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quantumphysicist
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Reged: 12/05/12

Homemade Mylar Parabolic mirror
      #5593600 - 12/29/12 12:00 AM

I came across this and thought it might be good for making a cheap Dobsonian telescope. Do you think this would work well for making the primary mirror?

Make a mylar parabolic mirror

I see a couple of potential problems with this. One being difficulty maintaining a focal length.


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dave brock
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Re: Homemade Mylar Parabolic mirror new [Re: quantumphysicist]
      #5593826 - 12/29/12 06:07 AM

Maintaining focal length is the least of your problems. A mirror for a telescope needs to be more accurate on the order of probably a hundred to a thousand times (off the top of my head). That mirror would have horrendous astigmatism even if it was somewhat parabolic.

Dave


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dan_h
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Re: Homemade Mylar Parabolic mirror new [Re: quantumphysicist]
      #5594240 - 12/29/12 11:52 AM

That mirror is a second surface mirror. It is also not a rigid mirror as it is only 1/4" or so thick. Not a premium optic.

dan


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Al8236
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Re: Homemade Mylar Parabolic mirror new [Re: dan_h]
      #5594264 - 12/29/12 12:04 PM

Quote:

That mirror is a second surface mirror. It is also not a rigid mirror as it is only 1/4" or so thick. Not a premium optic.

dan



I agree that it is not a premium optic, however the mylar "Space Blanket" is only .0005" thick and depends on which surface you put forward as to whether it is first surface.


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KerryR
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Re: Homemade Mylar Parabolic mirror new [Re: Al8236]
      #5594356 - 12/29/12 01:00 PM

This idea has been around a while, with at least one guy who actually made a telescope around the idea. A 4" version worked passably at long focal lengths. The larger version, 20" I think didn't-- If memory serves, the outcome of the 'study' was that the natural shape of a surface formed this way is hyperboloidal, not paraboloidal. Fine for making a solar furnace, but not for the extremely critical focus necessary for astronomy.

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*skyguy*
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Re: Homemade Mylar Parabolic mirror new [Re: quantumphysicist]
      #5594421 - 12/29/12 01:28 PM

This method to form a telescope mirror has already been tried ... it was shown at Stellafane back in the 80's, I believe. Unfortunately, the figure formed by the partial vacuum is no where near a parabola. The edge of the mirror is too steep and the center is too shallow. Star images will be spread across the FOV by approximately 2º. It looks cool ... but, it doesn't work!

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DAVIDG
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Re: Homemade Mylar Parabolic mirror new [Re: *skyguy*]
      #5594562 - 12/29/12 02:45 PM

As stated this idea has been around since the mid 1970's. There was an article in Sky and Tel by Maurice Gavin who posts here on a regular basis. Besides the fact the shape that is formed is not a very good parabola, one also has the problem that a minor change is air pressure causes the surface to change as well since you have a made a very sensitive barometer.

- Dave


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glennnnnnn
sage


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Re: Homemade Mylar Parabolic mirror new [Re: DAVIDG]
      #5595191 - 12/29/12 09:50 PM

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!


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Jon Isaacs
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Re: Homemade Mylar Parabolic mirror new [Re: glennnnnnn]
      #5595337 - 12/29/12 11:39 PM

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!




Potential for what? Do you understand that the rms error of a mirror is measured in nanometers deviation from a parabola over the entire surface?

Jon


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Pinbout
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Re: Homemade Mylar Parabolic mirror new [Re: quantumphysicist]
      #5595357 - 12/29/12 11:57 PM

Quote:

I came across this and thought it might be good for making a cheap Dobsonian telescope. Do you think this would work well for making the primary mirror?






that guy is crazy, did you see his other videos. before I clicked the link I thought I wonder if it's that crazy guy doing it and I clicked the link and viola.

He's so crazy. watch him cut a circle on a table saw. He's really crazy.


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Al8236
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Re: Homemade Mylar Parabolic mirror new [Re: Pinbout]
      #5595393 - 12/30/12 12:30 AM

Quote:


that guy is crazy, did you see his other videos. before I clicked the link I thought I wonder if it's that crazy guy doing it and I clicked the link and viola.

He's so crazy. watch him cut a circle on a table saw. He's really crazy.



Actually it is a very good method for cutting circles, have done it for many years IMO gives a better circle than a router.
What does scare me about his methods is cutting from the side of the saw opposite from the fence, Now That IS an accident waiting to happen!


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RingleaderO
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Reged: 11/21/12

Re: Homemade Mylar Parabolic mirror new [Re: Al8236]
      #5595455 - 12/30/12 01:34 AM

Hey quantumphysicist, it's EthanNino,the current top commenter on that video you linked:

Dan, are you going to continue making a telescope out of this?

