Making a 10" Dob

[Lognic04]

Hi all, :hi:
I have finally decided to take the plunge into mirror making! Going old school! :D
I am planning to grind my own 10 inch (250mm) f/6 (1500mm FL) Mirror to go into a 10 inch dobsonian tube that has been sitting unused.
Picking up the materials on Saturday, and am going to get help from the ASV all the way :thumbsup:
This is my first time mrror making! :O
Going to regularly update this thread on the progress!
:D

[Atmos]

Awesome!
This is one of the things to do on my bucket list.

[Lognic04]

:thumbsup:

[Lognic04]

Few questions:
Is there a way to measure my curvature relatively accurately without a spherometer?
I am seeing different methods of testing the mirror, is a ronchi/focault combination fine? Or is it better to map it with a PC?
Thanks!

[OzEclipse]

Hi Logan,

Good on you. I made my own mirror for my first scope when I was 15.

While you are grinding the easy way is to swish some water over the ground surface and focus the sun. I had a long wooden stick 2m long. I attached a piece of white card to one end and marked distance at the other end. You can use 100mm markers for the longer lengths then drop down to 10mm markers near your target focal length. Take the mirror outside on a sunny day, slosh water over the surface and focus the sun on the white card, read the focal length off your distance markers.

A 10" f6 has a 2.6mm deep sagitta or curve. If you don't have the sun, you can lay a straight edge across the whole diameter of the mirror. Then compare the centre depth to a depth marker like a 2.5mm drill.

You only use these methods when you are rough grinding. Get the focal length a couple of inches longer than you want by the end of the 80 grit grind. The fine grind will take the rest off. By the time you get to Ronchi or Foucault test (not Focault) you are polishing and the focal length won't change much during polishing. You can however use these tests to determine the focal length accurately.

Have fun mate.

Joe

[Baza]

Logan
there is a mirror making thread with lots of info, also have a look at https://stellafane.org/misc/links.html#Equatorial and http://www.bbastrodesigns.com/JoyOfMirrorMaking/ both have heaps of info burt you might have to dig a little.
http://www.mirror-maker.com/
awaiting the progress reports. Barry

[Lognic04]

Thanks for that Joe! :) I will definitely be using these techniques!



Thanks Barry! :)

:thumbsup:

[Lognic04]

Another newbie question, what should I make my tile tool with? Does it need to be curved?
Thanks!

[Baza]

double post

[Baza]

you need tiles of the same hardness all the way through. non glazed exterior tiles seem to work. they can be stuck to dental plaster. you will need to waterproof plaster of paris if you use that. details and photos on stellafane & numerous other sites.
It will curve with correct use.

[Lognic04]

DAY 1! 2 hours work - flattening the back of the blank with 80 grit, almost done. Lot of fun so far!

[xelasnave]

Good on you Logan should be good experience before you tackle a 20 inch f4:)
Good luck ( remember the harder you work the more good luck you get) and post often.
Alex

[xelasnave]

And you know we are all expecting a full length movie.
Alex

[Troy]

Are you hand grinding?

[Lognic04]

Hehe thanks Alex. I am hand grinding.
Look forward to that video! ;)

[Lognic04]

I am back to grinding, just wondering if i should regularly flip the mirrors? I am only flattening the back.

[OzEclipse]

Hi Logan

To make a perfectly flat (optically flat) surface, you need three blanks and they have to be regularly flipped top to bottom and swapped in and out of the pair.

I am understanding that you mean you are grinding the back of the mirror flat? Unless it is really uneven, it's not necessary to flatten the back.

However if you want to do it, then yes, do flip the 2 mirrors otherwise the top one will become concave. Flipping two mirrors, will make them roughly flat which is all you need.

Cheers

Joe

[Lognic04]

Thanks Joe. It is mostly done now anyway, so soon I can go onto rough grinding! Hopefully I don't use all my 80 grit on this! :P

[Lognic04]

Mirror right now. That is the back :)

[Lognic04]

Phew! The back is flat! I have started on the rings, they really were nasty at first but they are almost gone now - 4 or 5 more wets to go. Another question! How long should a wet last? Usually i let it go until it is a very light grey, but i feel like this is a huge waste of time, in the time i spend stretching out a wet i feel like i could have done 2 "normal?" wets in between.

