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Vacuum formed Mylar Mirror ?

DIY mirror making
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#51 DAVIDG

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Posted 16 January 2020 - 10:45 AM

 The technology to flex a large surface into an accurate optical surface does exist. It is used on the Keck telescope but it requires that the glass segments be made of Zerodur ( ultra low expansion glass). micro actuators to flex the glass at multiple positions,  a complex structural model of the whole system that predicts the flexing of the complete structure, a laser alignment system,  with active feed back and a computer system to control all of it. 

   There are many very smart people in this world and if there were a simpler ways of doing it, someone would done it by now. So when an solution to a problem looks to be simple and no one has done it yet,  what that means is that the problem is much more complex then one thinks and one needs to do more research to truly understand the problem.

 

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#52 GTom

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Posted 16 January 2020 - 11:26 AM

Actually what we need is a material lighter weight and easier to work on than glass. Advanced plastics might evolve to the sufficient characteristics with time.

Beryllium is not exactly cheap.

Edited by GTom, 16 January 2020 - 11:43 AM.


#53 davidc135

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Posted 16 January 2020 - 03:02 PM

Great thread.

I've been using Matt Considine's Schmidt calculator to get an idea of the vacuum/pressure difference needed to create a curve of 240 ins roc. For a film .010 in thick a metre diameter it's miniscule. I'm getting 10 E-4 to 10E-5 psi. (very approx.) depending on material. Unless I made a mistake! Can it be this tiny? Super sensitive to any little influences.

 

www.considine.net/mac/vacpan.html

 

David


Edited by davidc135, 16 January 2020 - 03:10 PM.


#54 dan_h

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Posted 16 January 2020 - 04:34 PM

Great thread.

I've been using Matt Considine's Schmidt calculator to get an idea of the vacuum/pressure difference needed to create a curve of 240 ins roc. For a film .010 in thick a metre diameter it's miniscule. I'm getting 10 E-4 to 10E-5 psi. (very approx.) depending on material. Unless I made a mistake! Can it be this tiny? Super sensitive to any little influences.

 

www.considine.net/mac/vacpan.html

 

David

The surface exceeds 1200 square inches and it is a thin film.  As DAVIDG stated back in post #36, it would be a very sensitive barometer. 

 

dan


Edited by dan_h, 16 January 2020 - 04:34 PM.


#55 Oberon

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Posted 16 January 2020 - 04:39 PM

Yes. The vacuum pump would be a simple screwed plug being drawn out of a sealed thread. Not unlike the methods used on Lunt’s range of solar telescopes for tuning the etalon.



#56 Oberon

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Posted 16 January 2020 - 04:46 PM

I’m more interested to know if a polished spherical glass mirror would form the desired parabola with a negative pressure applied to the back. Goodbye mirror support, hello variable f ratio’s?



#57 GTom

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Posted 16 January 2020 - 04:51 PM

I’m more interested to know if a polished spherical glass mirror would form the desired parabola with a negative pressure applied to the back. Goodbye mirror support, hello variable f ratio’s?

You'd need a quite stable vacuum pump to keep the focal length stable to micron levels and of course a mains plug next to the scope...



#58 Oberon

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Posted 16 January 2020 - 05:08 PM

You wouldn’t need a pump, a sealed enclosure will do, all you need to do is pull on the back wall of the sealed enclosure with a well controlled mechanism. Amateurs have built mirrors with a screw attached to the back for this purpose, buts as its never taken off I presume they were never a great success. I’m mildly curious if a vacuum interface would be an improvement or a waste of time.


Edited by Oberon, 16 January 2020 - 05:08 PM.


#59 GTom

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Posted 16 January 2020 - 05:38 PM

You wouldn’t need a pump, a sealed enclosure will do, all you need to do is pull on the back wall of the sealed enclosure with a well controlled mechanism. Amateurs have built mirrors with a screw attached to the back for this purpose, buts as its never taken off I presume they were never a great success. I’m mildly curious if a vacuum interface would be an improvement or a waste of time.

How much does the focus point have to move before the image gets blurry? I am afraid it would need an autofocus kind of setup, otherwise you'd be fighting with every 0.1 degrees temperature or even 0.01mbar local air pressure change...



#60 davidc135

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Posted 16 January 2020 - 05:40 PM

Yellobeard on the Cats and Casses forum who has made some large, high grade scts has varied the air pressure behind the primary to fine tune spherical aberration.  David


Edited by davidc135, 16 January 2020 - 05:41 PM.


#61 Benach

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Posted 17 January 2020 - 02:10 AM

Yellobeard is by no means comparable. He is a professional optics maker for several decades and he can do tricks and make optics that are beyond any of your wildest dreams.

How do I know this? I know yellobeard personally.
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#62 davidc135

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Posted 17 January 2020 - 01:32 PM

Yellobeard is by no means comparable. He is a professional optics maker for several decades and he can do tricks and make optics that are beyond any of your wildest dreams.

How do I know this? I know yellobeard personally.

You are jumping the gun if you think I was slighting him. I wasn't making a comparison, just pointing out an aspect of his work that should interest members! A mistake, you think?


Edited by davidc135, 17 January 2020 - 01:40 PM.


#63 Oregon-raybender

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Posted 17 January 2020 - 03:58 PM

OK Buzz kill here. I remember we tried very hard to make Mylar mirrors at Tinsley, back in the early 70's

We had so many variables, product processing, surface roughness, mounting issues, gluing issues, vacuum issues, pressure, both air (High and Low atmosphere changes, yes it did come into play) and vacuum holding and metering. We could not make it work. We came close, all we could do was use it for imaging systems, not for telescope quality. We had one system were we made convex and concave surface using pressure and vacuum pumping. I saw several companies at various OSA and SPIE industry conferences, no luck. But new tech always proves one wrong.confused1.gif

 

Not that I am saying it can't be done, BUT, I do mean BUT there is so many things to think about. If I was going to try a thin membrane material, I would suggest Gorilla Glass 6 that is now used for protection of phone and tablets. It bends easy and maybe the process has a surface roughness (polish) that may work. The question is how will a simple AL coat work. I doubt a layer coating would?  Just a thought.

 

One more add, Tinsley was the company who made the Keck Mirrors.

 

Starry Nightswaytogo.gif

 

https://www.youtube....h?v=kF-p9lCpwew

 

https://www.xda-deve...oldable-phones/

 

https://www.corning....la-glass-6.html

 

https://triaticinc.c...ASAAEgIU0vD_BwE


Edited by Oregon-raybender, 18 January 2020 - 12:51 PM.

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