Jump to content


Photo

Homemade Mylar Parabolic mirror

  • Please log in to reply
64 replies to this topic

#51 psi_chemie

psi_chemie

    Mariner 2

  • *****
  • Posts: 204
  • Joined: 01 May 2011
  • Loc: Leawood, KS, USA

Posted 06 January 2013 - 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?

#52 RingleaderO

RingleaderO

    Lift Off

  • -----
  • Posts: 5
  • Joined: 21 Nov 2012

Posted 06 January 2013 - 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:

Posted Image




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.

#53 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7553
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 06 January 2013 - 09:06 AM

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%. :grin:

Attached Files



#54 Dick Jacobson

Dick Jacobson

    Viking 1

  • -----
  • Posts: 898
  • Joined: 22 Dec 2006
  • Loc: Plymouth, Minnesota, USA

Posted 06 January 2013 - 10:18 AM

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.

#55 nytecam

nytecam

    Voyager 1

  • *****
  • Posts: 11411
  • Joined: 20 Aug 2005
  • Loc: London UK

Posted 07 January 2013 - 04:54 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.....

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. :o

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? :p

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

#56 glennnnnnn

glennnnnnn

    Mariner 2

  • *****
  • Posts: 257
  • Joined: 20 Oct 2009
  • Loc: San Diego, CA

Posted 08 January 2013 - 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!)

=)

#57 John Carruthers

John Carruthers

    Skiprat

  • *****
  • Posts: 3543
  • Joined: 02 Feb 2007
  • Loc: Kent, UK

Posted 11 January 2013 - 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.

#58 glennnnnnn

glennnnnnn

    Mariner 2

  • *****
  • Posts: 257
  • Joined: 20 Oct 2009
  • Loc: San Diego, CA

Posted 11 January 2013 - 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...?

#59 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1966
  • Joined: 10 Dec 2007

Posted 11 January 2013 - 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

#60 ccaissie

ccaissie

    Viking 1

  • -----
  • Posts: 630
  • Joined: 13 Sep 2010
  • Loc: Whitefield, Maine

Posted 11 January 2013 - 09:15 PM

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."

#61 ccaissie

ccaissie

    Viking 1

  • -----
  • Posts: 630
  • Joined: 13 Sep 2010
  • Loc: Whitefield, Maine

Posted 11 January 2013 - 09:20 PM

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 :roflmao: into something like glass and float a thin layer of metal onto it! Wow!

#62 John Carruthers

John Carruthers

    Skiprat

  • *****
  • Posts: 3543
  • Joined: 02 Feb 2007
  • Loc: Kent, UK

Posted 12 January 2013 - 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?

#63 glennnnnnn

glennnnnnn

    Mariner 2

  • *****
  • Posts: 257
  • Joined: 20 Oct 2009
  • Loc: San Diego, CA

Posted 12 January 2013 - 11:26 AM

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.

#64 careysub

careysub

    Surveyor 1

  • *****
  • Posts: 1894
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 12 January 2013 - 12:53 PM

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.composite...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).

#65 glennnnnnn

glennnnnnn

    Mariner 2

  • *****
  • Posts: 257
  • Joined: 20 Oct 2009
  • Loc: San Diego, CA

Posted 12 January 2013 - 02:02 PM

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






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics