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quantumphysicist
member


Reged: 12/05/12

Re: Homemade Mylar Parabolic mirror new [Re: DAVIDG]
      #5596985 - 12/30/12 09:50 PM

I suspected that this wouldn't work well. I didn't even consider the air pressure problem. Thanks for the knowledge though. It is a bit disappointing that it would not work well.

How could I forget that this would form a catenary surface and not a parabolic surface. Silly brain.


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Homemade Mylar Parabolic mirror new [Re: quantumphysicist]
      #5597553 - 12/31/12 09:05 AM

What is stopping you from making a parabolic surface and sticking on the reflective coating?
(That's a rhetorical question- I know the answer.)


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Homemade Mylar Parabolic mirror new [Re: glennnnnnn]
      #5597713 - 12/31/12 10:45 AM

Quote:

Good idea, at least the kernel. The specific application needs design to address each one of those problems. Just tune it up a little bit and make it really BIG.
Yeah, its not accurate to millionths of an inch but it has a 2 meter aperature! I think a camera at the focus would be the best method.
Mylar's got potential!




Before you can begin to evaluate the potential of mylar, you are going to have to eliminate the other sources of error. First and foremost is that the supporting rim of the mirror needs to be at optical tolerances if you expect the mylar surface to be at optical tolerances. A trash can lid is not going to do it. You need to machine and polish a rim to support the mylar accurately. Then determine how to manage the thermal expansion of this rim. Next you need to control atmospheric pressure, etc. etc.. Lots to work on before you can get to testing the mylar but in the meantime, you can cook some hotdogs with your setup.

I think Jon summarized it well.

dan


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ed_turco
Pooh-Bah
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Reged: 08/29/09

Loc: Lincoln, RI
Re: Homemade Mylar Parabolic mirror new [Re: dan_h]
      #5597825 - 12/31/12 11:53 AM

I'm not sure it is a catenary surface; for a cable, you do get this curve, but for a surface, you get something else, according to one of my math professors.

And before that, I always thought a catenary was the cat that swallowed the canary . . .

ed


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Homemade Mylar Parabolic mirror new [Re: ed_turco]
      #5597957 - 12/31/12 01:19 PM

Ed- it might be an irregular surface, sort of a sphere, sort of a catenary- because of the mylar's stretching.
If there is a foundation for the mylar all you need to do is attach it with spray adhesive and and hold the vacuum long enough to solidify the coating to the foundation. The foundation should be an accurate parabolic curve but it doesn't have to be glass, just solid and something that can be configured to that curve. You might be able to use a lathe to make it or just grind with a tool like you do a glass mirror, only it wouldn't have to be perfect, as in polished, because you have the film for that. (And also, unlike glass you could just fill in an area that was too low instead of taking the whole curve down to match the low surface.)
Still looking for a good material for the foundation. There are some rigid foams that might work...


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Homemade Mylar Parabolic mirror new [Re: DAVIDG]
      #5598077 - 12/31/12 02:26 PM

Quote:

Gavin article is in May 1979 SKy and Tel, "Aluminized Mylar as a Flux Collector"
Here is a link to more recent article about the idea. At conclusion of the article, the best figure that they could produce had 10 waves of error in it. http://www.gravic.com/graviclabs/pdf/papers-altz/Alt%20Az%2024%20Holenstein-P...

- Dave




An interesting article. Informative and easy to read. Thanks for the link.

dan


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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Homemade Mylar Parabolic mirror new [Re: glennnnnnn]
      #5598105 - 12/31/12 02:40 PM

The figure that is formed is a third order curve just like what happens when you use the vacuum method to make a Schmidt corrector plate by pulling a vacuum on a flat piece of glass. The 70% zone is high compared to the reference sphere.
As for making a parabolic mold it needs to be an optically accurate parabolic surface. So if you’re going to go to the trouble of making the mold that accurate you might as well just make a parabolic mirror using proven techniques. Even if you did use an accurate parabolic mold, you still have the problem that the Mylar film or what every material you use needs to be accurate in thickness to better then 1/4 wave i.e. a few millions of inch or the variation in thickness will distort the figure. Mylar or any commercial membrane type material is not accurate to that thickness. I know because I work for Dupont as a research chemist and engineer and use it and many other materials in my work. Even if you could find a membrane material that had a uniform thickness and also was optically smooth, you then have the problem of applying an adhesive between the mold and membrane that is it uniform in thickness to better then 1/4 wave. If not the curved formed will be distorted from the mold shape under it by the variation in adhesive thickness.
When I first started doing research for living many years ago, an senior coworker with a ton of experience gave me some great advice " If you think you found a simple solution to a problem that others seem to have missed, look a little deeper into the problem. You'll find it is not as nearly as simple as you think. There are many people that are smarter then you and I and if it was an easy problem to solve it would have been done long time ago"