[OzEclipse]

Hi Logan,

It's a long time since I last ground a mirror (33 years). I don't remember how long it lasts.

When on the course grits, you feel it stop grinding & become smooth when the grit wears down. That's when you change or recharge the grit.

Don't let it dry out. If it gets stuck, really hard to release.

Joe

[Rod]

Hi Logan

When rough grinding using 80 grit, you grind by sound. When the harsh grinding sound stops, you need to recharge the grit.

You will likely use a lot of 80 on your first mirror.

Rod.

[Lognic04]

Thanks all, getting closer to having those rings gone, hogging can start soon hopefully! I will use a piece of metal to rough it so that the tile tool that has been used previously gets less worn. :)

[troypiggo]

Hope you're taking lots of cool videos of the process for your Youtube channel ;)

[Lognic04]

:thumbsup:

[Lognic04]

Decided to fine grind the back of the mirror while waiting for a tile tool, Mel Bartels said it is best to do that (but mostly i wanted to have a go at fine grinding! ;) ) Have finished 120 grit, and am going to move to 35 micron tomorrow! :)

[OzEclipse]

Did he say why?
Joe

[Lognic04]

He says having a flat back reduces astigmatism.

[Lognic04]

Some pics!

[Lognic04]

Wow 25 micron is VERY FINE stuff!!!

[Lognic04]

Just picked up extra grit, a tile tool and a ring tool! Ready to hog out!

[Lognic04]

Have been Hogging out with a metal ring tool, and reached about 1.3mm deep. I have now switched to a tile tool to continue grinding! And I have decided to do f/5 :)

[Lognic04]

I have a tile tool that has a rather different (more curved) surface that doesnt match the mirror. I was thinking of doing normal strokes(CoC) to get the right curve/matching surfaces on the mirror, will this work?

[sopticals]

Depends how different the curvatures are. If close would be ok as a bit of grinding with #80 grit would soon bring the two surfaces into a match. If a big difference probably not.
You using ceramic or glass tiles on the tool? Ceramic will wear more quickly.

Stephen

[xelasnave]

Dropping the focal length is a confident move well done.

I got to thinking do you or others consider one could make a 2.2 fl?

Every one will probably say impossible but I wonder could you pull it off?

I mean ever one does a hand ground mirror with long fl s but a really fast mirror...It would be good if you could pull off such a feat.. the tube wont mind☺.

Then a fastar configuration?

Maybe you could send it someplace for final figuring so if its best that way it should become apparent.

I wonder ????

Alex

[xelasnave]

Opps we will need an equatorial mount...I have a mount design that I have been thinking about....more bearings than a car☺ and co sideeing building it in carbon fiber...better thermal stuff and I only have to fiber glass a balse model in effect ..no welding...ao the mount is covered maybe☺
RA and Dec moved by disks of 300 mm radius to make very precise ...at least that the idea.

I should explain it as you could build it I bet.

Plenty to do in astronomy eh☺

Good luck
Alex

[Lognic04]

I reckon it is possible!!!If you can make a mirror, then you should be able to make any f ratio!

[Lognic04]

I am really interested in this design alex! :)

[Lognic04]

Unsure whether to do normal stroke or chordal?

[xelasnave]

I have a model in cardboard to demonstrate the idea and I will use it to talk to folk about this and that so let me nail it down a little and I will present you with rather definite.
Needless to say it will be interesting if one could build it in carbon fiber...not because I am taken with carbon fiber but I have experience with making stuff in fiber glass and it seems carbon fiber just replaces the glass and you have a similar process with a suitable resin and hardner.
But I am thinking you could make it up in balsa and over lay that with carbon fiber...sooner or later depending on how many layers it sould be as sttring as steel.
You dont need to weld and it is almost a kitchen table project.
A wild idea but you never know.
Alex

[Lognic04]

:thumbsup: Any chance you could post the design? It sounds like it would be great for visual - could it be motorised for AP? :)

[Lognic04]

Wow, rough grinding will take quite some time!

