pitch. I will give it a try.

[Sunfish]

My first pitch lap (oversized 4.25 inch. ) on ply as per Bartels. The Stockholm tar worked. Pitch is soft enough to sink a 7mm rounded bar into 2mm after an hour under 500g. Now for the polish. Don’t think the 1 micron aluminium oxide will be right so waiting for Cerium Oxide.

[Sunfish]

A few Ronchi of my half polished little mirror inside and near focus and the owl eye outside of focus by nearly an inch.

You can see the outside owl eye focuses on the pyrex mould marks and edge rib on the back.

Sorry about the blurry ronchi.

The in focus look similar to Bartels part polished images except for the strange lines at the very edge . Perhaps the rib on the rear edge. Or the oversize lap , which is a bit of a handful to keep pressed.

Any more experienced thoughts welcome.

[sopticals]

Looks like you have a TDE. might be a good idea to post some images inside/outside ROC with less lines (say 4 or 5 dark lines). Inside 90% R looks close to spherical (less lines would give a better picture of where your at). Give higher sensitivity.

Stephen

[Sunfish]

Thanks for that reply Stephen.

When I look through the Ronchigrams on the Stellafane website it looks like a turned up edge gone slightly parabolic which also include those fine lines at the edge. There is also a hint of different turn in lines top and bottom which is problem i think.

I have trimmed up my lap a little reducing the oversize diameter hoping that will help but would like to find out more before i keep polishing.

Here are some images of out of focus owl eyes with less lines and close to in focus. Any further inside focus produces more and more lines as per the previous images.

[Sunfish]

My images look like this which are parabolic. Not sure I understand that when I am trying to get spherical. Or perhaps over corrected as per the second chart?

[Sunfish]

I found some old posts here on IS explaining a few techniques for moving along. So I trimmed and faceted my lap more often and pressed it often and took longer one third strokes to get down the turned up edge and it seems to be working.

Those turned up edges are starting to go and the Ronchi lines are starting to straighten up as the edge is more polished. Suppose to finish from centre out but perhaps my Bartels method and the oversized lap has over emphasised the correction at the edges. Still a loong way to go in polishing for such a little mirror but moving forward all the same.

[Spacecat]

Sunfish - the best strokes to get rid of TDE are very short 1/4 to 1/5 length W stroke with mirror face up , with some randomness thrown in to maintain or head better toward spherical .


Something like a 200mm mirror is much better fit for the hands and more stable , so you are setting yourself a task with a 100mm . The back should be ground flat or slightly concave. If you can see the mirror rock at all while you are polishing tool on top , this can contribute to a TDE .

[Sunfish]

Thanks for that.
I was directed to this site by the ATM who gave me the blanks and this blog describes working with a TDE .

http://atm-workshop.com/mirror14-log.html

Perhaps it is possible to be over corrected in the centre as per the stellafane table yet still have TDE. I did start off through the whole polish with short strokes But not much W . So I can try adding more W pattern stroke with the trimmed up lap.

I have an 8 inch to go on to which is down to f7 which I would like to deepen to f4 with a tile tool and 80 grit.

[mura_gadi]

Hello,

I am looking at trying to take my blanks from a large sheet of soda glass and I am after help from those who have done the same.

So before I re-invent an old wheel any pointers or tips?

Here's what I thought I could do:

Below is a youtube video showing a great sales pitch for cutting thick glass, while the video shows the product being fool proof, the price is not within this fools reach. The kit listed gives a 404 error but I've sourced the hammer at about $500 USD.

I thought I could modify a "Automatic Centre Punch Adjustable Spring Loaded Metal Drill Tool" (ebay) which allows for interchangeable springs for an adjustable striking force. Then all I need is a similar head component with a washer and blunted striker head. The hole punch can be had for under $10.

But I am sure better minds than mine have been here before. So links, tips, tricks and traps and any other advice would be great.

I don't have access to a slow speed drill press for a DYI glass cutter.


Thanks
Steve
Youtube link:
https://www.youtube.com/watch?v=x-84Sdq7jaE

[sopticals]

What's the size of the sheet? and thickness?

Stephen.

[mura_gadi]

Approx. 1300x690x25mm Slab of soda glass - ie green at the edge from an previous ATM so I assume non-toughened etc.

[Rainmaker]

Prestige Metal Works in Fyshwick can cut it into discs for you using their waterjet.....

[mura_gadi]

ACT Steel Works quoted $770 for 3 420mm, with 4 220mm and one 340mm cut from another sheet. Prestige Metal Works would not give a quote till they had cut a blank to work out the time and costs.

The waterjet saving is not having the disc plus 10% waste as the video showed, which is very nice. I have added extra as I am pretty sure I will have TDE and I'm just looking to bank/baffle the mirror down to 16", 12" and 8" etc.

PS. Matt(Rainmaker) I sent a pm regarding the estate purchase

[sopticals]

Had 4 sheets float (2.4m x 1.2m) waterjet cut back in 2014 by Aquajet NZ (Napier) sheets thicknesses from 34.1mm to 37.1mm. Cost was NZ$4000 to cut 39 blanks ranging from 12.6"-33" diameter.

[mura_gadi]

Do you know/remember if there was a limit on how close the discs can be together or from the edge of the sheet?



Steve

[sopticals]

Checking original cutting plans I see I allowed 1" from edge. Probably could have reduced that to less though. Some of the sheets had some edge damage so allowed a margin for that.

[mura_gadi]

Hello,

Well I tried Supaglass Industries out in Fyshwick and they were the cheapest quotes I could find in the Canberra region. $25 for the 8" cuts and $50 for a 16" cut on the quotes, best in Canberra by a long way.

So, I drove out there in a flash last Friday as I have been reading "How to make a Telescope" by Jean Texereau for way too long without anything to work with, had my grits since Christmas after an awesome estate buy.

Just got a call today, the 203mm is cut and the 220mm is on the machine deck. Cut those from my smaller bit of glass to test the services, the 220mm is my tool.

Going to try a 8"F9.x for a test run on a 1850/1900mm or so dob, and the F9 sits well with Rayleigh's rule. With a great deal of luck my difference from sphere to parabola will be next to nothing.

Happy chappie!
Till I hit the first of many expected hurdles no doubt...
Steve
Ps. Smiling till reality sets in at least
Pps. Hopefully Covid restrictions in Sydney will be eased and I can get my dental stone powder at the end of this month. Got a Tassie holiday lined up and the shop front is only a 5k detour, which SWMBO has approved.

Quote: Got the final pricing for the two 8" cut and they had mucked up the quote. Its now $42.50 each for the 8" and $85 for the 16" after they timed it on the machine. They're matching the original quote for the two 8" blanks, which is very nice, even at the higher prices they are still the cheapest option around the Canberra region though.

[Sunfish]

Sounds good. Great and economical blanks. I hope grinding goes well. I have stalled at the final polishing stage of my little F4 4 1/2 inch. I might have to follow your lead and move on to the 8 inch blank.

I am interested to find out how ATMers are planning to get a mirror coated. That seems to be an issue.

[mura_gadi]

Good question,

It all depends if Wayne Sainty from www.saintech.com is still in business or has managed to onsell the business to a new owner. wayne@saintech.com is the contact, I'm off to get my blanks.

I'm a fair way away from needing to know atm, but if you do get a response or answer to that question, please post. I know others are keen to find a astronomical mirror coating services here in Oz.


Steve

[multiweb]

:eyepop: That's bloody cheap. What type of glass and is it waterjet cut?

[mura_gadi]

Woohoo, blanks look really good! Great way to finish the week...

Sunfish - Those prices are priced at the cut only I supplied my own glass.

