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Homemade Mylar Parabolic mirror

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#26 dan_h

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Posted 31 December 2012 - 02:26 PM

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.co...Holenstein-P...

- Dave


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

dan

#27 DAVIDG

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Posted 31 December 2012 - 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

#28 glennnnnnn

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Posted 31 December 2012 - 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! =)


#29 Gary Fuchs

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Posted 31 December 2012 - 03:44 PM

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.o...r/Foucault.html

http://stellafane.or...erstanding.html

http://stellafane.or...test/setup.html

http://stellafane.or...ester-main.html

And a CN thread here.

Gary

#30 Jon Isaacs

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Posted 31 December 2012 - 04:50 PM

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

#31 glennnnnnn

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Posted 31 December 2012 - 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

#32 AB9MS

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Posted 31 December 2012 - 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.

#33 highfnum

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Posted 01 January 2013 - 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

#34 *skyguy*

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Posted 01 January 2013 - 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.c....gavin/flux.htm

#35 Starman1

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Posted 01 January 2013 - 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.

#36 dan_h

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Posted 01 January 2013 - 09:22 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.c....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

#37 *skyguy*

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Posted 02 January 2013 - 12:25 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.c....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!" ;)

#38 John Carruthers

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Posted 03 January 2013 - 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.

#39 dave brock

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Posted 03 January 2013 - 04:34 AM

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

#40 highfnum

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Posted 03 January 2013 - 06:36 AM

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

#41 RingleaderO

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Posted 04 January 2013 - 02:10 AM

Thanks for the link to Maurice's site! Interesting read, he managed to get some pretty good results just using bike wheels and scrap.

Also, that paper on Pneumatic Mirror Membranes was especially interesting. They got some promising results with their experimentation with the less-than-optimal-material-for-the-job Mylar, and pointed out a lot of areas for improvement. Who knew that Mylar is intentionally coated with particles to keep the sheets from sticking together. Also, I'd bet that a lot of the distortions were caused by imperfections in the manufacturing of the mylar sheet (because it's not designed to be a mirrored membrane surface for use in telescopes). At any rate, they thought enough about it to consider making a much improved next generation version.







On that note, Dan Rojas from GREENPOWERSCIENCE has been working on improving his mirror 1$ trashcan lid after getting a lot of posotive feedback and requests on his first video. Seen below, by blocking the outer 50% area with a cardboard diaphragm, he managed to drastically improve the clarity of the reflected image on a wall, and greater still when he blocked off the majority of the mirror leaving only a small hole:

http://youtu.be/bBXKbfqI49E

He says the center of any vacuum mirror usually has extremely good optics, so I will be trying this out as soon as I can.

#42 nytecam

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Posted 04 January 2013 - 04:50 PM

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.co...Holenstein-P... - Dave

Thanks for the many plugs - wow - it's 33yrs since my S&T article and still it gets reinvented. In case of the 1981 pro article [above] plagiarised for my OTA for the devise - flattery indeed :p

ps: I described my 'scope as a flux collector, because "10 wave performance" in above pro article is orders of magnitude [x100] inferior than expected from good optics :grin:

I was later invited to comment in New Scientist when Dr Waddle of Strathclyde Uni reinvented the devise - his comprehension of optics at the time left a lot to be desired :shocked:

#43 Gert

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Posted 05 January 2013 - 04:58 AM

Hi All,

Gavin article is in May 1979 SKy and Tel, "Aluminized Mylar as a Flux Collector"
...the best figure that they could produce had 10 waves of error in it. ...
- Dave


What about making the wavelength longer? Put an LNB in front of the trashcan-mylar-mirror and you can surely watch TV! That proves that this method can make a telescope (albeit a radio telescope!).

Clear Skies,
Gert

#44 psi_chemie

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Posted 05 January 2013 - 11:03 AM

Nytecam,

In 1992 I asked my astronomy professor for some reflective film to try and make such a telescope. He said to first do some research, then gave me some very high quality smooth gold mylar film. I never made it into anything.

The thing is, back then, I spent a couple days in libraries trying to find a publication on this.

I didn't think to check S&T.

Now with the internet, I find your article pretty easily.

I would have stopped my project had I found your article.

I wonder how many people now would not do a project because they can find earlier references on the internet, where before, you would have people trying anyway, and perhaps one of them makes something work. Not sure the scale of this situation..

#45 psi_chemie

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Posted 05 January 2013 - 11:07 AM

Would the curve formed by vacuum be the same as that formed by pressure? Think a tube with optical window on one end, and the reflective film on the other, and pressure in between pushing the shape into the film.

#46 psi_chemie

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Posted 05 January 2013 - 11:08 AM

Never mind, I can see it's the same. How much time I waste on stuff like this..

#47 Gary Fuchs

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Posted 05 January 2013 - 11:46 AM

I wonder how many people now would not do a project because they can find earlier references on the internet, where before, you would have people trying anyway, and perhaps one of them makes something work.


Wouldn't the greater availability of information make it easier to proceed productively; by avoiding previous errors and dead ends?

If information discourages innovation maybe no one with an idea should read anything?

Gary

#48 Pinbout

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Posted 05 January 2013 - 12:05 PM

Wouldn't the greater availability of information make it easier to proceed productively; by avoiding previous errors and dead ends?

If information discourages innovation maybe no one with an idea should read anything?




or at least it could help temper expectations, so the work could proceed more scientifically.

50% of 25"dia is 17.6"dia, or more specifically 25"dia divided into 2 equal areas...

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#49 nytecam

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Posted 05 January 2013 - 02:33 PM

I wonder how many people now would not do a project because they can find earlier references on the internet, where before, you would have people trying anyway, and perhaps one of them makes something work.

Wouldn't the greater availability of information make it easier to proceed productively; by avoiding previous errors and dead ends? If information discourages innovation maybe no one with an idea should read anything?
Gary

Perhaps in 1978, when I did my initial experiments, there was no effective internet and there was time to reflect on many aspects of membrane mirrors as subsequently reported in S&T. Today one hits the send button without thinking it through [I'm now as guilty as anyone] and likewise with replies :grin:

#50 careysub

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Posted 05 January 2013 - 05:30 PM

...
I would have stopped my project had I found your article.

I wonder how many people now would not do a project because they can find earlier references on the internet, where before, you would have people trying anyway, and perhaps one of them makes something work. Not sure the scale of this situation..


Old science saying: "A month in the laboratory can save you an hour in the library."

Finding old work that failed is very important - you don't have to repeat the same old mistakes. You can discover new ones!

Or to be more optimistic - studying the reasons for old failures may give insights to solutions.






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