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  #141  
Old 24-10-2018, 09:12 PM
Stefan Buda
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Originally Posted by Imme View Post
Looks great....what kind of time are you talking to go 1/4 to 1/10?
The closer one gets to the desired precision, say 1/10 lambda, the shorter the figuring spells become between testing. The problem is that after a short figuring run, that can get down to a few seconds, one has to wash and dry the optic before placing it on the test plate and then wait at least 15 minutes for the temperatures to equalize for a reliable reading.
After that the optic goes back onto the polishing lap where one also has to wait for the temperatures to equalize. So one hour goes by for a couple of short figuring spells. And many are required unless you have some supernatural ability to predict the behaviour of the polishing lap.

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Originally Posted by Atmos View Post
Excellent work Stefan, great to see this project slowly coming together.
Thanks Colin.

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Originally Posted by Lognic04 View Post
Wow, you seem to be very fast at this process?!
I hope to become faster with more practice.

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Originally Posted by multiweb View Post
Hi Stefan, what are we looking at? Parallel lines? Can you make the difference between 1/4 and 1/10 just looking at the patterns?
Yes the spacing between the interference fringes represent 1 lambda on the wave front regardless of the relative tilt between the two surfaces. In other words they show the size of the air gap between the test plate and the optic. If the two surfaces have the same shape, the fringes are straight.
It is very easy to see a deviation of 1/4 spacing, especially if one spread out the fringes. I kept the fringes rather tight for the pictures as it was easier to photograph them.

Last edited by Stefan Buda; 25-10-2018 at 02:52 PM.
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  #142  
Old 24-10-2018, 09:41 PM
sharpiel
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It's amazing to learn from you Stefan. Thanks.
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  #143  
Old 24-10-2018, 09:46 PM
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Originally Posted by Stefan Buda View Post
Yes the spacing between the interference fringes represent 1 lambda on the wave front regardless of the relative tilt between the two surfaces. In other words they show the size of the air gap between the test plate and the optic. If the two surfaces have the same shape, the fringes are straight.
It is very easy to see a deviation of 1/4 spacing, especially if one spread out the fringes. I kept the fringes rather tight for the pictures as it was easier to photograph them.
So the test plate has to be perfect? Did you make it as well?
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  #144  
Old 24-10-2018, 10:03 PM
Stefan Buda
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Originally Posted by multiweb View Post
So the test plate has to be perfect? Did you make it as well?
http://www.iceinspace.com.au/forum/s...&postcount=134
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  #145  
Old 25-10-2018, 07:02 AM
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Originally Posted by Stefan Buda View Post
Thanks. I understand now. All this polishing/figuring business fascinates me but I have no idea how it's done hence the silly questions. By the sound of it in your posts although there is a great deal of experience needed it seems to be an iterative process that can be tedious to achieve a high level of figuring, so there's no free lunch. What I find scary it that it's like sculpting. You can't add back if you remove too much and the potential to stuff up must be getting higher and higher as you're nearing your target figure.
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  #146  
Old 25-10-2018, 02:49 PM
Stefan Buda
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Originally Posted by multiweb View Post
Thanks. I understand now. All this polishing/figuring business fascinates me but I have no idea how it's done hence the silly questions. By the sound of it in your posts although there is a great deal of experience needed it seems to be an iterative process that can be tedious to achieve a high level of figuring, so there's no free lunch. What I find scary it that it's like sculpting. You can't add back if you remove too much and the potential to stuff up must be getting higher and higher as you're nearing your target figure.
Unfortunately figuring is a bit of a black art that cannot be learned from books. It is very difficult to get a full size polishing lap to do exactly what you want it to do. The polishing pitch is a very viscous liquid that is exposed to various amounts of compression and dragging forces that make its behaviour rather unpredictable. A sub-diameter polishing lap is easier to predict but it is not as good at controlling astigmatism. Figuring is not as bad as sculpting because if you dig a hole somewhere, then you just have to polish away the rest of the surface until the hole disappears - not too hard for spherical surfaces but a nightmare for aspheric ones.
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  #147  
Old 25-10-2018, 06:08 PM
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Originally Posted by Stefan Buda View Post
Unfortunately figuring is a bit of a black art that cannot be learned from books. It is very difficult to get a full size polishing lap to do exactly what you want it to do. The polishing pitch is a very viscous liquid that is exposed to various amounts of compression and dragging forces that make its behaviour rather unpredictable. A sub-diameter polishing lap is easier to predict but it is not as good at controlling astigmatism. Figuring is not as bad as sculpting because if you dig a hole somewhere, then you just have to polish away the rest of the surface until the hole disappears - not too hard for spherical surfaces but a nightmare for aspheric ones.
I reckon it's a skill you must acquire during a lifetime doing it and start young too. Mentoring sounds like a necessary step as well. There are tools to do the job but in the end it's a very organic process and depends on the individual.
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  #148  
Old 01-11-2018, 07:57 PM
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Today I received my corrector lenses back from Longman Optical in Tasmania and I' very happy with the coating job they did. I tried to photograph the residual reflection and it is hard, but you can see on the attached picture just how reflective the uncoated edge of the lenses is, compared to the rest of the surface. It cost around $90 per lens.