[xelasnave]

The idea is that it is to be the best AP mount out there.
Overkill on bearings 4 on eack of RA and Dec☺ and in effect huge gear diameter, although gearing is via motor driven rubber wheels on a disk 4 wheels and motors for each of RA and Dec..This part needs looking into ...computer controll of stepper motors etc
. But it should be equivalent to having a 600 mm dia gear on each of RA and Dec.

I am considering if gyroscopes could be used and if they would be a benefit.

The first, now lost design, draw by a professional was entered by him in some comp. and apparently won second prize judged by some high up astronomy folk in the UK.
I have no firm plans the avaialabily of material guide my builds. Size will change depending on what bearings are available but it was to use car or truck bearings because they were an over the counter purchase. I figure four should overcome any lack of precision....but size of bearing changes things all down the build...
I will post something when I can.

Alex

[Lognic04]

Awesome Alex, am really looking forward to seeing your design! Woud definitelty be interesting to see it in action! :D

[xelasnave]

This is one photo will require 1000 in explaination.
See if you can figure out the concept..
Alex

[Rod]

Hi Logan

Use a centre over centre stroke to mate the tool and mirror. Use a 1/3 centre to get to and maintain a sphere during grinding and polishing. Chordal strokes are for hogging out the centre rapidly. They are normally used only in the early stages of rough grinding.

Can you post a picture of your ceramic tile tool? It might help us give you clearer suggestions on how to adapt it.

I noticed your exchange with Alex re a short focus telescope. I would suggest you avoid very short focal lengths on your first mirror. There are some amateurs getting amazing results with ultra fast scopes - Mel Bartels is the most obvious. However parabolising a short focus mirror is very challenging. If you were successful, you would then need to pay for a high quality coma corrector (paracorr 2), an excellent focuser and high quality (ethos or similar) wide field eyepieces to take advantage of the short focal length. I suspect these additions could easily chew up a couple of thousand dollars.

Your original design with an F5 to F6 10 inch mirror is challenging in itself but doable. You can then get away with no coma corrector, modest eyepieces and something like a GSO focuser.

Hope that makes sense, I'm really keen to follow your progress and hope to catch up one day at the ASV's Instrument Making meeting.

Regards

Rod.

[Lognic04]

Thanks Alex, i think i get it, the ra/dec rotates and has a small bearing that rotates on the big fan shaped object?

[Lognic04]

Hi Rod, thanks for the write up. So i will do CoC right now, and that should give me an f6 curve by the time they are mated?
The fast mirror was just in theory :)
I was at the last IMS actually! :D

[Rod]

Make sure you are close to your target focal length with the ring tool and that the curve extends to the edge of the mirror. Then move to the ceramic tiles using centre over centre strokes.

Rod

[Atmos]

Making a 10” F/8.66 should be easier than making an F/5 as the tolerances aren’t as strict.
Adding a Faststar/Hyperstar would be an interesting experiment but the primary mirror in the Celestron is ~F/2.2 so it would only work if you made a F/2.2 mirror.

[xelasnave]

Hi Logan
I am glad others point out the fact that shorter the focal length the greater the challenge.
To understand the phot of the mount let me say that the "fan" bits can be thought of as a section of a large gear. The mount is designed to operate to observe 45 degrees above the horizon and with no need for a meridian flip.

The RA is supported by bearings either side of the Dec shaft running thru the RA shaft which is actually a tube with two bearings at either end.
A German Equatorial mount in effect only has bearings at one end of the RA shaft whereas this design is in effect an English Equatorial approach where RA shaft is supported either end and Dec run off in the middle.

Keep up your good work.

It certainly is best that you complete this project sucessfully before you start another and your experience gained may give you confidence to build the ultimate astrograph.

Alex

[Lognic04]

Hmm, i hogged it to 1.5mm with the ring tool, and i gave up on it because the tile tool seems to be MUCH faster. any ideas as to why?

[Rod]

Hi Logan

I suspect it's the stroke you are using with the ring but really if the tile tool is working for you keep going. I was mainly concerned that you not wear through the tiles. If it's a second hand tool, like the one I passed on, then they might be quite thin. How thick are the ceramic tiles?

Rod

[Lognic04]

About 4mm now. (did you pass on a 10 inch f6 tool? if so, i am using it now! :P)
I am at 2.1mm sag right now btw. Is there a way to check the FL other than physically and without the sun?