I'm an engineering student, and my dream is to design an inflatable space telescope using a similar pressure differential system. When I suggested an inflatable mirror to one of my professors, he warned me that it might not be parabolic enough to reflect an image properly, but I still want to try this out. Thank you for these educational videos!


I've already begun work on a dobsonian reflector mirror using his method of stretching out Mylar sheets over a rigid drum. That's to say I'm in the very early stages of designing the mirror that MIGHT work. I should warn you in advance, do not expect too much out of this project, for the reasons commenters here have already brought up here and my early forays. I can tell just by looking at it that there's a lot of wavy imperfections on the surface, it just ain't smooth. It looks pretty good when I stare directly into it, but that might just be because it's a mirror image of me, heh. The entire surface appears to be covered in these small wavy stretch marks that are increasingly apparent when looking at the mirror from greater than a 45 degree angle. It's hard to explain, I may post images of it later.

That being said, I wouldn't give up on the notion of an inflatable telescope mirror/inflatable space telescope. Mylar is NOT the ideal material to make a telescope mirror! I mean, I used a thermal blanket I had folded up for years, so it's not by any stretch the best setup (no pun intended). I'm sure if we got some big brains on the problem of developing a reflective membrane material, we could find something more workable. There's probably already something better, we just don't know it yet. Also, I'm kind of a noob when it comes to making mylar telescope mirrors, could someone please link me to how to perform a Ronchi/Foucault test, or something equivalent to them that I can do for cheap?






Also, Dan h, Dan Rojas from the Youtube vid said in the comments section that it was a 'first' mirror. I don't really know what that means, but here's what he said when asked by another commmenter if it would make sense to make a telescope out of one of these mirrors:

Maybe, I am testing them. Our mirrors on our site are second surface so while they produce a good image they have optical aberration for telescopes. Not a problem for solar but screws up good image. The mylar is first surface on one side. We are making some first surface mirrors from acrylic.

His previous video went a little more in detail: http://www.youtube.com/watch?v=FyCLOXF1188

Also, He recently responded to my post saying this:

The image quality is good from the center 50% of the mirror, the outer 50% screws it up. For solar cooking it does not matter as light is light so long as it covers the cooking vessel but, I have shrouded part of the mirror and it helps dramatically while also reducing the collection area. Since we are using cheap materials, I am thinking a 40" mirror could make a working 20" mirror 50" = 25" and so on. CPF bulb images very good.


Granted, I don't think he's constructed an entire reflector telescope since yesterday for his trashcan lid mirror, I'm pretty sure he tested the image quality using objects at close range, nevertheless it gives me hope!

Wouldn't it just be, just, awesome if we could make gigantic reflector telescope mirrors for under 12 dollars?


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highfnum
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Re: Homemade Mylar Parabolic mirror new [Re: RingleaderO]
      #5595714 - 12/30/12 09:05 AM

still kind of interesting
reminds me of my multi mirror experiments


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Homemade Mylar Parabolic mirror new [Re: Jon Isaacs]
      #5595731 - 12/30/12 09:21 AM

Henry Ford never hired experts, because they were always telling him what couldn't be done. Instead he found those who weren't aware that a task was impossible so they could do it. Think about it. Its not impossible. There's a solution for each of those problems.

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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Homemade Mylar Parabolic mirror new [Re: quantumphysicist]
      #5595733 - 12/30/12 09:23 AM

The problem (with focusing) is using air pressure to maintain the shape. Since air pressure changes that won't work except for short intervals when you have to refocus each time.
But using air pressure to create the shape is a good start.
Then there's the shape itself, which needs to be correct.
The cool thing about this type of mirror is that you don't have to send it out to get aluminized, you have a reflective layer that you can deposit easily or remove and change.


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Jon Isaacs
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Re: Homemade Mylar Parabolic mirror new [Re: glennnnnnn]
      #5595783 - 12/30/12 09:59 AM

Quote:

Henry Ford never hired experts, because they were always telling him what couldn't be done. Instead he found those who weren't aware that a task was impossible so they could do it. Think about it. Its not impossible. There's a solution for each of those problems.




There are plenty of failed projects out there, you can be sure that Henry had his share. I have worked on a few myself.

I am no expert but have enough common sense to recognize a doable project from a wild goose chase. That's why I am called an engineer and not a scientist.

Jon


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*skyguy*
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Re: Homemade Mylar Parabolic mirror new [Re: Al8236]
      #5595927 - 12/30/12 11:16 AM

Quote:

Quote:


that guy is crazy, did you see his other videos. before I clicked the link I thought I wonder if it's that crazy guy doing it and I clicked the link and viola.