Happy New Year,
- Dave


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Homemade Mylar Parabolic mirror new [Re: DAVIDG]
      #5598155 - 12/31/12 03:10 PM

Well that's certainly good advice on simple solutions, but another pragmatist wrote KISS.
The thing about these mylar mirrors is that they could be HUGE. Then all this micro-measuring wouldn't matter so much. The extreme accuracy required for a TINY 8" mirror would vanish when that mirror was the size of the one on Mt. Palomar (the Hale 200") because the aperature would win and cancel the errors of the less than micro-correct surface. Even if it was only 50% accurate you would still get a whopping eye-full of astronomic goodness! It wouldn't be perfect BUT you would have the opportunity to improve it in stages, unlike the current state of an optical mirror that never gets touched, because once its polished and aluminized it can't be altered, and is fragile.
Because the reflective coating is such an easy stage to do with the mylar film, if a zone of your mirror didn't measure-up you would be able to change it. This is a different kind of mirror and its a mistake to use the same standards used for glass. Granted, the optics are the same but the mylar mirror could be configured while you're using it, using adjustments to focus and repair bad sections. The biggest problem I see is finding a large enough place to store it.
EDIT: I forgot making it heavy enough so that it wouldn't get blown away in a gust of wind! =)


Edited by glennnnnnn (12/31/12 03:56 PM)


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Gary Fuchs
Pooh-Bah
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Reged: 05/22/06

Loc: Easton, PA, USA
Re: Homemade Mylar Parabolic mirror new [Re: RingleaderO]
      #5598224 - 12/31/12 03:44 PM

Quote:

Can someone please tell me or link me to instructions on how to do a Ronchi/Foucault Test or something equivalent that I can do for cheap?




You might start with these:

http://www.atmsite.org/contrib/Harbour/Foucault.html

http://stellafane.org/tm/atm/test/understanding.html

http://stellafane.org/tm/atm/test/setup.html

http://stellafane.org/tm/atm/test/tester-main.html

And a CN thread here.

Gary

Edited by Gary Fuchs (12/31/12 03:50 PM)


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Homemade Mylar Parabolic mirror new [Re: glennnnnnn]
      #5598341 - 12/31/12 04:50 PM

Quote:

The extreme accuracy required for a TINY 8" mirror would vanish when that mirror was the size of the one on Mt. Palomar (the Hale 200") because the aperature would win and cancel the errors of the less than micro-correct surface. Even if it was only 50% accurate you would still get a whopping eye-full of astronomic goodness




I think you have that backward. A large mirror requires the same absolute accuracy as a small mirror, this why large mirrors are so difficult to make, why they ate so difficult to mount.

Have you ever made a mirror? You discuss correcting the polymer mirror as if were something new. The reason a glass mirror can be accurately made is the combination of the ability to accurately polish glass combined the the ability accurately test the figur/shape. The process of making a mirror involves many, many tests of the mirror as it progresses towards a finished optic.

Read the Perfect Machine, the story of the 200 inch. The final figuring was done in a temperature controlled basement. The optician would test the mirror, polish it for a few moments, and the let it cool for a week, test it again...

I suggest studying up on your solid mechanics as well as looking into the thermal properties of polymers. Study a bit of optics...

Put it all together along with realistic values for the material properties and then consider the effect of small thermal differences. An FEA program properly meshed would be the tool.

I am a big believer in KISS but I also know that telescope mirror is almost certainly the most precisely made object manufactured by man.

Most technical folks ate aware of a certain story with the punchline, "Well, I have solved the problem for the case of the spherical cow."

A real cow is a much larger job.

Jon


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Homemade Mylar Parabolic mirror new [Re: Jon Isaacs]
      #5598621 - 12/31/12 07:31 PM

Jon- I have ground and polished between 8 and 10 mirrors depending on how you count doing them over (and over!) until they're done. Testing as well, both Ronchi with my own design telescopic tester and Foucault, under varying conditions with other testing as well, and I believe in accuracy. When I was young and too stupid to understand optical theory I worked in a lab grinding lenses. Some of that soaked in, and I find myself at a sort of intuitive level with regard to lenses and mirrors and such optical things. Then there's some theory and reading. Sadly, I think lots of the people making telescopes get so involved with being accurate that they never see anything! I worked in Aerospace tooling for years, and I can honestly say that real accuracy is important but often misunderstood.
Not in any way trying to say that I could ever make a mirror so grand as the Hale, but maybe as big, just for example. And it would have to be big, because the aperature would make the difference with a mylar mirror, which wouldn't have the accuracy of a ground and polished glass mirror, but could be made to work anyway.
Edmund has 2 54"x84" pieces for $8, which you have to admit is a real po-folks telescope or two! Like Mr. Dobson's creation of an easy to build and use telescope, an easy to make and install giant mirror could be another step in that same direction. So far, it hasn't been any more than just the idea of it. When I actually get some big pieces of that silvery stuff maybe I can make something happen. I know that I've been looking at samples of it for years, thinking, "Wow!" Its such amazing stuff!
It would be too bad if it really wasn't good enough for high-end optical applications, but I think that at this very moment there are attempts being made to improve that quality!
-Its a completely artificial mark in time but a sincere Happy New Year to all!
-Glenn