[Rod]

Yes that sounds like it could be mine. I used it 20 years ago to make a 10 inch F6. I would need to see a photo to be sure.

You can test the focal length approximately with a torch. Place the mirror on a stand. Hold a torch parallel with your eyes. Wet the mirror. Locate the torch reflection in the mirror and bob your head up and down. When the torch reflection moves in the same direction in the mirror you are inside the radius of curvature. When the reflection moves in the opposite direction, you are outside the radius of curvature. The point at which the torch reflection does not move is the radius of curvature. Halve that and you get the focal length.

Some people do a couple of wets with 120 grit and then 240. This helps give a clearer reflection and the water seems to stay on the surface longer.

Hope that helps.

Rod

[bojan]

It can get stuck even when wet..
It that case try to immerse the whole thing in hot water, than cold, then hot again.. it will sepatare suddely, so don't drop it.

[Lognic04]

Mirror and tool, plus 10 cent coin (2mm) fitting under the steel ruler! :)

[Rod]

Hi Logan

Yes that's my old tool.

Assuming a sag of 2.1mm and a diameter of 250, your focal ratio is currently 7.4 according to the stellafane calculator:

https://stellafane.org/tm/atm/mirror-refs/sag2fl-calc.html

So there is a little more hogging to do, but I am impressed by how quickly you are getting to your target!

I am a little nervous that you may grind through those thin tiles at some point. I would feel better if we glued some new tiles over the top. At 80 grit new tiles will conform to the curve you are grinding very quickly. I could bring some epoxy (jb weld) and tiles to the meeting next Saturday if you would like to do that.

Happy grinding!

Rod

[Lognic04]

Nice to know who used this tool before! I am at about 2.3mm now. I dont hink the tiles will actually get that thin, its at about 3mm thick still. :)

[dave brock]

I've come in late here but I see nobody has mentioned the issue that F/2.2 would need a 45-50% secondary obstruction.

Dave

[Lognic04]

I have just passed the 2.5mm saggita mark, when should i be switching to normal strokes?

[Rod]

You are at about F6.4 then. You could stop deepening the curve now if you wish as that's pretty close. With a chordal strike, you often wind up with a deep centre and and edge that lags behind. I suggest you start moving the stroke in towards the centre and start to shorten the stroke. This will help to even up the curve.

I'm assuming you are going for F6 not F5? If you want F5 keep on with the stroke you are using a bit longer.

You can test how good your contact between the tool and mirror are by using the pencil test:

https://stellafane.org/tm/atm/grind/fine.html scroll to the bottom of the page.

Rod

[Lognic04]

I am aiming for F5 - although i miscalculated and switched to normal already... :help:

[Rod]

There's no problem switching back to chordal stroke. You could try normal stroke for a while and see if it is still deepening the sagitta at a rate you are happy with.

F5 will be more portable and easier to mount. I think it's a good choice. It will be more challenging to parabolise.

Rod

[Lognic04]

I'm not really sure how to know how fast it is going - all i know is i did about 5-10 wets with normal strokes and nothing changed really, though now i realise i was doing strokes FAR too long. if i were to go back to chordal, when should i switch? :thanx:

[Rod]

I would change back to chordal stroke now then. No sense continuing with something that's not working. Test often so you don't overshoot.

Rod

[Lognic04]

After some consideration, I actually think I will do F6 - easier to parabolise, and smaller secondary for planetary, although the tube may be taller I am going to make a truss tube, so it is easy to move around

[Rod]

Hi Logan

There are certainly advantage to the longer focal length but I don't think you'll get significantly better planetary images. The advantage of a smaller diagonal is very over rated. The quality of the mirror and good collimation are what I think makes the differnce.

Consider carefully which scope you can most easily build and transport. You have some excellent mirror makers at the ASV who can help you with the parabolising if that is what worries you.

BTW I have the cardboard tube for my 10 inch F6 sitting in my garage. Let me know if you would like it. It could be used as the tube or as a form for building a more exotic tube, mirror box or secondary cage.

Rod

[Lognic04]

I have decided to sperise now. I think that the mirror will end up being about f5 by the time this is done anyway! I have a 10mm ring that has not contact, any ideas on how long this will take? Thanks!
:D

[Rod]

Hi Logan

So you have the outer 10 mm not contacting when you use the pencil test? That's to be expected when using the chordal stroke.

