With the help of a forum member and some P.M's, patent copies and further research in to the understanding of the mechanics and physics involved to make this work, I would say that progress is slowly being made.
I have U.S patent number 4,822,155 which is the original patent by Peter Waddell who is the inventor in the photos I posted earlier.
This patent is very useful as it explains how the whole concept comes together to a workable optic.
Of some significance this week, after talking to Australia's DUPONT films supplier is that Mylar ain't Mylar in many cases
I was informed that many inferior metallised films called ''MYLAR'' are nothing like the actual product at all and actually breach copyright by doing so!!!!!!!!
I have no doubt that any crazed experimentors like myself have in the past, given up or just plainly failed due to the lack of realisation that the film needs to be what it's supposed to be and that the mylar frame holds a few key/special requirements to stretch that film evenly and correctly.........or else!
Obtaining an optically flat surface before applying a vacuum is critical also otherwise it just won't work............period.
So, things are coming to the surface as I dig deeper..............just hope I don't hit sewerage on the way
Anyhow, research and work on this will continue.
Any input is welcome
In the late 1970's the chief engineer at the AAO, Ed Simmonds, had the same idea and made an almost identical set of rings to deform circular glass disks cut from old glass plates from the UK Schmidt telescope. Why ? Because the Schmidt plates were optically flat to quarter wave, on both sides and plane parallel, as well as being very thin so they could be bent to fit the curved focal plane of the telescope.
Ed found that using an 18" aluminised glass disk, the central 6" or so might be useful at about f/20 but the rest of the plate produced so much spherical aberration as to be useless for imaging anything. It was only fit for use as a solar cooker.
I was a physics student at ANU at the time and solved the maths to work out the curve achieved... it's no good for telescopes... The thought of aluminised mylar occurred to us too but the surface quality was simply dreadful compared to the thin Schmidt plates.
There is also a much older reference to the same idea somewhere in one of the ATM books from the 1950's.
Why Waddell wanted to waste money on a patent lord knows, the idea is useless...
Spherical abberation is a result of inadequate edge clamping..........that's a known fact, no matter what the material.
If the Mylar is no good as an accurate surface for a mirror, why have there been small working telescopes made and pictures of such published?............I have seen them on the web site of the stretchable membrane mirror under Strathclyde University and Ethereal Technology.
I for some reason at the moment, can't post the evidence of the PDF I have on my work computer, but will get our I.S dept. to assist me next week.
That site also has a graph of the actual curve. It differs from a parabola, but I'm not convinced that it''s totally opposite or wrong to what is required by a telescope??
Just adding to my last post, lf you read the patent I have listed, it doesn't take much to realise that blaming the Mylar surface is an easy way out to establish a reason for failure.
The patent, when carefully studied, clearly indicates the immense importance and methodology applied to clamping the film and the processes that follow.
That ''perimeter'' which comes in to contact with the stretched Mylar is where many companies that have spent large sums of money in R&D on this idea, have NOT taken in to consideration.
This, in conjuction with the very thorough adjustment points to flatten the film before a vacuum is applied is what makes this design work.
Stretching Mylar over drums, bicycle wheel rims and other stupidity will indeed produce only a ''shaving mirror'', if that.
I have been told in person by someone involved in this project all those years back, that the above mentioned ''tricks'' is what sent many researchers and developers in to bankruptcy.
I was also told that holding a vacuum is not diifficult at all with ''just a thin smear of silicone grease'' and that ''a simple regulator had easily been deployed at very little expense to hold the vacuum constant''.
We can sit here and **** can this as much as we like, but read the following link, about half way down in particular, where a paragraph starts with ''In 1985''
Well..
In the meantime, we have GSO, and their good and affordable mirrors..
I fully understand the inventors' "buzz" from adrenaline when something seems to be working.. but.. we have to be practical as well.
And, I have seen a lot of B.S., many times produced with perfect, genuine honesty..
I was also playing with the mylar idea in '64 and onwards.. and today I still have and happily use my Coulter Optical 10" mirror, purchased in '82, for 105 US$.
Se here you go..
Well..
In the meantime, we have GSO, and their good and affordable mirrors..
I fully understand the inventors' "buzz" from adrenaline when something seems to be working.. but.. we have to be practical as well.
And, I have seen a lot of B.S., many times produced with perfect, genuine honesty..
I was also playing with the mylar idea in '64 and onwards.. and today I still have and happily use my Coulter Optical 10" mirror, purchased in '82, for 105 US$.
Se here you go..
Bojan/ALL, who are ''GSO'' ?..............can you provide some details?
I'm not trying to convince everyone on this forum that Mylar mirrors can be made to work as good a chunk of expensive ground and polished glass.
What I AM trying to indicate is that it's very easy to make a solar collector or shaving mirror...............as have many,........ who have then come to the conclusion and spread the word that the principle doesn't work and never will...............as is obviously the case from some of the reactions I have received..........and expected.
If you DO take the time to look at the patent, which I don't think anyone has bothered, you will realise the complexity that goes in to the holding/streching mechanism that I know for certain NO ONE in the years past has followed.............not even close.The number of bolts and screw adjustments in that area alone shows a serious effort at fine tuning the film in to a flat opticallly perfect film surface.If you don't have that, you have nothing and can stop right there.
Of course, many did not, and concluded that this just doesn't work.