Sydney Glass (Ph 02 97902830) supplies and cuts 25mm glass, they might be worth a try.

A.E.C in South Australia who supply all the ATM'rs needs also offers mirror coating services last I read, so they might be worth a try as well.

I can supply you with a nice 6" Pyrex blank - to meet Rayleigh's rule/criteria that would be a 6" at F8.2 or little under 5' with mirror cells etc if you're looking to mount on a tripod that might be a better option.

Two arguments for that:
1) Why both going to all the hassle when you can get a 6" F8 anyway
2) You can get a 6" F8 very cheaply second hand, that means all the build requirements for the scope are taken care of, tube, base, mirror cell and secondary mirrors holders etc.

While everyone says Pyrex is better due to better thermal properties, soda glass actually takes a better polish/finish than Pyrex does. I am hoping that the better finish will be worth the little extra wait in cool down times.

[mura_gadi]

Multiweb

I have updated the quote price in a PS. but still very cheap, my own glass supplied and yes water jet cut, real nice finish - I think anyway. No bevel on the edges etc though, that's my step one.

See my comment about Sydney Glass Pty Ltd below.

SupaGlass Industries Canberra
Quote: Got the final pricing for the two 8" cut and they had mucked up the quote. Its now $42.50 each for the 8" and $85 for the 16" after they timed it on the machine. They're matching the original quote for the two 8" blanks($25 each), which is very nice, even at the higher prices they are still the cheapest option around the Canberra region though.

Sydney Glass Pty Ltd:
Quote to supply 25mm clear float Disc 420 dia , $290.00 + GST + Each
Quote to water jet own glass $65.00 + GST + Each

So, its a lot cheaper if you can source your own glass and the wetjet at Sydney is slightly cheaper than my best price in Canberra.


Ps. Barnes in Sydney for Hydrostone TB at around $70 for 25kilo tub, hoping to make that detour later in the month. HQ is only 5k's off the route but currently closed due to Covid, Newtown store is still operating though.

[sopticals]

If you want thicker float, I still have a few pieces left. Contact me if interested.

Stephen.

[mura_gadi]

Hello,

Thicker glass sounds nice, with my 25mm I'm happy to go to 12", but the 16" at 25mm has me doubtful... When I get half an idea I may feel differently.

I have just popped my glass pushing cherry, got the table built and the glass tool beveled to 1-2mm. After nearly 3 hours I have a nice little bubble of trapped air in the center of the glass which I am pretty happy about.

Hopefully now that I am starting to make a wider contact between surfaces it will progress a lot quicker. On #120 grit it does seem to take a long time to get started as a complete newb.

But happy I've started, see what the next 3 hours gives me, hopefully a measurable curve!!!


Steve

[legswilly]

Hello. This is my first post and I have two questions.
Is there a mirror maker in the Port Macquarie/Wauchope area? I hope to get come information on testing equipment and tests.


Why are the secondary mirrors on reflectors made an oval shape? The reflection from the primary mirror creates an oval shape on the seconday, it being at 45deg angle. Could a round mirror, or even a rectangular or square one of sufficient size be used and would it be cheaper to make? Has anyone tried to use a piece of a normal mirror like a bathroom one?

[astro744]

Welcome. Not sure of where any makers are and perhaps others will answer. In answer to your second question:

Any shape mirror will do but the elliptical if correctly sized is most efficient at producing a fully illumination circular field and without introducing odd diffraction effects in the process. The reason is that an elliptical secondary will show a circular reflection through the focuser when intercepted at 45 deg. which if sized correctly will show you the circular primary when you look through the focuser (no eyepiece).

When buying a secondary you need to know the minor axis of the elliptical since this will in effect be the diameter of the reflection at 45 deg. and equal to the minor axis. The major axis of the elliptical is 1.4142 x the minor axis; 1^2+1^2=1.4142^2 but it is not necessary to know this for any calculation involving the secondary.

[Sunfish]

Normal mirror glass will not work as the mirror surface is on the back of the glass. Telescope mirrors are first surface so the mirrored surface is in the front of the glass. Technically difficult but necessary.

Go to any amateur mirror making site such as Bartels or download a book and the geometry will be made clear.

[legswilly]

Thank you Ray and Astro.

We have an observatory at Port Macquarie and I will contact the person there and they might know someone in the area, were I could have a look at a setup for testing mirrors. No hurry because I ordered a 200mm mirror from China. It most likely will be only spherical and not suitable for a telescope, but I thought I could use it to find out how testing works. I may have squandered some money, it won't be the first time. I am waiting on some books and they might tell me a bit more.

[legswilly]

I am looking at how to test a mirror and I would like to build an autocollimation test rig.

The one I have read up on had a round flat mirror. Would it make a difference if the mirror is square? Surely a square one would reflect the round image of the Primary as a round. How is a mirror made flat?
I have tried to print a ronchi screen on the laser printer. It did not come out as well and everyone that makes a mirror would use a ronchi. Where can I get one at a reasonable cost.

[mura_gadi]

Hello,

I am currently looking at ronchi eyepieces and an artificial star setup for my testing. From looking around the ronchi screen on glass slides are fairly expensive.

Some smaller printers/startups do offer printing to glass, I haven't got any feed back on pricing for those.

Try and print the whole transparency as A4, then scan over the sheet and cut your best two square inches out.


Steve

[Saturnine]

Hi Werner

Try looking up the Stellafane site https://stellafane.org/stellafane-main/tm/atm/index.html

Plenty of other mirror making and testing sites on the web as well

[Sunfish]

Look on the Stellafane website in the testing section for a video lecture regarding auto collimation testing. The mirror is required to be optically flat first surface . The video describes the specification of flatness required and the history and design of these kinds of testing rigs.

[legswilly]

Thank you for the link. I did have a look at the auto collimation test setup and will build one as soon as I can get hold of a reasonably priced 300mm flat mirror, or 300mm blanks.

Sydney glass quoted me $319.00 for one 25mm blank, and $154.00 for polishing.
Telescopes and Astronomy sells 305mm x 25mm for $162.00. Need three blanks.

My concern is the thickness. It should be OK for a flat mirror, but I would eventually want to use the remaining flats to make a mirror 300mm and it might be too thin.
One guy tried to laminate, but said he was not successful with epoxy. Have to look him up and see what went wrong.
I am looking around on AliBaba and will get some answers maybe on monday.
Where else could I look?

[legswilly]

Why did you have to use glass pillars? Why not poor the resin in the centre of the bottom glass and place the the top on make sure the glue squeezes out everywhere and let it sit for a few hours with the slow curing epoxy.

[sopticals]

Hi Werner, I tried the "pillars between" experiment because I had past experience with the Hubble Optics "sandwich" mirrors (3 x 14") and they performed well. Anyway, was to subsequently obtain a large quantity of thicker (35-37mm) good quality plate glass so never had to try any alternative ways of bonding together thinner glass sheets.

Stephen.

[JohnF]

I certainly hope there are still amateur mirror makers as am still trying to sell a pair of 6 inch blanks.

[legswilly]

Thank you for replying Stephen. Have already bought some glass and will try to epoxy together. I also want to build a auto collimation test stand and needed a flat mirror for it. Could not find one to buy, so will make one. Still need to go to Sydney to pick up the glass which is waiting for me.
Found a Ronchi eyepiece maker https://www.gerdneumann.net/english/ronchi-okular-ronchi-eyepiece/ronchi-okular-5l-mm.html. He makes a 10lines per mm, which equates to 254 lines per inch. Is 254 lines too many? It's a lot cheaper.

[sopticals]

254 lines per inch should be ok. I use a 133 lpi eyepiece and it also works well for testing the whole system using an actual star. At 254 lpi sensitivity to optical defects should be higher. Shouldn't be any negative diffraction issues.

Stephen.

[legswilly]

I have got all the grit I need for grinding and polishing. Bought from Naxos in Sydney which I know from the time I was working.
Now I am looking where to buy pitch from. Is there someone in OZ? Last time I got a quote for things from the USA, the postage was quite expensive and I would try to get is somewhere else. Is there somewhere in the UK? I looked also in Germany, but they use a different classification for it. So I am still looking.
Bite the bullet and buy from the USA ?

[Merlin66]

PM sent.

[dave brock]

I find 250LPI ok in the scope on a star but too fine on the bench.
Dave

[legswilly]

Too late, it's already on order from Germany. I did not know where to get one with less lines.

Where did you get your 250 line one from. What make is it?

[dave brock]

I got my latest ones from ronchiscreens.com but they are no longer available.

[mura_gadi]

Hello,

Try looking for "Gerd Neumann (jr)" products, there are 10L/mm for 250 per inch for around 35euro, 5L/mm as well, but can be a little more expensive, $55euro.



Steve
Ps. https://www.gerdneumann.net/
PPs. https://www.gerdneumann.net/deutsch/ronchi-okular-ronchi-eyepiece.html (a few shops hold them as well)

Should add Ronchi slides are available in Oz from places like Edmunds optics and Proscitech, but expect to pay between $250-$500+ depending on L/mm and the size/material of the slide.

[legswilly]

[QUOTE=mura_gadi;1522464]Hello,

Try looking for "Gerd Neumann (jr)" products, there are 10L/mm for 250 per inch for around 35euro, 5L/mm as well, but can be a little more expensive, $55euro.

Thank you for the link. I got the 10L/mm and it's coming. Reading about testing most people use closer to 5L/mm. Being a skinflint, I bought the cheaper one. I'll see how it goes.

I am nowhere near testing. At the this time, I am building a turntable for grinding and polishing. I thought I needed a flat mirror for the auto collimation test as is written on the Stellafane site. Now I know I don't need a glass mirror. Too late. Got most of the stuff for the turntable.

[legswilly]

I got some square glass from Sydney and wanted to cut the rounds with a glass cutter as shown on youtube. Would not work with the cutter I bought and homemade tools. Got my grinding/polishing turntable built and cut the rounds on the turntable. First one was not too bad and the next 2 turned out OK. Chamfered them and I thought the polishing should be OK to do.

I read about grinding/polishing flats in Making your own telescope by Allyn J. Thompson. With the three blanks method and float glass, 600 grit is recommended, because it should be reasonably flat. I bought an optical flat and calibrated a homemade spherometer and it gives me a difference of appr. 0.0005". That is over 75mm, I only got a 80mm optical flat. So I thought that is not too much.
I have never polished glass and what happened is that after a very few rotations (4 rpm) the top blank seized on me. I used plenty of water in which I had mixed the 600 grit. It was like I was wringing precision steel flats together. Very hard to shift. Has to stop the turntable and slide it off. On looking at the blanks after cleaning, some dull areas showed where it was ground. If I would keep going, I could not hold onto it, too much grabbing.

What am I doing wrong? Is it not possible to do it this way? Would I have to make a lap and selectively grind/polish it flat?
Sorry for the long post.

[legswilly]

Would I have to make a lap and selectively grind/polish it flat?
[/QUOTE]


I persevered and changed to 400 grit. Went through a whole cycle with the four disks and finally have some success.

What was wrong with the 600 grit, nothing, I used more water to dilute it and it worked, but slowly. Wanted all the shine off and I am nearly there. One more cycle with the 400 and I start the 600 cycle again.

[Stefan Buda]

Werner,

You don't need an optical flat for setting your spherometer.
Use your spherometer as a comparator: If you get the same reading on all three surfaces, then you are well within what is achievable with your spherometer. In fact you can grind a good flat using only two discs but you need three later on when doing the figuring.

[legswilly]

Hi Stefan, What do you mean by figuring? To get a flat, I would proceed with 600 grit, 800 grit and finally cerium oxide to polish it.
Still using the three disks, I will only polish one. I planned to make a pitch lap for the cerium oxide.



Today I ground the three disks with 400 grit and nearly finished. One disk has still an area that shows as not being ground, very small. Once I have this disk showing all ground, I go back to the 600 and 800 grits. Should I go to a finer than the 800 before polishing with cerium oxide?



I am very careful not to chip the disks when changing over in the sequence, still getting chips on the rims. Is it just the glass I am using? It's float glass. Maybe there is better quality float glass. I will use the best disk for the flat for the auto-collimation test rig and cut the remaining disk down to 300mm.

[Stefan Buda]

Werner,

You need to get to much finer grit than 800 before the polishing with cerium oxide. I would finish the polishing with 9 micron aluminium oxide.

You must understand that no matter how flat you manage to grind your disk, it will not remain the same during polishing. That is why it needs figuring like any other optical surface. For that you will need to polish all three surfaces so that you can compare them during the many figuring stages. Unless you have a reference flat the same size that you are making.

[legswilly]

I will ring tomorrow to get #1000 (6.8 - 9.3microns) and #1200 (4.5 - 6.5microns). Have to get it sent up from Sydney. I hope he will post it out to me.
The three disks are 350mm and initially I thought I would make a pitch lap of 75% that is 260mm for the polishing. I did the grinding with a 1/4 stroke. Should I make a 350mm lap, there may not have to be a lot figuring to be done. I can cold press on one while I polish the others. I don't know if at the end of the 1200, there is going to be enough reflection to look at it with a newton interference meter. It works with the un-ground side of the glass. The fringes are all over the place. I got an optical flat, but only an 80mm one, should have bought a bigger one (maybe 100mm), but it would not have made a lot of difference.

At the end of all this, I will get the 1000 and 1200 grit. Should I make the pitch lap 350mm, do I need to make a pitch lap, Or can I use the three to polish against each other?

[Rod]

Hi Werner

Welcome to the hobby! Mirror making is engrossing and from a quick read of your previous messages you are obvious enthusiastic.

I gather making flats is your first project and you want a flat to be able to null test a mirror. I think that’s a great idea but making an optical flat as a first project may be a bit of a challenge - partly because not that many amateurs do it and so you don’t have a lot of expertise to draw on. Stefan is the obvious exception here and it’s great he’s following your thread.

I don’t want to discourage you but I’d like to suggest a simpler first project. If you built a long focus smaller mirror say a 6 inch F10 you would learn the basics of mirror making and have a lot of expertise available to you on web sites such as stellafane and through this thread. A 6 inch F 10 can be null tested with a simple ronchi or Foucault test as the mirror can be left spherical so you avoid the difficulties of figuring and testing a parabola which many first timers find challenging. A well made long focus Newtonian is a versatile instrument.

After you have this experience you, you’ll be able to tackle more difficult projects. I could supply you with a 6 inch Pyrex blank if you decide to do what I suggest.