It probably won't take long to come into contact at 80 grit but I can't give an estimate. It depends on your technique which I can't see.

I know you are probably sick of rough grinding but you are getting close. I find the early stages of grinding a bit tedious but it gets a little more interesting and quieter as the grits become finer.

Keep up the good work.

Rod

[Lognic04]

Thanks Rod. yep the outer 10%, I was hoping to pour the pitch lap at the ATM section on Sunday but i don't think that will happen in time :(

[Rod]

Hi Logan

You are doing much better than I did at your age. I tried making a 6 inch when I was 15. I gave up during rough grinding and finished it in my 20s. I've made quite a few mirrors since but I really admire young people who stick at it. Hogging out a 10 I think is a great achievement.

I wouldn't worry about the pitch lap yet. It's better to do at the end of fine grinding. Pitch dries out and becomes hard so you don't want it sitting around not being used. Aculap (synthetic pitch) might be ok. I think it maintains its consistency better.

The meeting is Saturday BTW, not Sunday. I look forward to catching up with you then.

Rod.

[Lognic04]

Thanks Rod :D BTW, down to a 7mm ring now! how to tell when it is completely done?

[Lognic04]

Should I be worried about this bubble? Will it go away?

[Rod]

Hi Logan

You are done when all the pencil lines grind out evenly and you have the Sagitta you aimed for. Then you move on to 120.

I'm not sure what bubble you are referring to? Is it in the Pyrex? How big is it and how deep down?

Some bubbles in Pyrex are common. Usually they are small and below the curve you are grinding.

Rod

[Lognic04]

I have tried to outline it here - it is an air bubble inbetween the disks. It changes shape though so im not sure if i should worry about this?

[Rod]

Ok now I understand. Short answer - don't worry about it. Its what you expect at this stage. It's more evidence that the disks do not match each other. The centre is deeper on the mirror than the corresponding hill on the tool. As you make the disks more spherical, you should see a more even distribution of bubbles. Don't get hung up on the bubbles though. Use the pencil test to monitor your progress.

Rod

[Lognic04]

Ok cool :)

[Lognic04]

4.5mm left! sag is 2.8mm. Hopefully i can move on to fine grits tomorrow or the day after! :D

[Rod]

Seems to be going well Logan. Keep up the good work!

[Lognic04]

I ran out of 80 grit! the ring is VERY nearly gone - about 1.5mm left. Not sure what the best thing to do here is? Switch to fine grit?

[Rod]

I suggest you take a break and get more 80 grit on Saturday. Progress will be slower with 120.

Rod

[Lognic04]

Hi Rod, I think that I am so close to it being done that after a few 120 wets it will be spherical - the ring is only about 2mm big now anyway.:)

[Lognic04]

Switched to 120 grit now, and the ring is near gone. Boy, was that an ordeal to clean up all the 80 grit mess! :P

[Lognic04]

Caught up with everyone today at the IMS! While I was there, i got the mirror and tool mated perfectly, and now it just needs a few more wets to finally spherise it and then on to 320 and finer!

[Lognic04]

I did a few more 120 grit wets, used an eyepiece as a magnifier and can confirm that there are no 80 pits left! On to 320 grit, as i dont have 240.

[Lognic04]

Looking for pits after 120 grit!

[Lognic04]

Grr, just finished a wet only to find quite a few bad scratches!!!

[Billyboy78]

Make sure you maintain a nicely rounded chamfer around the edge of tool and the mirror. When stopping you move the tool 3/4 of the way off then lift it up, leaving the edge of the tool on the mirror.......SCRATCH.

[Rod]

Hi Logan

Frustrating I know! But at least it's right at the beginning of a finer grade abrasive. A couple of wets at 120 will remove them quickly. You might even be able to grind them out at 320 but I think you'll find it quicker with 120.

Bill is right. Be careful how you lift the mirror off the tool. When you start a wet, gently place the mirror or tool on top and make a few very short strokes with no pressure. This helps reduce the size of any larger bits of grit.

