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Old 24-06-2010, 03:58 PM
robz (Robert)
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Mylar vacuum mirror

Hi everyone, I have actually spoken to Peter Waddell back in 1989 on his development of the attached ''strechable membrane mirror'' pics.

Has anyone considered trying this?............I know I have considered it many times(but never had the guts to try it) and also have quite a few secrets on how to make it work. This has been collected from various sources over all these years.

It has been used in a smaller form as a telescope mirror and was once presented at the U.K Astronomical society with an apparently enthusistic response.

I know it sounds like vaporware,and was originally designed for 3D imaging, but it really does exist and produces extremely high quality images........good enough for astronomical purposes from my information at hand..................no proof of such however........so who knows?

May be worth discussing as an alternate method to those giant and incredibly expensive glass mirrors?

Comments welcome
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Last edited by robz; 24-06-2010 at 04:10 PM.
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Old 24-06-2010, 04:14 PM
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I've read the occasional piece on these over the years. The main problem is vacuum deformation of a mylar sheet (greatest stretch on the edges) ends up giving nearly the opposite form of a hyperbolic mirror (ie stronger curve in the centre).
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Old 24-06-2010, 04:19 PM
robz (Robert)
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Hi Peter, I have some more pics that indicate the actual curve generated.
Standby..........................
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Old 24-06-2010, 04:57 PM
robz (Robert)
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Having trouble. File size too big. I'll work it out when I can.
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Old 25-06-2010, 10:13 AM
robz (Robert)
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For some reason, I can't download the image of the plotted curve.

Howvever, it DEFINATELY follows a PARABOLA with only a slight deviation........a deeper curve which ends up meeting the ideal parabola in the centre.

So, is this a hyperbola?

Can this be used for astronomy?
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Old 25-06-2010, 05:18 PM
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How much does temp variation of the partial vacuum change the curve?
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Old 25-06-2010, 08:09 PM
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Quote:
Originally Posted by robz View Post
For some reason, I can't download the image of the plotted curve.

Howvever, it DEFINATELY follows a PARABOLA with only a slight deviation........a deeper curve which ends up meeting the ideal parabola in the centre.

So, is this a hyperbola?

Can this be used for astronomy?
No ...just me using RC's for too long ....I should have typed Parabola.....but I still understand the same problem applies....vacuum deformed Mylar is too steep at the edges and not enough in the centre

A hair's width will do it...just talk to the Hubble team

It would also be interesting to see how the system handles wind gusts
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Old 26-06-2010, 12:34 AM
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Vacuum mirrors

I went to uni in Scotland with Peter, and have copies of all his reseach notes on the mylar mirrors.
In the early 90's I made two 16" frames and a vacuum system to trial the concept. The issues were lack of surface accuracy; the edge wrinkles distorted things well into the centre. I tried pre-streatching before pulling the vacuum; modifying the edge radii and clamp rings etc etc. I could not get any worthwhile imaging results. The ronchi testing showed the surface was all over the place.
I then tried with a 2mm thick sheet of plate glass and managed from memory, to pull it to f9; it sat on an O ring edge seal... looked very promising.
I'm overseas at the moment so don't have access to my notes etc.
It's certainly a VERY interesting concept, but one which doesn't seem to work with optical accuracy.
(BTW I also tried some samples with slumped 3mm glass disks.... you can get the radius but they still need polishing...not an easy feat with such a thin mirror)

Ken
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Old 26-06-2010, 12:13 PM
robz (Robert)
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Thanks Ken and all.

This has answered my questions that have been bugging me for so many years.

It's obvious now that trying to make something like this work is nera on impossible for the average diy telescope enthusiast.

On an interestng note however, I am talking with a team here in Australia that have manged to created small telescope mirrors using a gel/resin........weird stuff :

http://www.cfree.com.au/lightweight-mirrors

wadayarekon?????
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  #10  
Old 26-06-2010, 04:54 PM
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Gday Robert

You should grab a copy of "Unusual Telescopes" by Peter Manly.
It actually has a bit on Waddels mylar mirrors,
as well as heaps more, like obsidian mirrors, mercury mirrors, "plastic" lenses.
Full of all the weird and wonderfull optics and mounts people have come up with.

Andrew
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Old 27-06-2010, 07:20 AM
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I always thought this was my secret idea from back in early '60-ies

Mylar was not know to me then.. but, as Ken suggested, my conclusion after some experimentation with various materials and methods was that it would be impossible to obtain the quality, not to mention the coping with variability of the optical surface shape due to temperature and air pressure changes (I imagine even air circulation may be detrimental to the resolution of such a mirror).

While playing with this, one friend of mine suggested electrical field as a mean to obtain curvature (high voltage, in other words - couple of kV.
BTW, electrostatic loudspeakers use the same principles for sound generation).