Rod

[legswilly]

@Stefan and Rod
Thank you for your interest and I appreciate your advice.
I finished today with 800 grit grinding and await the 1200 grit.
I calibrated the spherometer with its legs at diameter of 70mm on a glass optical flat.
I did measure how flat my 350mm disks are. At 0, 90, 180 and 240deg and at each coordinate at the centre, 70mm, and 140mm and found two disks consistent at each measurement point to be concave by 0.0003”. the third disk is concave by 0.00025 (half a graduation on my spherometer). Which means nothing other than that they are more consistent overall and nowhere near flat-. I am sure the concavity was produced by the way I ground. Tomorrow I will try and increase the stroke and see what happens after one cycle. Maybe next week I will get the 1200 grit and try polishing one disk with cerium oxide. Want to see what it looks like with the interferometer.. I might make a larger spherometer and get it calibrated at the local machine shop. They should have a surface plate.
Thank you Rod for the offer of a blank. At the moment I want to see how far I can go. Like a dog with a bone.
Still need to find someone uphere with a test setup, ronchi and focault. Tried the local Port Macquarie club, but no joy.
p { margin-bottom: 0.25cm; line-height: 115%; background: transparent }

[Stefan Buda]

Think about it until you understand what I said, otherwise you may be wasting your time.

If you grind two discs against each other, you can get the same reading with the spherometer, on both discs, only if both surfaces are flat. We are talking about matched surfaces. If one is slightly convex the other one will be slightly concave and your spherometer will show the difference. That means you can detect an error that is twice smaller than what you could detect by comparing one disc with an optical flat as reference.

[legswilly]

Quote: If you grind two discs against each other, you can get the same reading with the spherometer, on both discs, only if both surfaces are flat. We are talking about matched surfaces. If one is slightly convex the other one will be slightly concave and your spherometer will show the difference. That means twice the the error that you could detect by comparing one disc with an optical flat as reference.End quote


There must be something wrong with my measuring. All three disks are showing to be concave. Two are consistently on all measuring locations 0.0003" concave. The third less at 0.00025". I checked it today again thinking there may be a difference in the way I measured, but it is the same result. grinding one on top of the other would produce concavity/convexity. I checked the specified sequence of which goes on top or bottom and I copied it OK. I kept check on the sequence of grinding by marking it off on a sheet. I counted the strokes for each combination. I washed the disks each time a sequence was finished. As you say, if the measurements are the same for all the disks, which they are more or less, I was splitting .0005" graduations to arrive at 0.0003 and 0.00025, then the disks are flat. That leaves the optical flat. Over ebay, a Russian one, Comes with a certificate:P, could be the culprit. So I need to find out what is going on. I am making a bigger spherometer, cant do any more anyway

until the grit comes for polishing. I hope I can get it calibrated at the local machine shop.
The grit I was quoted for #1000 and #1200 is silicon carbide. Would I be wrong to ask for aluminium oxide?
Thank you Stefan

[Stefan Buda]

You still don't get it: You don't need to do any calibration of your spherometer because you are not making a sphere, where you would need to measure the sagitta. You just want to compare two surfaces that have been rubbed together. If you get the same reading, it means they are flat, no matter what the actual reading is.

I have not seen/used silicon carbide finer than #800. All the fine grits that I use are aluminium oxide.

What thickness are your discs? If they are not thicker than 30mm, better not waste your time. Even at 30mm thickness, plate glass of that size would not make a precision flat even in experienced hands.

[legswilly]

I got it, thanks to you. I just wanted to find out why the discrepancy between the optical flat and the disks I ground.


Since you are using aluminium oxide, I will ask him to supply me with it instead of the quoted silicon carbide.
The disks I got were square float glass 19mm thick. So they are not precision optical flats, although they should be reasonably flat according to the measurements made. I want to make an auto-collimation test rig, as is on the Stellafane web site. The best one I want to use as the flat mirror with the hole in the centre.

[Stefan Buda]

Werner,

Please don't go any further with this idea because it is practically impossible to make a useful flat with the diameter to thickness ratio that you describe.
I understand that you intend to make an optical flat with a D/T=18 - way beyond the realm of amateur telescope makers.

You can epoxy two of the discs together and use it as a grinding tool for making a 14" mirror (but not using the third disc - you would need a thicker blank for that).

[legswilly]

What is meant with D/T=18? I WILL find a site that has translations of commonly used acronyms, but what is it?
I would like to cut the disks to 12" and see if I can produce an F4.5 mirror. Or maybe even a 10" one.

[Stefan Buda]

Diameter divided by Thickness.

[legswilly]

Thank you

[legswilly]

What would one use to look at the ground/polished surface. I would be interested to see it as it gets smoother with each size of grit.Looking at the surface after say the 5micron grind, could one see what imperfections are left?

[mura_gadi]

Hello,

Normally you just turn a 25mm plossl around and look at the glass close up in a strong light for the 1st few grades. After that a 10mm reversed around for your seeing glass, but that may just be because most ppl have those ep's.

Its about seeing the same size imperfections across the mirror to know that your done with a grit. Also check to see that the center, middle and edge are all progressing together evenly. You should see no variation in the surface finish from edge to center, neither smoother or rougher in any zones.

I'm still on my first mirror and can only say I can see the difference between 750 and 1100 under a 25mm plossl seeing glass(24-16micron) with fairly old eyes.


Steve
Ps. You generally grind at half the grit size, so, the 750/1100 is going from 12-8micron smoothed surface.

[legswilly]

What brand eyepieces have you got and what focal lengths. I will buy some, but which is the question. There are so many to choose from.

[mura_gadi]

Hello,

Sorry I only mentioned the 10/25mm plossl as they come standard with most startup scopes and a lot of ppl have them already. You can find them fairly cheaply and readily normally because of ppl upgrading, or borrowed from a astroclub member etc.

All you need is a strong light and a magnifying glass, jewelers loupe what ever you can get. The eps suggestion was based on an assumption you'd have the plossls.



Steve

[legswilly]

I just bought a set of eyepieces from Amazon. Really cheap, but all I want to do with them is look at the disk I am grinding at the moment.
So, thank you, I didn't know one can use them like this. I vaguely remember reading about it, but I am getting too old and can't remember. I like this forum, good advice and hints. Once I have my telescope made, I can use them and upgrade to something better.

[legswilly]

I checked after grinding with 1200 grit and the surface looked OK. Started to polish with cerium oxide. I had some sessions and checked the progress. It looked much clearer and did a test with a mini mag light and came across some very fine scratches. At first I thought it is from the pitch lap I made and I was worried (the first one I made). Then I wanted to see if it is indeed the pitch lap, by doing another 2 sessions of polishing, this time not by hand but on the turntable. There were a few less scratches, with the very faint ones gone. It was not the pitch lap? Tried to polish a few more sessions on the turntable, but the few scratches would not go away, but would not increase in number. I will go back to 800 and 1200 grit and get the scratches out before I start polishing again.
Question, My cerium oxide is of a reddish brown colour. What I see on the net is that people are using a white one. Is my cerium oxide of poor quality? Would it still polish out, but take longer? I bought this from a supplier which we used when I was still working.

[mura_gadi]

Hello,

Not sure about the reddish side, should be a pink or salmon at best. I had a quick look at CN and got the post below.

I have read that cerium oxide is used for industrial use in polishing glass, rouge is preferred for large precision instruments.

Throw some of your cerium oxide in a glass with water, it should settle cleanly in the water, you should not have a greasy surface with scum on top.

Can I ask how your water is out there? You might have tap water that is very hard or has impurities in it. You should be able to use the tap water for warming and cleaning the glass. But after drying and cleaning ready for a session, maybe try warmed bottle water with the rouge.



Steve
Ps. Try the jeweller's supply shops for rouge, or any lapping supplies companies if you want to try something fancier.

Microgrits are great, very good for the min/max grain sizes, highly recommended.

(not sure of naming companies on IIS but "Gemcuts" and "Aussiesapphire" are two that might be able to help you with rouge. For lapping suppliers try for zirconium oxide if you can find any, even better than rouge.)

https://www.cloudynights.com/topic/668386-red-cerium-oxide-polishing-8in-f27-pyrex-mirror/

[legswilly]

Thank you, did not think of the water supply. I was using tank water. Could have had some bits in it. Change to town water and wash down after each session. A session lasts four lots of 15 min polishing with 20 min cooling of the motor. I will change the motor setup, so it won't heat up and I can polish/grind as long as I want.