Scratches look ugly but even if you have a few on your mirror at then end they make almost no difference to its performance. I would get rid of these since you are only just out of rough grinding but if I had a couple at the end of fine grinding, I would just leave them.

Rod

[Lognic04]

Thanks guys - how should i be taking the mirror off safely?

[Rod]

Hi Logan

Just try to limit the pressure downward and lift it rather than drag it off.

Your mirror bevel is quite large and you are using a tile tool so I doubt that's the problem.

Also check the grit at the start of the wet. I usually gently run my fingers over the grit after I have added water to it. if you feel something sharp and even, clean off the grit and try again.

Hope that helps.

Rod

[Lognic04]

How do i check how deep a pit is? i have a 0.5mm long fracture or something from 80 grit - i want to know if i should go back to 120 grit.

[Rod]

Hi Logan

Some people check with a magnifier.

I normally use 240 after 120 but I suspect 320 will just take a bit longer to remove pits etc. I suggest you continue for 30 minutes or so with 320. See if the pit is getting smaller. If it doesn’t reduce noticeably, do a few wets with 120 to get rid of it.

Rod

[Lognic04]

I think i might need to go back to 120 - really annoying! but i have only done a few wets since last post.

[Lognic04]

Also i think a bit of an issue is i didn't do #240 grit?

[astro744]

You may have got some contamination from 80 grit. You must ensure that when moving onto a finer grit all traces of the old grit are removed and no containers that were used to house the old grit be simply washed and used for the finer grid. You need new containers. I used different sauce bottle to hold and dispense the wet grit.

Use some plastic drop sheet over the whole surface and under the mirror on the grinding bench. Then when going fine get new drop sheets,

The items that must be reused, such as mirror and tool must be washed thoroughly and again do not use any brush used for coarse scrubbing for the finer powders, get a new brush.

A couple of fine scratches here and there won't hurt too much but too many finer ones and your mirror will suffer in contrast. Of course if your anything like me you'll keep going until that last scratch has gone! Just might take a while. I think 240 grit would have been good to have but of course one uses what one has available and can get by.

[Lognic04]

The scratches are gone now, now its only some pits hanging around from 80 or 120!

[Lognic04]

Ok, i'll admit it - ive been a bit lazy lately! Soon, it will be done! NOT giving up no way!!! :D

[Lognic04]

Had a great ASV meeting yesterday! Gout an hour and a half of #320 done. Today, about 40 minutes. Another hour and a half or so is what it needs maybe until 25u. Gotta beat Stefan! ;)

[Lognic04]

What the heck is this pit! It is the last major pit left after 320 grit?? :help::sadeyes:

[Rod]

Hi Logan

How big is it? Hard to tell from the photo. There looks like another smaller one off to its right. I can't tell you what it is. It does look unusual. Perhaps Stefan might have some idea?

If it's a one off and nothing else that size is there, I would probably ignore it. I would suggest you remove all pits the size of the smaller one I mentioned above.

But get another opinion before following my advice.

Rod

[Lognic04]

It is quite small, unlike the photo suggests, and yeah it is a one off. I will do a few more wets tonight and see if it changes, if not move to 25u :)

[Lognic04]

:mad2::mad2::mad2::mad2::scared: :doh: :scared2:

I reckon its tool bits flaking off - sealant around the edge.

:help2::stupid:

[Rod]

OK so you have a small scratch. Has the previous apparently large pit changed?

I'm not sure I would blame the tool yet. More likely you have some contaminant in the wet - dust, rough section of bevel etc. Always look for the simplest explanation first. Check your bevel. Clean everything and check the powder when you add it with your finger tips. If you feel roughness, clean up and try again. It might also be worth working mainly mirror on top.

If it's only one small scratch I would leave it and move on but if you have many, you will need to go back to coarser grit to remove them.

Hopes that helps.

Rod.

[Lognic04]

Thanks rod, there has been some construction in the area I work so maybe some wood chips got under? I will definitely work mirror on top for now as the FL won't change.The scratch seems to be fine so I will leave it and hope it grinds off with 25u :thanx:

[Lognic04]

And no, the pit did not change that much.

[Lognic04]

Starting 25u alox! The scratch is shallow enough it *should* go away

[Lognic04]

Back to 120 grit :sad::sad::sad:

WHY?!!?!!!!!!!? :mad2::mad2::mad2:

[Lognic04]

IIS is eating my images, but whats happened is something has given me an 11cm long scratch on my mirror, and it is very deep and wide ...