However, I also remember one S&T article on ATM - there was a description of pre-distortion of flat glass plate by means of attached low pressure chamber, used in process of figuring of Schmidt correction plate.

Last edited by bojan; 27-06-2010 at 08:31 PM.
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Old 27-06-2010, 04:18 PM
robz (Robert)
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G'day guys...............thanks for your confirmation and personal experiences.

I'm allways thinking outside the circle, but there's a limit to what can be done of course.

Any thoughts on the link I provided?

I know absolutely nothing about this link/product, but in an email conversation, I was told that their PRIMARY INTEREST was in astronomy applications as a telescope mirror.

Rob.
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Old 28-06-2010, 11:21 AM
robz (Robert)
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No takers??????????

O.K...................looks like I'll start saving up for my 12 inch Meade LX90
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Old 28-06-2010, 11:31 AM
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No takers??????????

O.K...................looks like I'll start saving up for my 12 inch Meade LX90
Wise decision
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Old 28-06-2010, 01:44 PM
robz (Robert)
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Thanks bojan..................I think so.................damned mylar!
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Old 28-06-2010, 06:11 PM
bobson (Bob)
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Mate, I understand you. I was very tempted to send my wife's big glass lid of the cooking pot to be aluminised

cheers

bob
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  #17  
Old 29-06-2010, 11:18 AM
robz (Robert)
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YOU TOO????............and I thought it was only me!
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Old 29-06-2010, 12:15 PM
robz (Robert)
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Quote:
Originally Posted by Merlin66 View Post
I went to uni in Scotland with Peter, and have copies of all his reseach notes on the mylar mirrors.
In the early 90's I made two 16" frames and a vacuum system to trial the concept. The issues were lack of surface accuracy; the edge wrinkles distorted things well into the centre. I tried pre-streatching before pulling the vacuum; modifying the edge radii and clamp rings etc etc. I could not get any worthwhile imaging results. The ronchi testing showed the surface was all over the place.
I then tried with a 2mm thick sheet of plate glass and managed from memory, to pull it to f9; it sat on an O ring edge seal... looked very promising.
I'm overseas at the moment so don't have access to my notes etc.
It's certainly a VERY interesting concept, but one which doesn't seem to work with optical accuracy.
(BTW I also tried some samples with slumped 3mm glass disks.... you can get the radius but they still need polishing...not an easy feat with such a thin mirror)

Ken
Ken, just a curiosity that you may be able to answer?

Looking at Peter Waddell's giant 1.2 meter mirror, would I be correct in assuming that even though he succeeded to make a large mirror using vacuum deformed mylar for ''3D IMAGING'', it was however no good as a telescope mirror?..........or, was your attempt unsuccessful due to precise machining required etc?

Cheers,
Rob.
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Old 29-06-2010, 02:28 PM
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IMHO, this mirror could not support itself.. I bet even the weight of mylar is deforming it enough to render it useless.. we are talking here about 1/10 of visible light wavelength.. 0.04um.
But then again, I might be wrong..
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Old 29-06-2010, 04:17 PM
robz (Robert)
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Quote:
Originally Posted by bojan View Post
IMHO, this mirror could not support itself.. I bet even the weight of mylar is deforming it enough to render it useless.. we are talking here about 1/10 of visible light wavelength.. 0.04um.
But then again, I might be wrong..
bojan, I see where you're coming from and understand the accuracy required.
What gets me is the documentation I have read and what was said to me by Peter Waddell all the way back in the late 80's.

Up until joining this forum (fabulous by the way.great blokes!),I was lead to believe that the mirror is allways used in a vertical position, so how can the figure be of any use?

Secondly,the U.K Astronomical society used a mylar prototype mirror made by Waddell(who presented it one night) and were impressed sufficiently by the images it produced...apprently.

The spiel on the mirror technology ''applications'' lists ''high quality telescope optics'' at a fraction of the weight and cost amongst other uses
There was also an interrview with Waddell where he stated that ''indivudual hairs'' on a person's face could easily be resolved?

Now, I don't know where the B.S. starts or ends and where the truth lies here

I do know that if it takes ultra high precision machining and polishing of the edges that come in contact with the mylar, and that the method of clamping and selecting the ''appropriate mylar''(another Waddell statement) are all critical to the thing working, I'll forget about it instantly

By the way, got some more info from the ''cfree lightweight mirrors'' mob(link I provided earlier on).
They have made a 20inch mirror but it suffers from abberation caused by inperfections on the mold and dust particles in the air.They stated that a ''clean room'' is very important in making good mirrors.
They are at the early stages of research and development.

I would suggest that we keep an eye on these guys as they have a revolutionary(we'll see?) system in process here and they have asked me to keep in touch with them..............something about September this year was mentioned, so I guess that this is when they will begin the serious part of the development?

Cheers,
Rob.

Last edited by robz; 29-06-2010 at 04:28 PM.
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