Looked at the CN link and it gave me some idea on how long it takes to polish and I am nowhere near the times they say. Thank you for that. Might ask the cerium supplier the particle size of it.

[mura_gadi]

Hello,

With grits they tend to have all the smaller grits in them till you get to the next grit grade. Ie 120 grit will have grits from 115-175 or so, when the 180grit kicks in.

The smaller grits within the range tend to act like mud, interfere with the grinding action of the normal sized grits and aren't good for great finishes.

That's were the microgrits and such shine. You have a very small range of grit sizes within the grade. The zirconium is 1-3um for example, so, ask about the particle size range within the grit/oxide more than the particle size itself.


Steve
PS. Remember to keep your channels open on the lap, and try micro-faceting the lap as well. As per Mel Bartels site - https://www.bbastrodesigns.com/JoyOfMirrorMaking/Polishing.html - MOT polishing tends to be better as the channels and groves can hold the larger particles away from the mirror surface leaving only the embedded rouge to work on the lap surface.

TBH I would think you would be better going manual at 1200 grit. Use the 1200 to learn the "I" stroke that you will need later for the polish to parabola. The "I" stroke will setup the mirror surface better for polishing and you'll have better control if you practice the stroke first. But I am a newb and my knowledge is only what I can remember from bits of reading.

[Sunfish]

My cerium oxide is a pinkish colour. Polishes very well. Scratches at the polishing stage can be left over from previous grits or from dirt and impurities in the air. Such as dust falling from a shelf above or blowing in. Rinse every thing thoroughly before starting a session and keep your lap in a sealed box. I have had a scratch when I was a little careless about dust and rinsing off . Improved after better attention.

[legswilly]

I will need to be more careful with the washing/cleaning after each grit. Made myself a wash stand covered in black plastic just outside the shed entrance. After my session today, I found some small scratches and a few little pits. Tomorrow I will clean everything before I start another session. Keep on going with the 800 grit until I get it right.
Another problem I have is the foucault/ronchi test. I built the autocollimation test stand as in the movie on the Stellafane site. Until I have a mirror, which I am making now, I thought I would try and do the ronchi test. They say it is easy, for me it is not obvious.I got some slides with ronchi grids, But the grids are just a small area, maybe 10mm square and one can't shine the light source through them and at the same time look through the grid. Made myself a ronchi grid out of fishing line to shine the light through and at the same time look through it. Cant see any pattern.
I bought a cheap mirror from China solely for the purpose of learning how to test a mirror. It is a 200mm mirror and the specs say it is 750mm focal length. Today I measured the saggita as 0.1325" and that works out to 742.8mm. So I am within the range of the tester set up to 1500mm. What do I do wrong?
Wish I had a telescope/mirror grinder here.

[mura_gadi]

Hello,

Keeping things clean is a bugger, change all your clothes, change the rags you use, shower, check your fingers nails... list is nearly bloody endless (if your machining - all your pillars and seals/gaskets/grooves etc.). I try and keep anything I touch to one side, then I know it needs to be cleaned. Saves me missing stuff.

What thickness is the fishing line? You only want about 3-4 lines a mm after you have unthreaded the 2nd cord. I have some .105mm line on the way, it's thinner than needed, a mistake from me. You'll end up with a little under 125 lines per inch with that so should still be useable. I can send you a couple of meters after it arrives if you want. (6weeks +??? no idea with OzPost going postal)

Your using a LED diffused light(if its not, just rub it with some very fine wet and dry sand paper)? Strong 30,000mcd rating etc? You'll not get much feed back on the Ronchi till you at least get a flash polish on the face I think...

I could be wrong there, hopefully someone will say at what grit you can expect to see the bands on a blank from a Ronchi test.



Steve
Ps. Another way to test your grits is to try a elutriation process, very effective and fairly easy.

[legswilly]

I am using a Krypton bulb as it is shown on the autocollimation test movie from Stellafane. It is not diffused. It is a very bright light and I have to dim it to look at the 8" mirror, which is aluminised. What I am not clear about is, do you shine the light through the screen AND look through the screen, or do you shine the light through the screen and look at the mirror but not through the screen, or do you shine the light on the mirror and look at the mirror through the screen. Different sites say different things.

Thank you for the offer of some fishing line. I did it with a 10lb line, which worked out to 80 lines per inch, but what I would like to do is go on a web site like Microsoft or GIMP and see if someone would know enough to make a file with a grid that can be printed on a laser. That would be ideal, I could get it printed at a print shop on transparent film. I came across a site that had a pdf file full of 2 inch 100 grids, but on trying to print it was not good. Surely someone would know enough to produce a file like that. The Stellafane site also has one that gave me no joy.

[Rod]

Hi Werner

There are two ways commonly used to set up the ronchi test:

1) use a very compact light source and shine it through one end of the ronchi screen while you observe the reflected image from the mirror through the other.

I think a krypton bulb will be too big. The stellafane video on autocollimation is dated 2009. These days most people get a bright small white LED.

2) The other way is how it was always done on the past and may suit you if your grating is small. Create a small slit - eg with two razor blades. I prefer to use the blades from pencil sharpeners. They are small, only sharp on one side and have a hole for a screw.place the LED behind the slit. If your mirror is of moderate focal length, a bit of separation between the slit and grating won’t hurt. You need to have the grating angled the same way as the grating - ie as vertical as possible. In the other method, this happens automatically.

I usually place a torch in front of the slit to help me roughly line things up.

Rod.

[mura_gadi]

Hello,

Printing at 600dpi should not be an issue, I tried stellafane's pdf at Office Works and got 8 copies done, all were crap. They printed them on the smooth side not the rough side!

Mind you they need to change printer settings from paper to transparency and run a cleaner sheet through the machine first. That may not have been done either from the look, but I wasn't going back again.

If you know anyone who has a 600dpi printer, they can print the Stellafane pdf etc for you. Just buy good quality transparencies, use the cleaner sheet that comes with it first and check which side you print to.


Steve
I gave up on the printing for now and moved to the fishing line, if you want edmunds optics in Oz sells the 5lpmm Ronchi - 2"x2" for $165 1x1 for $120.

Ps - You can use a monocular or such to look at the screen as long as it focuses close enough. That may help you with your eye spacing.

[Sunfish]

Clean everything rinsing under fresh water every time. Clean clothes . Clean hands. Clean work space , no air movement.

The ronchi screen downloaded from stellafane printed on a acetate on a laser printer works. Easiest way to start. Use the Bartels light box or a tiny tube on a stand with an led in it at centre . The auto collimation test is far too complex for the first stage of polishing.

[legswilly]

I printed the stellafane ronchi screen and this time it looked OK after I shrunk it to 60% AND printed it at he highest quality setting on the laser. Did not change the setting the first time and it looked bad. Now I am looking for a transparency film to print it on. Rang Officeworks, but they could not help me.
Will have a look at the Bartels light box and see if I can make out how to build it. There are a lot of examples of test stands, but the very detail where the screen is placed, where the light is placed and where and what to look through escapes me. Sometimes the simplest thing won't get into my head.

[legswilly]

Had a look at the Bartels light box and it could not be simpler. Next I want the grid.
Thank you for the answer.