[Rod]

Hi Logan

I am sorry to hear that. It's very frustrating after all the effort you have put in.

I noticed you said there was a bit of construction going on in the house. Perhaps a piece of dust got into your work. Did you hear or feel anything when it happened? Was it during a wet or just at the start?

Some people add a small amount of detergent (eg dishwashing liquid) to the grinding slurry. This can help stop grit clumping together. It's important just to add a drop. Too much reduces grinding action and you have suds everywhere.

Check your mirror bevel and your tool tiles carefully. See if there is any evidence of tiles chipping. I think this unlikely but check anyway. Are the tiles getting very thin?

I have some 240 grit. Send me a PM with your address and I can arrange to get it to you. Depending on the scratch depth you may be able to get rid of it with that.

When you get back to 25 micron remember that is where scratches are most likely. So it's important to take extra care keeping your grinding area as clean as you can. Scratches during polishing are less likely.

You have done really well maintaining your motivation. Don't let this discourage you.

Rod.

[Lognic04]

Thanks for that Rod. It is annoying.
I made special precaution to move into my room, maybe that's got contamination? I also scrubbed the tool and mirror, and replaced the cover on the grinding stand. I think it must be from the tool? Some tiles at the edge have very sharp pointy edges. I didnt feel anything strangely enough! I also do use dishwashing liquid at the moment. I will check out the tool and report back.
240 grit would be great Rod!!! Thank you so much for your offer.
How come they are less likely in polishing? I would have thought that that work is so fine any scratches would pop up.

[Rod]

Hi Logan

Sounds like you are doing your best to keep things clean so that's great. I'm not really worry about the tile edges at this point. However you could bevel the outside ones if you are worried. I don't normally bother.

I am glad to help with the grit. It's not a large amount but I think enough for your needs.

During polishing you use a pitch lap which is much softer than glass or ceramic tiles. So rough material does not abrade as much. You can get problems with sleeks in polishing but let's leave that discussion for now.

Rod.

[Lognic04]

Just bevelled the mirror and tool, and thanks to rod begun with 240! After 6 wets you wouldnt know a scratch was there unless you looked for the faint line of pits!

[Lognic04]

Scratch is gone! On to 25u!

[Lognic04]

Almost done with 25u! Should be able to finish it tomorrow, then 12, 9,5!

[Rod]

That's great Logan!

Take your time with each step and clean everything very well in between grades.

Rod

[Lognic04]

I'm pretty sure i am done now - no pits out to the edge!

[Kunama]

Great work Logan !!!

[Lognic04]

Thanks! :)

[Lognic04]

I'm having issues with 12micron - it is EXTREMELY sticky, like, hard to move! What is the issue here?!

[Rod]

It sounds like you are adding too much grit and not enough water. It should be quite runny, the consistency of water. Try mixing it up in a squeeze bottle - eg old shampoo bottle and add a dash of detergent. Squirt it on the mirror and then lightly run your clean fingers through it to check for sharp bits.

If you look at the start of this video, you'll see how runny it is:

https://m.youtube.com/watch?v=noBWSeozpkI

A fine grinding wet should last 5 minutes or more.

Rod

[Lognic04]

Thanks Rod, that fixed it!
However, it still loves to hug one particular edge region. Any ideas as to why this occurs?

[Rod]

It may be drying out in that area. Add a little more of the mixture and see if that helps. Don’t force it when grinding. Be gentle.

Rod

[Lognic04]

It is like that from the start however?

[Rod]

Maybe do a sharpie test just to make sure the tool and mirror are well matched. Use a felt tip pen.

[Rod]

Also try adding a little more grit to the mixture

[Lognic04]

I feel it might be best to skip directly to polishing at this stage as I am having sticking issues. I will use polishing pads so the time spent will be marginally different from 12 to 5 micron. Should I skip?

[Rod]

Hi Logan

I don't like the idea of jumping to polishing before we know what's going on. You will be polishing for a long time, even with pads and if you have a problem where the mirror and tool are not spherical it just wastes time. I would prefer to at least finish with 9 micron and preferably 5.

