Flexible membrane mirrors?

[Merlin66]

Did anyone follow up on the earlier discussions?
Any success?
http://www.wikipatents.com/US-Patent-4822155/mirror-assembly-with-flexible-membrane/Page-3

[Wavytone]

Haha just reading it raises an old memory.

An old friend in the 1970's tried exactly the same thing, using very thin circular glass disks which were cut from old plates from the UK48" schmidt at Siding Spring, then aluminised. They were perhaps 2-3 mm thick and of pretty reasonable optical quality in terms of flatness etc..

After making a rather magnificent assembly of two large rings with about 40 caps screws to gently apply pressure around the permitter of the plate, he was rather disappointed to find the resulting concave mirror wouldn't form a decent image.

At the time I was studying maths and was able to solve for the shape of the surface - its a nasty shape not even close to spherical, let alone parabolic.

Reading the abstract suggests this fellow has tried to do the same.

He then went on to try sealing the plate against the ring using an O-ring, and using a partial vacuum behind to deform the plate. This also produces a concave mirror (better than the first attempt) however still nowhere near good enough for anything much, from memory the figure this produces is some sort of quartic.

Just goes to show who does - and doesn't understand what a paraboloid is (or isn't) and how accurate it must be to produce a decent image.

Lastly, I am surprised he was awarded a patent for this as there is plenty of "prior art" on this topic in mathematics circles - its a classic 2nd year differential equation problem in Applied Maths. It seems lately that the US Patent office will issue a patent for anything on the basis of "wait and see" with the expectation someone else may challenge the patent in court, rather that attempting to do any serious searches or critical research first.

[MrB]

Ah memories :)

Back when I was young and naive, SN1987a was big news and being a kid of 12, I was trying to make a telescope to see it better.
(Didn't know then that there wasn't any more of it to see than there was naked eye)

After borrowing a book from the local public library and seeing how 'simple' a Newtonian was, my first attempt at a primary mirror was to pull my one and only CD (Dire Straits - Brothers in Arms) into what I thought would be the right curve. This was done by resting the CD over the opening of an appropriately sized plastic bowl with a bolt and nut thru the spindle hole.
Ofcourse it didn't work so plan B was hatched.
BTW the CD survived and I still listen to it to this day :)

Plan B involved some Christmas gift wrapping plastic, kinda like Aluminised Mylar (it may be?), which was stretched over the same plastic bowl and a partial vacuum was pulled(by mouth) thru a hose and needle valve that I had from my tropical aquarium. Again, this was never going to work but I didn't know that at the time :lol:

I do remember being a bit upset at the time that neither worked and I couldn't afford even a cheap supermarket Tasco 60mm refractor, but I soon got over it and kept using mum's expensive 7x50 binoculars for years.

I had observed Halley's quite a lot the previous year, but never got hooked until SN1987a and the realisation that this stuff happened all the time.
So, the best thing that came from the experience was a love for astronomy :)
I started buying astronomy magazines and dreaming about all the equipment that I would never own.
Adverts for mirror grinding kits would fuel my dreams for years!

[OzEclipse]

No doubt a corrector could be designed for it but my feeling is that the complexity of the corrector may exceed the difficulty of making a conventional large mirror.

I suspect that the flexure of the thin mirror as it is pointed in different directions would almost certainly change the figure significantly as you point it to different parts of the sky.

A disk of liquid will spin up into a perfect parabola Mercury will work but once spun, can only point at the zenith. Telescope mirrors have been made this way out of optical plastics which eventually set hard and can then be aluminized.

Joe

[Poita]

I remember reading ages ago about a technique to melt glass and spin it to get the parabolic shape and then cool it slowly, the idea was to minimise the amount of grinding required to get the parabolic shape.

Don't know if it was ever commercialised though?

[Merlin66]

Spin casting has been around for many years and has been employed on most of the large mirrors in use today.
http://www.npr.org/2012/01/26/145837380/want-to-make-a-giant-telescope-mirror-heres-how

[Poita]

Thanks for the info, I remember reading about it and being excited by it as a kid. A shame the predictions for affordable 1 metre mirrors for amateurs didn't eventuate though.

[mithrandir]

Define affordable Peter. Not spincast but have a look at http://normandfullumtelescope.com/mirror_en.html
They have a 100" kiln and have a made a test 61" blank.
You need to ask for a quote for 30" and above.

[Poita]

Thanks for the link, a (40 ") mirror comes in at just over $50,000, which is 1.4 times my gross yearly salary.

That is outside most non professional budgets I would guess, especially by the time you built a scope around it, but I see your point, for some of the upper league of amatuers a 1m mirror would be affordable.

Interesting though that I can't find a spincast one any cheaper, which was the whole point of the original article that I read.
The hope that after being proved on big observatory mirrors, the process would be commercialised down to the point that 1-2m mirrors would be far more affordable than ones made by a traditional grinding process. This was going to be due to the mirror being closer to the finished shape straight out of the kiln. Affordable in the article meant that a 1m mirror would be around the same cost as a 12" mirror was at the time. i.e. still not pocket money, but affordable to just about anyone that really wanted one. It is a shame that hasn't eventuated.
Great scopes are amazingly affordable now compared to 20 years ago, but the article had me convinced that when I grew up , with a bit of saving and scrimping I could own a 1m scope.

And holiday on the moon and have a hover car if I had a good job...

[Poita]

I'm still holding out hope for this group.
http://tech.dir.groups.yahoo.com/group/AltAzInitiative/