[sopticals]

Back in the very 1960's (1961-62 from memory) being unable to source a Ronchi screen (and of course before all the modern tech [printers and such]) i built my own Ronchi screen using 2 x 100 threads per inch brass rods and the fine copper windings from an electrical coil salvaged from an old radio, phone or such. Took awhile to do "the wrap around" of about 2" or a thereabouts, but the end result was worth the effort. Was wide enough to receive return beam through the screen back to the eye. Cant remember what I used as a light source.

Stephen.:)

[Sunfish]

Here is my first attempt using the printed 50x50 screen on film on a glass plate and a repurposed kids wood project box and a light bulb , like the Bartels box. Rough but worked.

The next iteration was the LED in a tube and glass ronchi so the eye can be closer to the light source and a view looking through the screen above the light source . On the table with the mirror at centre of curvature in a black box , looking though the screen which is on an adjustable music stand in either case.

Very sensitive to exact position so the stand needs to be movable and adjustable.

[legswilly]

This is great and I can see how it is working. I asked at Officeworks to photocopy the screen that I printed on paper, but it came out with a lot of moire. Forgot my USB drive with the file on it, but they said they can print it on their printer on transparencies. So I will go on Monday and get it done. Part of the copy of my file was OK and I cut a 0.5" strip and wanted to try it. Had too high a voltage on the krypton bulb and blew it. So I will get some LED bulbs, 33000 mcd. I dont know if it is bright enough. What is a Cree LED as opposed to a LED? It comes in 45000 mcd. Thank you for posting the photos, that is great.
Have cleaned up and started to grind again. The motor on the turntable is conking out more often now, overheating and I ordered a new one with a gearbox and a variable drive.
What resolution are the photos you posted?

[Sunfish]

Not sure about the specs of the led . A standard led from Jaycar . I had a red one lying around. Does not get hot and runs off a penlight battery pack. Does not matter much how bright with the ronchi.

[mura_gadi]

Hello,

I have just made my Ronchi tester, but not the stand. I used a LED from a cheap $10 headlamp from Jaycar, the printed Stellafane pdf transparency and fixed it inside a 35mm slide. Worked really well, unbelievable the detail on the blanks surface...

The one thing I found tricky was the alignment, I used a A3 sheet behind the tester to locate the returning beam. Once you have the returning beam it is a lot easier to locate your RoC and to align the tester/blank so the returning beam lands on the correct part of the transparency/Ronchi bands.

What ever the design just make sure you can get your eye in really close to the screen. That is probably the single most important thing to get right.


Steve
Ps. My LED is really bright, I was thinking of using a milk bottle cutout as a diffuser. The board is about 8mm thick, and the bulb is inset to about 3mm away from the screen. I was having trouble with light leaks, the insert was the easiest way for me to cancel them and it got the light source close to the screen.

I found that if you move the stand forward after getting the bands right, you can lose the bands and get a Foucault like test image from the blank as well.

You can get a LED head lamp torch for around $15 posted on ebay, that will get you the battery pack and LED light and a switch if you want it. I had already bought the headlamp for the hands free red light feature and had removed the white bulb. The battery pack was $2 something from jaycar.

Get your eye position as close as possible
Get the light as close as possible to the grid
Get the return beam as close as possible to the source

[Sunfish]

A similar setup.

I suppose that is what is good about a two wire led inside a small tube. The light can only be thrown in one obvious direction creating a circle of light and does not need to be blinding bright.

My tube is a drilled out threaded rod given to me by another ATM and can be threaded in to a plate or bolted on.

[mura_gadi]

Hello,

I was worried about the optical quality of the transparency in such a sensitive test. The second 35mm slide will use .105mm fishing line for 120+ lines per inch for zero interference from the substrata.

Sunfish, I see your Ronchi slide looks to be very thick though. I can see why the slides are so expensive if they are on optical glass that thick.


Steve

[Sunfish]

The ronchi is very old and was given to me, the problem is the small size.

The 50x50 laser printer transparency from stellafane works fine but is better clipped onto a piece of etched glass to keep flat. I should try one on sticky-back transparency.

[legswilly]

I have been a while polishing and I found the glass is reasonably flat. It is difficult for me to take a photo, but it should be able to be seen on the attached photos. I have polished two sessions on the turntable and every time the fringes look a little different. I thought that it was not polished out, because I could see very small glittery flecks on checking with a laser light. That is the reflection on top of the glass. After each session of half an hour, I hose down the glass with town water, wash down with distilled water in a squeeze spray bottle and then with 70% alcohol from the spray bottle. I let this dry in the air and put it down flat. I use lens clean from my eye glasses supply and wipe it down with a very soft cloth, like velvet. I could see not difference in the polish after the last time. I tried shining the laser on the glass and wing it with a lens tissue and found that the flecks can be shifted with the tissue, they move when wiped. So I have lint after all the things I did.
Could you tell me how to get rid of the lint to see if the glass is indeed polished out. What do you use to wipe down the glass?

[Sunfish]

Don’t wipe the glass . Don’t touch it.

Wash it with plenty of running water and stand on its edge to drain and dry
away from dusty drafts like you would a mirror . I lean mine against the tile wall on a laminate block resting on tissues or in the bath.

[Sunfish]

Ps. If you are impatient , use the hot water tap. Dries quickly if the glass is warm.

[mura_gadi]

Hello,

If your finding lint after the wash/rinse etc, the lint is coming from your lens cleaning cloth. I use well launder cotton, washed several times with no detergents. Keep them in a container so you don't pick up containments from benches etc.

Are you using warm water on your polish? Both the blank and mirror as well as the polish bottle water etc need to be warmed to help with contact during polishing. For hand polishing you would simply use ToT to move the polish out to the edge - this has a greater chance of scratching as the lap channels no longer take away the larger particles from the glass.


Steve
Ps. I cut up old tea towels and towels, anything that's was good cotton and made a lot of them. If you put a cloth down in the wrong spot, or touch it with dirty hands etc... throw it in the wash basket and get another one...

[legswilly]

Thank you Steve and Ray.

I washed down the glass this morning and most of the specks are gone. I do it at the shed which is about 100m away from the house, that's where the boss wanted it. I don't have a warm water tap, I could heat water in a pot, but that seems a little too much. So I will just let it dry by itself. I could buy a hairdryer, that might work. What do you think. I need one anyway when I silver the mirror.

At the moment, I think that it needs at least another 3 sessions of polishing to get it fully polished out. The fringes are not really straight, I will see if they are getting worst after another session. Before I start polishing again, I will cut the hole in the centre. Plan to use a little dam of blue tack for the water and cover the glass with a couple of layers of glad wrap.

[mura_gadi]

Hello,

Just take a kettle out with you, no need to take it to the boil though. Keep topping up your wash basin prior to rinsing etc, to get the water warm. Store the polishing powder slurry bottle in the wash basin as well, you don't want the glass contracting/changing shape with cold water.

Good luck on your drilling, use a pilot and don't jump size too quickly. You might want to keep some 80 grit at the drill bit once the well is started, that will help at low speed.



Steve
Give the side of the mirror contact as well to help with vibrations.

[legswilly]

Did not think of a kettle. Have to get my mind around that and see how I could set this up. At the moment I built a washstand, just a tilting board with two wooden pegs and I wash it outside the shed. Hose it down.
I cut a 40mm hole with a diamond cutter, cheap on ebay. Did it on the drill press with a weight on the handle to apply even pressure while cutting. The entry on the polished side looks good and I will chamfer it a little. As it cut through, it broke off some small bits. They will be on the back of the mirror, so it wont matter. Gave it one more polish session and it is now drying standing up until tomorrow.

[multiweb]

Did you make yourself a custom interferometer? Do you use a sodium light or diffuse the light from a laser?