I would really like to be sure your mirror and tool are properly mated. That's why I suggest the sharpie test as described on the stellafane site.

My other theory is that you may have now added too little aluminium oxide to the water. That would cause it to grab. Make the mixture a little richer and see if that changes things.

Perhaps you could post a video of what you are doing somewhere. That might make it easier to spot the problem.

Rod.

[Billyboy78]

If you have sufficient lube, as in watery grit, then the only reason they will stick is that the discs aren't matched up perfectly from rough grinding. Possibly they match up COC and thus you get a good sharpie test, but trying to move off centre air can't get in as they mis-match and a vacuum is formed locking everything up.

You have 3 options, go back to 25 micron and grind slowly and carefully aiming for 100% match, persevere with 12 micron, it will eventually come good, or put a flash polish on the mirror and do a KE test to see where you're at.

[Lognic04]

Maybe I could try channelling the tool? It has absolutely no gaps so that could be a problem. I will also add grit, see if this continues and if it does post a video

[Rod]

Hi Logan

Don't try channeling the tool at this point. It's only going to invite scratches. Plenty of people make telescope mirrors with a glass blank as the tool - no channels.

Remember to try the easiest solutions first. Vary the mix a bit as discussed. See if that helps. Try a sharpie test. It's easy to do.

When you have done the above report back and then we can discuss what to do or try next.

Rod

[Lognic04]

About to shellac and begin polishing! Yes only after 12u, as Mark (from ASV) said that it would be best. By the way, the sticking is pretty much gone after letting it sit for a few days and i presume thermally equalise.

[Lognic04]

Polishing pads on! And so begins the 3rd and final part of mirror making!

[Lognic04]

Polishing is going well!

[Lognic04]

Forgot Stefan was watching this!!! :P
Did polishing with the pads for 6 or so hours and I realised fine grinding wasn't done, so I went back to 12,9,5 and then begin polishing again, by the way, I am 2hr 20 mins in and have already a beautiful shine across the entire mirror and the edge is near invisible! If I am not pouring the lap at the asv i will be finishing Polishing on the awesome spinny machine :)

[Stefan Buda]

I give you another 6 weeks before I re-join the race. :lol:


You may not realize that you are only about half way trough this job. By far the hardest part is still ahead of you.

[Lognic04]

29 jul
begin figuring
1hr COC to reduce ring

1hr blending, ring nearly gone, bad tde

diagnosis tot short strokes

20min tde tot edge improved, slightly oblate
diagnosis mot tde strokes

Just chucking my log here.

[Lognic04]

HA! I beat you Stefan! :lol:
I just finished my mirror at 1/10 wave p-v, 0.98 strehl!

[Lognic04]

Thanks for all your help everyone! What a fun journey!

[Billyboy78]

Congrats Logan, well done.

[Lognic04]

Thanks!

[Atmos]

Congratulations!
Now you’ll have to custom build the rest of the dob around it ;)

[OzEclipse]

Congratulations Logan. Very impressive effort mate.

What's the program/apparatus you're using to analyse the wavefront error?

I didn't think Ronchi could be used to analyse this or is it a composite of a ronchigram and some other sort of interferometric test?

Well done!

Joe

[Lognic04]

Thanks guys!!! :D
The Figurexp software uses a couder mask for its data. :)

[Lognic04]

Scope is done!!! (minus mirror coating)
Thanks everyone for helping!!! :D:D:D
Venus: big crescent, nothing interesting
Jupiter: 4 moons, bands, small bands, festoons!!!
Saturn: Cassini division, obvious surface banding
Mars: orange blob!
Stumbled on the lagoon nebula, could actually see nebulosity uncoated!

[troypiggo]

That’s awesome mate. You must feel so proud and a real sense of achievement.

[Lognic04]

Thanks Troy!!

[xelasnave]

Well done Logan.
Absolutely wonderful effort.
Alex.

[RickS]

Congrats, Logan! That's quite an achievement.

[multiweb]

Legend. :bowdown:

[Stefan Buda]

Congratulations, Logan, I admit defeat!
(Just as well I did not make it a bet. :D )

[GrahamL]

Well done young man !

[Lognic04]

Thanks so much everyone!!!