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AB9MS
member


Reged: 10/19/12

Re: Homemade Mylar Parabolic mirror new [Re: glennnnnnn]
      #5598890 - 12/31/12 11:10 PM

How about a mylar sheet made for this job. Varying the thickness outside to the inside so that a true parabolic curve
would be created. No idea myself on the engineering involved to accomplish that, but I'm always amazed at what some people come up with. Secondly, perhaps not AP quality, but how about photometric (if thats the right word for it) quality. Huge aperture to measure faint star light magnitudes. Or could a computer program take a picture of the mirror then post process out the imperfections? I seem to recall a mirror made out of rotating mercury that needs to be post processed to take ripples into account.


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highfnum
Carpal Tunnel
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Reged: 09/06/06

Loc: NE USA
Re: Homemade Mylar Parabolic mirror new [Re: AB9MS]
      #5599152 - 01/01/13 06:54 AM

there is a book called "Unusual Telescopes" by Peter Manly
pages 15-16 discuss mylar mirrors
a very complex holding cell was developed yielding some success


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*skyguy*
Carpal Tunnel
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Reged: 12/31/08

Loc: Western New York
Re: Homemade Mylar Parabolic mirror new [Re: highfnum]
      #5599662 - 01/01/13 01:49 PM

Here's Maurice Gavin's website that details his 21" aluminized Mylar mirror telescope that was featured in the May 1979 issue of Sky & Telescope:

http://home.freeuk.com/m.gavin/flux.htm


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Homemade Mylar Parabolic mirror new [Re: *skyguy*]
      #5599683 - 01/01/13 02:03 PM

Look, on the micro-surface level, mylar isn't smooth at all. It's rough. And it varies in thickness a lot. And the reflectivity wouldn't be good enough.
And a million other problems.
Just because something is shiny doesn't mean it would make a good mirror.
Radio telescope--maybe. Optical telescope? No way.


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Homemade Mylar Parabolic mirror new [Re: *skyguy*]
      #5600359 - 01/01/13 09:22 PM

Quote:

Here's Maurice Gavin's website that details his 21" aluminized Mylar mirror telescope that was featured in the May 1979 issue of Sky & Telescope:

http://home.freeuk.com/m.gavin/flux.htm




Note that Mr. Gavin doesn't refer to his creation as a telescope but rather as a flux collector. It is stated in the article that this scope is not suitable for viewing. If you look at the image of the sun shown in the article you can see why.

dan


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*skyguy*
Carpal Tunnel
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Reged: 12/31/08

Loc: Western New York
Re: Homemade Mylar Parabolic mirror new [Re: dan_h]
      #5601162 - 01/02/13 12:25 PM

Quote:

Quote:

Here's Maurice Gavin's website that details his 21" aluminized Mylar mirror telescope that was featured in the May 1979 issue of Sky & Telescope:

http://home.freeuk.com/m.gavin/flux.htm




Note that Mr. Gavin doesn't refer to his creation as a telescope but rather as a flux collector. It is stated in the article that this scope is not suitable for viewing. If you look at the image of the sun shown in the article you can see why.

dan




Gavin actually does refer to his mylar mirror design in this article as a telescope:

"The accompanying pictures show a novel telescope of 21-inches aperture that I designed and built for about $1 during July, 1978"

However, I agree that a much better name for it is a "Flux Collector!"


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John Carruthers
Skiprat
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Reged: 02/02/07

Loc: Kent, UK
Re: Homemade Mylar Parabolic mirror new [Re: Starman1]
      #5602332 - 01/03/13 04:01 AM

While working as a glazier I made a 36" double glazed unit with a thick (6mm) back and a silvered (2mm) front, then partially evacuated it. It gave a sort of image, I could project the sun and moon onto a wall, that's about as good as it got.
I later tried a similar experiment with Baader solar film on a smaller scale, similar result, not good.


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Homemade Mylar Parabolic mirror new [Re: glennnnnnn]
      #5602349 - 01/03/13 04:34 AM

Quote:

The biggest problem I see is finding a large enough place to store it.





I think you're in for big disappointment if that's the biggest problem you see.

Dave


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highfnum
Carpal Tunnel
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Reged: 09/06/06

Loc: NE USA
Re: Homemade Mylar Parabolic mirror [Re: dave brock]
      #5602420 - 01/03/13 06:36 AM

Improvent can be made using a mask to hide edge of mirror
Also
Using electrostatic points behind Mylar to fix errors


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