[legswilly]

I made a newton interferometer, simple construction with a strip of red leds, the self adhesive ones. The power supply of 12 volt is from an adjustable power supply I bought for the ronchi test rig and a led from jaycar. I later will use it for an autocollimation test stand as described on Stelllafane. That is for me the simplest. I started out with diffusing a laser light, but promptly zapped it when I forgot what voltage I had the power supply on. I can make you a drawing of the interferometer if you would like.

[multiweb]

Yeah that'd be great! :thumbsup:

[legswilly]

The box is big enough to hold a 14" disk. You can make it whatever size you want.
The top compartment has a lid that the LED strip is stuck on. The adhesive will not stick well on the mdf I used, so I put a track of superglue, the cheap stuff you buy at the cheap stores. Use a glove to spread the track of the stuff. The 5 metre strip of red LED's is cut to lengths https://www.ebay.com.au/itm/183879660663 to fit the lid.

Each strip has marked where to cut it and you need to carefully cut and scrape away the silicon cover to expose the contacts. Stick down the strip and solder copper wire from + to + and - to -. Connect you power supply to the daisy chained + and -. the LED's point downwards.

The bottom of the top compartment is plain glass 3 or 4 mm with a sheet of plain white paper over it to act as diffuser.
The testing piece on the bottom and an optical flat on the top. https://www.ebay.com.au/itm/323769219170.
The semitransparent 45deg glass is supposed to be so one can see the reflection of the test piece. The sheet of glass is to look at the test glass from the front. It did not work for me, it was too dim and did not show the fringes. I increased the number of leds and doubled it, but it did not help. When I get the time/inclination, I will remove the glass, no point having it.
Sometimes sodium light is recommended, but it is harder to get and more expensive and for my purpose the LED's work fine.
I hope that is enough for you, if not I go down to the shed tomorrow and take some photos.

[Sunfish]

Interesting construction. A hairdryer would just blast airborne dust at your mirror. You want negative clean air flow or no air movement in a clean room.

[legswilly]

I didn't think of taking the mirror up to the house and rinsing it off in the bathroom. It has to be the most dust free room in the house. I polished it once more yesterday and after rinsing it and drying overnight it was mostly dust free. There were a few dried marks from the water. Not many, so I could test around these. When I take it up to the bathroom, I could rinse it with distilled water after rinsing it under the tap. The polish looks good, when looking at it with the laser pointer. It has gone a little more concave, so I will do another polish and move the lap a fraction to the outside. It is flat enough for the mirror I want for the autocollimation tester, but I want to see what happens.

[multiweb]

Nice :thumbsup: Will check it out.

[legswilly]

did another two polish sessions and the surface is a lot better. On the outside is still a little to be polished. The flatness has also improved. So I will do one more session tomorrow.
I looked at the glass through one of my eyepieces I bought and can see what looks like dried in puddles. There are a few very faint marks on the outside of the mirror and I think they will come out at the next polish.

With the tiny glowing flecks using the laser light, I thought that they might be very tiny bubbles in the glass, even looking at them through the eyepiece I could not make out what they are. Is it possible to have tint bubbles in the glass? It is only cheap float glass that I bought to practice.
One more polish, I think and I will start the silvering process. Need to read up again, it's been a while since I bought the chemicals and the silver. Not quite clear about the priming of the glass. Anyone doing this?

[Sunfish]

Sometimes the CEO dries in small patches which are hard to remove except with lots of running water and finger rubbing. Do you have hard water? Or scratches create small circles. I have only experience with Pyrex.

My problem is I still have TDE on my small f4. The straight lines are creeeping toward the edge but I will be 100 before I finish at this rate. Perhaps I need to go back to fine grinding 600 or polish with with pads on the glass tool to get spherical. There are many conflicting techniques.

[legswilly]

I found that I could get rid of most of the spots by rubbing them out with the fingers. After the last session of polishing, I found that it has gone a little convex. For the purpose of the autocollimation test rig, it would be well within the limits given. So I am letting it go until I can get the stage of the test stand re-built. I don't like the way I have done the existing one. More or less copied the Stellafane one, but with the stuff I had, it did not work well. Ordered parts on ebay, but did not come. Also waiting to get some tin for priming the glass for mirroring.
I don't have any experience in fixing a turned down edge. Looked at a video https://www.youtube.com/watch?v=tMp0sI3aYEU&t=204s. It might not be applicable to you. Going back to spherical with grinding and then polishing sounds like a good solution.

[mura_gadi]

Sunfish,

"Going back to spherical with grinding and then polishing sounds like a good solution." I'd second that, from my reading you need to be spherical and polished out before parabolizing the mirror.

I am still at a very simple mirror for learning, the F4 will be a beast.

Try Mel Bartels site for how to make different pitch laps for the parabolizing of fast mirrors.

https://www.bbastrodesigns.com/JoyOfMirrorMaking/Parabolizing.html

https://www.bbastrodesigns.com/PitchLapCalc.html

PS. I got a flash polish on my mirror enough to test. Revealed some nasty flaws, dropped back to 400 and fine tuned the F/l, then life got in the way for the last few weeks... But no matter what you do, you can always remake the surface of the mirror and go again.

[Rod]

Hi Ray

I have had good luck reducing turned edges with short straight strokes mirror on top with a slightly oversized lap. Same idea as Mel Bartels. This technique should work with a standard lap, it will just take longer. You may need to work for a couple of hours to see a significant reduction. You’ll likely get some surface roughness and a central hill in the process but these are easily fixed.

A well pressed lap is essential. Make sure the lap is micro faceted and not glazed. If the lap looks shiny, get a brass brush and give the surface a light scrub. Gordon Waite shows this in one of his videos. The micro faceting and brushing improves contact significantly.

When you examine the edge, look at the image outside the ROC, it tends to show the edge defect more clearly.

Hope that helps.

Rod.

[mura_gadi]

Rod, tyvm for the wire brush idea, I had been using the Stanley but that makes a lot of sense.

[Rod]

Hi Steve

The one I use is from bunnings and in this set:

https://www.bunnings.com.au/wire-brushes-sc_p6114348?store=6180&gclid=Cj0KCQjwqp-LBhDQARIsAO0a6aLVz7tHLJt-ZyfEBo5YxSHb046TIFEglarwGaOXQT8Eukc 3kHFPU9UaAr2BEALw_wcB&gclsrc=aw.ds

Only $3.50

Rod

[Sunfish]

Thanks Rod , Steve and Werner. I have been using the Stellafane quarter circular stroke with a sub diameter but I will give the short straight a try . I have one of those wire brushes and it is true that the lap looks a bit shiny.

[Rod]

Meant to write short straight strokes mirror on top earlier. I’ve corrected my earlier post. TOT will probably work too but these days I’m finding MOT works best for me, probably because I use slightly oversized laps - usually about 6 mm larger.

Rod

[Sunfish]

:thumbsup:

On another point , I have been wondering if the rib around the back edge of the cast Pyrex piece makes difference. The larger pieces I have a flat , but this Pyrex piece is cast in a tapered mould with a bottom with a ridge like a cup or bowl would.

[Rod]

Blanks with a rim like that used to be common. It shouldn’t pose any issues. By the look of it, it’s a standard 1:6 thickness ratio so what’s on the back is not much of an issue.

Rod.

[Sunfish]

Thanks Rod. Gave the extra cut and wired star sub diameter a go as I had the lap on hand . Did not seem to be any different so I will try the MOT oversize Bartel again which I also have from the initial polishing.

[mura_gadi]

Hello,

When using a Ronchi tester, is the diameter of the returned beam on the Ronchi screen related to the diameter of the illuminated field given in "Newt for the Web"?


