Telescope Reviews: Homemade Mylar Parabolic mirror

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RingleaderO
journeyman

Reged: 11/21/12

Re: Homemade Mylar Parabolic mirror new

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
Postmaster

Reged: 08/20/05

Loc: London UK

Re: Homemade Mylar Parabolic mirror new

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
super member

Reged: 04/15/08

Re: Homemade Mylar Parabolic mirror new

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[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
sage

Reged: 05/01/11

Loc: Leawood, KS, USA

Re: Homemade Mylar Parabolic mirror new

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
sage

Reged: 05/01/11

Loc: Leawood, KS, USA

Re: Homemade Mylar Parabolic mirror new

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

Reged: 05/01/11

Loc: Leawood, KS, USA

Re: Homemade Mylar Parabolic mirror new

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
Pooh-Bah

Reged: 05/22/06

Loc: Easton, PA, USA

Re: Homemade Mylar Parabolic mirror new

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
Post Laureate

Reged: 02/22/10

Loc: Montclair

Re: Homemade Mylar Parabolic mirror new

new

[Re: Gary Fuchs]
#5606460 - 01/05/13 12:05 PM Attachment (16 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

Reged: 08/20/05

Loc: London UK

Re: Homemade Mylar Parabolic mirror new

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
Pooh-Bah

Reged: 02/18/11

Loc: Rancho Cucamonga, CA

Re: Homemade Mylar Parabolic mirror new

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
sage

Reged: 05/01/11

Loc: Leawood, KS, USA

Re: Homemade Mylar Parabolic mirror new

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

Reged: 11/21/12

Re: Homemade Mylar Parabolic mirror [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
Post Laureate

Reged: 02/22/10

Loc: Montclair

Re: Homemade Mylar Parabolic mirror new

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[Re: RingleaderO]
#5607922 - 01/06/13 09:06 AM Attachment (14 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

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
Postmaster

Reged: 08/20/05

Loc: London UK

Re: Homemade Mylar Parabolic mirror new

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[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: West Coast High Desert

Re: Homemade Mylar Parabolic mirror new

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
Skiprat

Reged: 02/02/07

Loc: Kent, UK

Re: Homemade Mylar Parabolic mirror new

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

Reged: 10/20/09

Loc: West Coast High Desert

Re: Homemade Mylar Parabolic mirror new

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[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

Reged: 12/10/07

Re: Homemade Mylar Parabolic mirror new

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[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
sage

Reged: 09/13/10

Loc: Whitefield, Maine

Re: Homemade Mylar Parabolic mirror new

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[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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