Short cutting this intricacy mentioned, will indeed produce a total joke image quality wise. A distorted and edge wrinkled pile of garbage is the usual prize for an incorrect attempt at this........guarenteed.
On the subject of Mylar, the patent states 125 micron film thickness which is 4.92mil or nearly 5 one thousandths of an inch.
That is ''if'' you have ''real'' Mylar that is
On a closing thought, I'm sure many of you have seen mylar used over the fronts of SCT's for solar viewing?
I know I have(10inch) and observed a sun spot at high magnification. I then turned off the drive and watched the image move accross the field of view.......not once, but many times.
I NEVER saw any thing to indicate that the mylar was anything like cheap window film. The image was distortion and ''wavy'' free.
If Mylar is good enough for this purpose, why should it's surface accuracy be lousy for a mirror?
If Mylar is good enough for this purpose, why should it's surface accuracy be lousy for a mirror?
Well
you just answered your question with the question itself:
Reflecting light from surface is not the same thing as passing the light through the same surface (doubled, if you like).
Mylar foil acts as a plan-parallel plate, so it is suitable for window, even if it is not perfectly flat and stretched. But to use it as mirror is something entirely different.
And, the very complicated method of adjusting the mylar film is just too complicated to be practical.. simple junk of polished glass does the better job, in a cheaper and more stable way, it is not temperature and air pressure dependent.
So why bother at all with all this?
GSO is Guan Sheng Optical company, a Taiwanese manufacturer of cheap (but good) and conventional glass mirrors.
Hey Bojan!...............you're a tough nut to crack LOL !!!
Yes, all that complexity is really a deal breaker when you consider what has to be manufactured and accomplished before you even stand a chance of making a ''BIG'' mirror.
By the way, what nationality is ''Bojan''?
Thanks for the company details.............I'll check them out!
Rob.
That's interesting.....................I was born of Polish parents here in Australia.
They settled in Western Australia after WW2. They had a hard time all those years ago...............especially with a one and only son who would not stop looking through telescopes each night
My first scope was a ''slider''....you know...........the sailor's spy scope..........now that was fun!..............not!
If Mylar is good enough for this purpose, why should it's surface accuracy be lousy for a mirror?
It isn't - as anyone who has used a scope with a GLASS solar filter will know - Mylar scatters a ****load of light, the surface is quite rough at the microscopic scale compared to properly glass. And a mylar mirror scatters just as badly, having tried it.
But lets not go there because the surface shape is quite simply wrong, anyway; useless for a decent aperture where it might have been worthwhile. On small apertures under 20" it's easier to us a glass mirror.
Wavy, you have every right to stick to your guns............I'm not going to debate what you believe in or your ''maths'' all those years ago.
I'll just refer to all here, on what I have found during my ''digs'' through the documentation on hand. What anyone thinks doesn't really matter as the ''anomalies'' in everyone's theories still stand:
1. a telescope mirror WAS made in 1985 by Braithwaite. It was tested and a telescope built with a sliding secondary mirror/focusser. Had the curve been all ''wrong'', the mirror would not have made it past that stage.........let alone a fully built reflector.Black and white photo of telescope mirror and reflector exists.
2. the much hated(?) Peter Waddell, built a small scale 8 inch ,perspex bodied, mirror (as per the patent number I posted)designed as a space telescope primary optic and NASA was very interested in it after testing and looking in to the design parameters.Colour photos of this mirror exist.He then went on and built a 12 inch, 24 inch and finally a 1.2 metre that can resolve the thin light hairs on a person's face (and I'm not talking about a beard)in 3D imaging applications.
3.The ''curve'' as published, compares favourably to a parabola, with a small deviation............how that affects the image, ''telescope'' wise I don't know?(still can't post the *******!!!!)
4. To enable a Mylar mirror useable, it seems that considerable effort needs to go in to the mounting frame and consequent adjustments.Together with this is the curious fact that THICK Mylar is specified in the Patent?..........why not the thin stuff
5. there is no point in listening to anyone's own personal experiences with Mylar if those attempts involved stretching film over dish pans,tupperware, potties or skinless drums etc. We know that this doesn't work.
I would not have started this post if I wanted a mirror 20'' or less.....I'd just go out and buy one.
I'm talking about a considerably larger one that can operate at f 2.0 or even less.
Now, about that ''curve'' that's ''wrong''..............anyone care to enlighten me on how it's ''wrong''??............I'd be very interested, even if this subject is laid to rest as a result
Yeah,.............................. ......................... ...hmmmmmmmmmm???.................. ........................ ................................... ................................. ............................. ............I'm gonna get me sum glad wrap, sum silver spray paint and a cookin pan and pulll me a mirra!!!!
Robz,
Drop the "theoretical parabola" to match the sag at 150mm and the curves match very well; there will be some error at the centre (covered by the secondary) and the extreme edge ( mask it off)
Question: do you have similar curves for say a f4 or f5 mirror - this ratio would be of more interest to the average amateur astronomer....
Re your 'Curve'. Mylar is relatively nonstretchy material, very little give. Yo could never pull a curve anywhere near that deep from a theoretically flat stretched sheet of the stuff.
........ ................................... .
........................ ............I'm gonna get me sum glad wrap, sum silver spray paint and a cookin pan and pulll me a mirra!!!!
peter waddell contact
