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
Quote:
Originally Posted by 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?
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]).
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!).
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.
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.
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.
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.
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
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 $?)
wishing dark skies to all!
Hamish
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.
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"?
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.
Quote:
Originally Posted by 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.
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.
Quote:
Originally Posted by 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"?
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?
#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.
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.
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).
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:
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.
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.
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.
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.
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.
Another 33" mirror making project
