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Found this chart on the web, not sure how reliable it is but you can see that your mirror is close to the red line indicating the difficulty in using a Bath interferometer for this project.

Hope you find this of interest.

John

bath3.png

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This is how I do it with large mirrors. Hogging out is done with this 'machine' the rest is done with my MoM. It took me about 1.5 hours hogging out this 20" f/5.6 mirror using a angle grinder with

As a mirror maker with many years experience I wonder if I might share some thoughts on your ambitious project. When making a big mirror the first thing to consider is how do I test it? If you can’t t

Work continues with #800grit:     Aiming at #1000 then #1500 to prepare a pitch lap and check if we have something that resambles a sphere.

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5 hours ago, Glasspusher said:

Found this chart on the web, not sure how reliable it is but you can see that your mirror is close to the red line indicating the difficulty in using a Bath interferometer for this project.

Hope you find this of interest.

John

Thanks John, very appreciated. This is line with the feedback from the Bath group - difficult but doable

https://groups.io/g/Interferometry/topic/bath_for_a_800mm_f_3_3/74609578?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,74609578

We'll start with Ronchi and do the optimization wwith the BAth if we can make it work. I always had thought that Bath gives that topographic feedback that makes addressing errors effectively.

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  • 7 months later...

An update on the mirror side of the project as a part of the club has been working on the mirror for a few months now. The bulk of the material has been ground off at a machine shop that works on granite.
Subsequently, the hogging was done with a half tool to smooth everything out with #80 grit.

a.JPG.840c2610443783d90931b4311f94e679.JPG

You "may" notice that the route we took is the ....most impervious...but cheaper and readily available. Also not very recommendable which is a flat  25mm float glass.

All in all, I'd consider this a practice run for the part of the team who works on the mirror - we'll gain some experience.

aa.JPG.f868efb01cb6f860c96f5c542ecf7a1a.JPG

Since we are not expencing to reach lambda/4 we are still exploring some alternative solution for a potential 2nd mirror. I'm getting more and more convinced that everything taken into consideration a 25mm thick float (?) slumped meniscus is the way to go. The big deal is that it looks pretty hard to find a suitable place in Europe to slump the glass - it's probabaly me not looking hard enough.

Currently we are looking at 2800mm FL 

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  • 2 weeks later...

This is how I do it with large mirrors. Hogging out is done with this 'machine' the rest is done with my MoM.
It took me about 1.5 hours hogging out this 20" f/5.6 mirror using a angle grinder with a diamond disk. The surface is rather bumpy and to remove these irregularities it took me a 2 hours(or so) using a MOM(mirror-O-matic) and carbo #120. It is not futuristic looking I know, but this device saves me lots of work.
When I need another ROC, I make me another set of curved side panels(using a router), that's all there is to.

 

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Thanks for posting the video, interesting. From what I can see the tool is not free to rotate during the grinding process which it should be. Also to help with controlling astigmatism a larger grinding/polishing tool is advisable, probably around 80% the diameter of the mirror and best used in a 'spin' grinding mode. Hope this helps.

John

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9 hours ago, Chriske said:

This is how I do it with large mirrors. Hogging out is done with this 'machine' the rest is done with my MoM.
It took me about 1.5 hours hogging out this 20" f/5.6 mirror using a angle grinder with a diamond disk. The surface is rather bumpy and to remove these irregularities it took me a 2 hours(or so) using a MOM(mirror-O-matic) and carbo #120. It is not futuristic looking I know, but this device saves me lots of work.
When I need another ROC, I make me another set of curved side panels(using a router), that's all there is to.

I saw this several times and it is saved in my favorite yt folder. Did you try to use any water or was it messy?

This is another good example:

https://www.youtube.com/watch?v=eRdr86gXBLI

Although it started like this....:

https://www.youtube.com/watch?v=lonAulU-kWA

 

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  • 3 weeks later...

When using a sub-diameter tool for fine grinding it is normal to work the tool off-centre in order to give an even grind. From the video it looks like your stroke is through the centre which will give preferential grinding to the central regions at the expense of the edge. The tool also looks a little small,  the standard is around 75-80% the diameter of the main mirror.

Hope this helps.

John

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Hi John, you're right the stroke is dead center and the tool I think sub-radius - like 45% of the diameter. We tried a full tool but the friction was way too high. The main grinding was done with a CNC diamond tool.

I'm definitely not an expert in making mirrors - I've only done few , long time ago. For sure I'll ask about your consideration and come back to you.

Btw I've cobble up another video - not advisable if you're seasick ;D

https://www.youtube.com/watch?v=w4HIsp1-zXw

 

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