He's so crazy. watch him cut a circle on a table saw. He's really crazy.



Actually it is a very good method for cutting circles, have done it for many years IMO gives a better circle than a router.
What does scare me about his methods is cutting from the side of the saw opposite from the fence, Now That IS an accident waiting to happen!




This guy is the perfect poster child for the old saying: "A little knowledge is a dangerous thing!"


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Homemade Mylar Parabolic mirror new [Re: Jon Isaacs]
      #5595969 - 12/30/12 11:41 AM

Yes, his first 4-wheel vehicle was 12 years and several attempts before the Model T in '08. (From wiki research this morning.)
This may have been why he didn't put much faith in experts. Without regard to any of that, a forum is where ideas fly and perhaps find appreciative members as well as critics who cite good reasons. This is a healthy environment!


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RingleaderO
journeyman


Reged: 11/21/12

Re: Homemade Mylar Parabolic mirror new [Re: glennnnnnn]
      #5596690 - 12/30/12 06:31 PM

PS. 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?

Also, KerryR, do you know the guys name? Or do you know if any of his work is online somewhere? It sounds interesting that he at least got a 4" mirror working.

Skyguy, do you know the guys name who tried this? Or if his work is online somewhere? I'd be interested in that Sky and Tel article too, what's Maurice's screen name here?

Dan Rojas from the YouTube video said in his reply to me that his mirror's center 50% works, while the outer 50% does not, so if you had a 10 inch mirror, only the inner 5 inches would work. I don't think this means he made a reflector telescope already out of it, but might not using the outer 50% area, would this help reduce the astigmatism?


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DAVIDG
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Re: Homemade Mylar Parabolic mirror new [Re: RingleaderO]
      #5596886 - 12/30/12 08:24 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.com/graviclabs/pdf/papers-altz/Alt%20Az%2024%20Holenstein-P...

- Dave


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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
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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
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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

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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
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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
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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

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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
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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
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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
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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
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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*
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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
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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
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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*
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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
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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
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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
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Re: Homemade Mylar Parabolic mirror new [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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RingleaderO
journeyman


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Re: Homemade Mylar Parabolic mirror new [Re: highfnum]
      #5604006 - 01/04/13 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.


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nytecam
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Re: Homemade Mylar Parabolic mirror new [Re: DAVIDG]
      #5605193 - 01/04/13 04:50 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


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

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

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


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Gert
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Re: Homemade Mylar Parabolic mirror new [Re: DAVIDG]
      #5606027 - 01/05/13 04:58 AM

Hi All,

Quote:

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


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psi_chemie
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Re: Homemade Mylar Parabolic mirror new [Re: Gert]
      #5606328 - 01/05/13 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..


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psi_chemie
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Re: Homemade Mylar Parabolic mirror new [Re: psi_chemie]
      #5606333 - 01/05/13 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.

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psi_chemie
sage
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Re: Homemade Mylar Parabolic mirror new [Re: psi_chemie]
      #5606335 - 01/05/13 11:08 AM

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

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Gary Fuchs
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Re: Homemade Mylar Parabolic mirror new [Re: psi_chemie]
      #5606418 - 01/05/13 11:46 AM

Quote:

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


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Pinbout
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Re: Homemade Mylar Parabolic mirror new [Re: Gary Fuchs]
      #5606460 - 01/05/13 12:05 PM Attachment (23 downloads)

Quote:

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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nytecam
Postmaster


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Re: Homemade Mylar Parabolic mirror new [Re: Gary Fuchs]
      #5606736 - 01/05/13 02:33 PM

Quote:

Quote:

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

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careysub
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Re: Homemade Mylar Parabolic mirror new [Re: psi_chemie]
      #5606995 - 01/05/13 05:30 PM

Quote:

...
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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psi_chemie
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Re: Homemade Mylar Parabolic mirror new [Re: careysub]
      #5607676 - 01/06/13 01:33 AM

I am still curious about this. My shaving mirror provides maybe 3x, if this would give a decent image at super low mag why not the mylar? How am I thinking wrong on this?

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RingleaderO
journeyman


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Re: Homemade Mylar Parabolic mirror new [Re: psi_chemie]
      #5607723 - 01/06/13 02:53 AM

Fellas...

Mylar IS NOT manufactured with the intention of being used in precision optics, why should we expect it to work? It just so happened to be a convenient, reflective, inexpensive, and readily available membrane material.