Also the primary mirror has reached the final stages of figuring. So far I did it all with the full lap, MOT, and the figure is beautifully smooth.

Tomorrow I'm off to VicSouth so there will be no progress for a few days.
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  #149  
Old 03-11-2018, 01:06 PM
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When I saw the weather forecast, on Friday morning, I aborted the VicSouth trip even though the car was almost fully loaded. Pushing glass instead.

Longman Optical supplied a reflectivity graph for the coatings.
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Last edited by Stefan Buda; 03-11-2018 at 06:03 PM.
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  #150  
Old 11-11-2018, 08:06 AM
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Yesterday I finished sculpting the primary mirror and that means all the optics are done.
Figuring an f/2.7 mirror is quite an adventure. I did most of it guided by a Ross Null setup and when it became difficult to make more progress, I switched to the Bath interferometer. At that point I was within one lambda wavefront error but it was a long road from there to 0.035 lambda rms.

The attached report is based on only 5 noisy interferograms taken at 72 degree rotation so the mirror is probably better than the report indicates as air currents, vibrations and other artefacts have not been properly averaged out.
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  #151  
Old 11-11-2018, 09:35 AM
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Originally Posted by Stefan Buda View Post
When I saw the weather forecast, on Friday morning, I aborted the VicSouth trip even though the car was almost fully loaded. Pushing glass instead.

Longman Optical supplied a reflectivity graph for the coatings.

Stefan,


Sometimes you just have to ignore the forecast, only Sunday night was a complete writeoff, Saturday was basically clear all night, Friday and Monday had a few hours each.


Paul
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  #152  
Old 11-11-2018, 06:44 PM
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Paul,
I intended to go for two nights only and it looked like the second night was going to have high cloud, so I thought it was not worth the long drive.
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  #153  
Old 11-11-2018, 07:00 PM
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Paul,
I intended to go for two nights only and it looked like the second night was going to have high cloud, so I thought it was not worth the long drive.

Stefan,


I agree, it's a long drive for two nights, but at its best is a very dark site.



Paul
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  #154  
Old 17-11-2018, 06:45 AM
Stefan Buda
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I spent a couple of days vacuum coating telescope mirrors - including the LOGAN #1.
All the optics for the CDK are now complete.
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  #155  
Old 17-11-2018, 08:58 AM
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  #156  
Old 17-11-2018, 09:34 AM
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The Logan #1 is almost a plug for the Stefan

Looking good
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  #157  
Old 17-11-2018, 12:42 PM
Stefan Buda
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Colin,

You obviously have not followed Logan's project. His first mirror is the same size as my CDK primary and it is not shown on my photo. The smaller mirror is the secondary of the CDK.
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  #158  
Old 20-11-2018, 02:20 PM
Stefan Buda
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The housing for the corrector lenses is done.
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  #159  
Old 20-11-2018, 02:37 PM
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The housing for the corrector lenses is done.
So first lens goes in, then spacer, second lens and last ring to secure? Is that how it works? Does the orientation or order of the lenses matter?
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  #160  
Old 20-11-2018, 03:58 PM
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Stefan, the mirror works amazingly!!!
The moon was beautiful at high power and the homonculous nebula was visible too!!! wow!
Thank you so much!
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