Thanks

[Rod]

There’s no relationship that I can think of. The illuminated field given in newt is mainly determined by the diameter of the diagonal mirror and to a lesser extent if you have a narrow aperture stop in the system (eg. small diameter tube).

Rod.

[mura_gadi]

FYI

Recently I posted about trying a thin heated taper metal tip to reset the grooves in my lap which had formed poorly.

The results were very good, using the heated tapered tip based on John Upton use of a hot channel cutter. There are some waves formed, but very few and easily cleaned off with the Stanley knife. Certainly made chipping away inside a plastic bag both easier & faster and the channels are far better formed and cleaner. Material loss from the lap is maybe 1-2 percent compared to the Stanley.

I used a long tapered metal screwdriver I had hit with the grinder wheel and a blow torch(small kitchen variety) to heat for 5-7 secs. I tried using boiled water in a cup but the screwdriver was too thick and the heat was lost after about 4"'s of channelling.

Using a boiling water pot to reheat the tip would reduce the chances of volatiles being released from overheating the tip, slightly longer but safer I'd guess.

A good size round skewer with an insulated handle might be perfect.

Normal reset on the cold press required etc.

The heat loss using the water in a cup was due to not being able to heat the whole length of metal. I would guess if you can immerse the required length of metal in near boiling water and re-press the lines in one go it would work very well.

[mura_gadi]

Hello,

Just a note on picking laminates and sliding and static frictions etc. I've been trying to think of a test rig that would be easy to try various laminates with small weight on a telfon base.

Using angular friction will give you an easy way of calculating the difference between sliding and static friction. Just means you need a ramp that you can measure the lift on or a "side by side" test of what starts to slip 1st.

**I was going down the path of pulleys and weights over a table edge etc... Using the angular friction might make getting a few bits of laminate tested at a store/workshop a lot easier. A clipboard and a inclinometer would be good, or a simple a good sized board and a ruler to measure the height with.

Easy way to get the best laminate available for you. If you're in a shop with test boards, your teflon based weight and an inclinometer, life could be sweet. At least you might get to test a few more laminates before being asked to leave...

Ps. A good bench mark to test against would be to record the incline need against an old LP record, which are a good cheap start to beat for the azimuth bearings on smaller dobs.

*********************************** *****

Part 2: I wanted to incorporate vibration/knocking into the "sled" to help reduce the static friction grip. Turns out "Jaycar" has a little pill case with a vibrating alarm. It takes 3AAA batteries and has a compartment for pills/weights and is pocket sized.

There normally around $12, Jaycar is clearing them out at $5 so I'm off to get one to try later. Should be a very close to useable after a few teflon feet, depending on the power of the vibrator. - Missed the sale, thought to make the sled with a small PC fan with one side of the blades removed.

[CharlesM]

My current project is 2x12" mirrors that will be flexed for a binocular.
Hopefully they will work.

[mura_gadi]

My optic tester got a nice step forward for my newt.

Bought a solid tube GSO newt the same diameter as my blank, old and trashed for $50.

Plans are basically cut out the middle section. 2mtr poles attached to the base and have the secondary cage slide as required. Gives my star tester a mirror cell, bob's nobs and a full secondary cage and hopefully rapid acclimation for $50 and some dowel/brackets.


Ps. GSO and SW are 200mm and 203mm respectively in the 8". SWer's optical assembly would be better as you can always shim down.

[Sunfish]

Back to the polishing . Still have TDE on my little mirror. I have gone back to the glass tool and Al. Oxide with short circular strokes, alternating with the star lap in the centre and the lines are creeping toward the edge. I would go back to grinding but worry that I will end up back in the same position, but I suppose if I use the same straight circular stroke with the glass tool and step up a grade from aluminium oxide I will get there quicker.

[Rod]

Hi Ray

Pleased to hear you have found a mix of strokes that are dealing with the edge. However, I’m worried about you polishing with aluminium oxide. How do you plan to clean your lap when you are ready to return to cerium oxide? Any residual grit will likely cause scratches and sleeks.

Rod.

[Sunfish]

Thanks Rod. I thought of that as soon I started. The little star lap is disposable I suppose and I was intending to replace my pitch as it is the tar based one and Marc has given me some synthetic pitch. I should not use the oxide on my full size lap as you say.

The aluminium oxide is 1 micron 14000 grade . I have some 800 grade to see if I could speed it up with the tool and then repolish .

I would like to make a small kick wheel which would speed up the polishing, like the Belgian optician with his tiny double mirrors.

[mura_gadi]

+1 on the kick wheel, I have a pergola as my wood supply, plenty of good wood shows up in gumtree amazingly cheap.

I'm planning on sandwiching bricks between the two boards(discs) over one concrete slab for the base weight. Should be easy to disassemble and relocate if needed.

Lots of people sharing good ideas on youtube, also mentions which bearing types you need...

https://www.youtube.com/watch?v=iJEr1M9QGGc

Ps. If the weather ever cracks 17degrees again I can restart my blank...

[Sunfish]

I was thinking more like crank arms and treadles with a lighter flywheel but there is a single bar design The Leach Wheel, which is the gold standard kick wheel for pottery.

https://makezine.com/article/craft/the-leach-treadle-wheel/

[mura_gadi]

Looks like the mustard, vnc.

I'm building mine with a view of it living outside, a removeable top and easy to hose down, and probably a lot closer to my level of handy man work... I am certainly KISS when it comes to making stuff, tbh.

[mura_gadi]

Hello,

Thought I post this as I found it very useful and easy to apply.

Previously I have been using the squirt bottle and fine grits 1600+ using a milky mixture and spreading the mix out by finger tips. Then slightly lifting the tool/mirror and twirling to even out the grits out further. It was fairly obvious that even with the twirl the grits were still concentrated in places. As the wet progressed it become more obvious and the wet normally stopped due to the areas of concentrated mix.

Using a thicker mixture closer to a single pouring cream and place a coin sized puddle# in the middle of the mirror, lower the tool down to spread the mixture out to the edge. I have a glass tool so it very easy to control the spread out visually and found that the mix was very evenly distributed. The wet lasted a lot longer due to not having concentrated mix areas. Using this technic I was able to use 14000 grit as the final pre-polish grit and that sped up the flash polish by a huge factor. *

Another bonus is the initial hydroplaning dissipates a lot faster and the grits start to work a lot sooner. A technic well worth trying imo.

Wouldn't work with tiled tool, but a +1 for a glass tool.


Steve
*This is not my idea but I can't find the text I was reading to quote the original author
#I was using about a $1 coin size puddle for an 8" mirror.


Ps. Well I have been polishing for under three hours now, mostly MoT and the mirror passes the laser test to within about 15mm of the edge. Will change over to ToT for the next session. Well under 3 hours to pass the laser test start to finish so far, (Big smile), so much less time than it took me from 2000 grit first time round.

Might get the first born under the tester before I go back to work in the New Year... Will keep going on the polishing after I get a good laser test edge to edge as the inverted 25mm EP still shows the occasional spot.

Hopefully TDE will be bearable and not take an age to work out.

[Sunfish]

Sounds like it works. I am still using the squirt bottle but swirling around seems work to spread the grit.

I also went back to 14000 pre polish to fix my turned down edge using the circular edge technique and that was working all be it slowly. So I built the rotating part of a mirror o matic 14/ 20 and back to 800 grit, fixed problem in an hour. Then went slightly too deep then back to slightly to shallow in 20 min. Very effective short circular motion on the rotating platter at 35rpm for getting TDE down and the polish is still good enough to test.

Next , install the mirror o matic arms and on to the 12 inch once I can work out how to figure the 4 inch F4.