Nytecam, do you think that maybe somewhere, perhaps still undiscovered, there's a membrane material vastly superior to Mylar for making these kinds of optics? Surely if it was the goal of science to create a material specifically for this task, we would have it soon.

Thanks for making that point Gary, No insult intended to those who've experimented with this idea and have yielded discouraging results, but I wouldn't be so easily discouraged by such rudimentary forays. Again let me point out that those who, in my opinion, had the most technically sophisticated setup and promising results thought enough of the notion of vacuum mirrors to design a next generation mounted trial mirror:






Pinbout, I meant an area formed by 1/2 the radius. I debated in my head if it would be worth the effort of making this distinction considering how well this idea is faring. At any rate, I provided a link to Dan's videos, where he demonstrates how placing a shroud over the periphery of the mirror increasingly improves the image quality when more is covered. He said that with the 21 inch trashcan lid mirror, the best shroud had a circular opening of 8 inches in diameter, so 38.095238095238095238095238095238%, heh.

I've tried this out, and there's a clear improvement.


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Pinbout
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Re: Homemade Mylar Parabolic mirror new [Re: RingleaderO]
      #5607922 - 01/06/13 09:06 AM Attachment (21 downloads)

Quote:

He said that with the 21 inch trashcan lid mirror, the best shroud had a circular opening of 8 inches in diameter,




interms of area 8 of 21 is 14.5%.


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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Homemade Mylar Parabolic mirror new [Re: RingleaderO]
      #5608011 - 01/06/13 10:18 AM

Quote:

Nytecam, do you think that maybe somewhere, perhaps still undiscovered, there's a membrane material vastly superior to Mylar for making these kinds of optics? Surely if it was the goal of science to create a material specifically for this task, we would have it soon.



Graphene is an atomically perfect material that's currently being experimented with. Conceivably some time in the future one would be able to obtain graphene sheets that are durable and flatter than the best optical window. Couple that with a precision diamond-machined ring to hold it, and you could have a quality mirror that is spectacularly lightweight.


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nytecam
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Re: Homemade Mylar Parabolic mirror new [Re: RingleaderO]
      #5609638 - 01/07/13 04:54 AM

Quote:

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.....

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....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.


The thread title is overly ambitious eg "Parabolic mirror" - I deduced 33yrs ago the 'natural' shape was an oblate spheroid eg an ellipse rotated 90 degrees - the opposite from required parabola. Of course stopping it down will improve things - it usually does but with the huge loss of aperture.

BTW - I greatly enjoyed my membrane mirror experiments and never considered them a failure within my expectations and note the trumpeting of Dr Waddell never materialise into a multimillion dollar venture - wonder why?

I'm sure better results than mine are possible so keep plugging away


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Homemade Mylar Parabolic mirror new [Re: highfnum]
      #5611667 - 01/08/13 10:24 AM

I found a piece of exceptional quality mylar. It was used as the 90 degree mirror inside a projection TV. Still not a guarantee of the kind of precision needed for a telescope mirror, but optical-quality mylar. You probably can't buy this kind of mylar, but you can take it from an old projection TV if you have your Swiss Army Knife or a Phillips-head screwdriver. In this part of San Diego, when they can't get another futball mundial or basketball game out of their old TV they just park it out in an alley, and there are often 2 or 3 of these projection TV's (as well as any number of tube-type or LED/plasma) within a short distance just waiting to be picked over for usable parts.
Probably hundreds of them being discarded locally every year as their technology fails. The first stage of recycling is to scavange any usable parts or materials. In this case finding a material that was specifically made for a particular use. Good Hunting!
If the TV doesn't use mylar-under-tension it will have a gigantic piece of front-surface glass mirror for free!
(WARNING: Razor-Sharp Edges!)

=)

Edited by glennnnnnn (01/11/13 09:51 AM)


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John Carruthers
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Re: Homemade Mylar Parabolic mirror new [Re: nytecam]
      #5616858 - 01/11/13 08:54 AM

just a thought, how about inflating a lenticular 'pillow(s)' with liquid of suitable RI ?
Loads of clear film around (Baader Turbo film?)
Would work in a vaccuum, might sag under gravity though.
Never tried it for real.


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glennnnnnn
sage


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Re: Homemade Mylar Parabolic mirror new [Re: John Carruthers]
      #5616952 - 01/11/13 10:01 AM

That's another good idea worth exploring!
It would expand/contract with temperature, but you could stabilize that.
The gravity part seems more serious, but if the "liquid-filled pillow" was in a fixed position and you used a rigid plano mirror to adjust the angle of viewing...?


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dan_h
Carpal Tunnel


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Re: Homemade Mylar Parabolic mirror new [Re: John Carruthers]
      #5617082 - 01/11/13 11:26 AM

There was a felow about two years ago playing around with liquid lenses. He just filled the mylar and let it sag under the weight of gravity. He did get soem images but I don't recall ever seeing anything posted.

dan


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ccaissie
professor emeritus


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Re: Homemade Mylar Parabolic mirror new [Re: Jon Isaacs]
      #5618031 - 01/11/13 09:15 PM

Quote:

There are plenty of failed projects out there, you can be sure that Henry [Ford] had his share. I have worked on a few myself.




Now, that's a knee-slapper.

I see the Mylar parabolic idea as a solar concentrator, not suitable for imaging at my level of technical expertise.

Murphy's law...."Everybody has at least one idea that won't work."


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ccaissie
professor emeritus


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Re: Homemade Mylar Parabolic mirror new [Re: glennnnnnn]
      #5618041 - 01/11/13 09:20 PM

Quote:

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




Wait dude! you're onto something!

Like grind and polish the shape into something like glass and float a thin layer of metal onto it! Wow!


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John Carruthers
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Re: Homemade Mylar Parabolic mirror new [Re: ccaissie]
      #5618415 - 01/12/13 03:22 AM

or cast the shape in resin then apply a film?
I doubt it would get to the quality we are used to obtaining but it's worth a try?


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Homemade Mylar Parabolic mirror new [Re: ccaissie]
      #5618856 - 01/12/13 11:26 AM

Quote:

Quote:

There are plenty of failed projects out there, you can be sure that Henry [Ford] had his share. I have worked on a few myself.




Now, that's a knee-slapper.

I see the Mylar parabolic idea as a solar concentrator, not suitable for imaging at my level of technical expertise.

Murphy's law...."Everybody has at least one idea that won't work."




All sorts of people working to create and invent something better, and you know that so many things come about because a guy was just doofing around and... (example: A falling apple hits him on the head!)
In between Great Ideas you just go through the motions.

It might be a workable plan.
Grinding the base to attach the mylar is in the works, along with a whole zoo of other projects!
-An alloy bicycle rim for a strong edge, and many possibilites for the base-material.
-Grinding the surface out, just like grinding a mirror would probably work better.
-Once the surface is good enough you can make a mold and cast duplicates.
-The vacuum part is just to attach the mylar to the base with spray adhesive.

Edited by glennnnnnn (01/12/13 11:46 AM)


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careysub
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Polymer Mirrors new [Re: John Carruthers]
      #5619018 - 01/12/13 12:53 PM

Quote:

or cast the shape in resin then apply a film?
I doubt it would get to the quality we are used to obtaining but it's worth a try?




This thread has touched on a number of proposals for exotic mirror technologies - the Mylar/air pressure system of the original post, liquid pillow mirrors, this one about using resin with a reflective film (though you could metallize it with more standard techniques), and even ultimate mirror technology using graphene film and diamond cells. All of these are types of polymer mirrors (if you rate graphene as being technically a polymer).

I wanted to point out that there is work being done now on carbon fiber/epoxy mirrors that are showing promising results:
http://www.compositemirrors.com/pub/spie/ULTRAOptics.pdf

What they are doing is (in effect) using a glass "inverse mirror" mirror as a mold, applying a layer of commercial pre-impregnated CF fabric/epoxy that is cured at 121 C under 15-30 psi. The composite mirror surface is generated by direct contact with the mold. The CF mirror surface thus produced is then glued to a composite "egg crate" cellular back for support.

They perfected their techniques with 16" F/4 mirrors, consistently getting 1/8 or so wavefront error mirrors, and are working on scaling up to a 1 meter mirror for a real telescope to be built.

Interesting side-note, the 16" mirror mold was made by Royce, their first system was a 6" made by Pegasus.

This technology is producing good mirrors of interesting size and focal ratio now - it looks like a promising area for the bold ATM'er (perhaps hoping to go commercial if successful) to take a crack at to see if these techniques could be adapted to the needs/budgets of amateur astronomers.

Since normal mirror making naturally creates a spherical inverse mirror tool you could start with this for experiments, without worrying about parabolizing.

Another possibility is to use a regular mirror as your starting point, and cast a mold that is then used for the mirror lay-up. Some research and experimentation would probably be called for to find a good castable mold material that will transfer the mirror surface accurately (and not stick to the CF prepreg).

Edited by careysub (01/12/13 01:55 PM)


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: Polymer Mirrors new [Re: careysub]
      #5619127 - 01/12/13 02:02 PM

As the idea(s) take form there are more materials available. Hooray for R & D!

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