Whats the Maddest scope you've ever seen?

I had thought of that Philip........vacuum induced paraboloid.
Peter.

The problem there is that the difference between a spbere and a parabola on that scale is much smaller than the surface changes you would see due to atmospheric pressure changes from minute to minute! Even if you could pull a perfect vacuum-parabola you couldn't hold it for more than a few seconds. Far easier not to bother and just go for a long focus (>f/12) system (if you want good resolving power)... of an f/1 system if you want to flash-grill someone :twisted:

Phil.

PS - The Vatican telescope in the US (an f/1 Gregorian system) was spin-cast using molten glass. It was kept spinning while the glass cooled down (anealed) over several weeks!

Mr. Eamonn Ansbro (Space Exploration Ltd) in Boyle Roscommon has already completed research and manufactured a working miniature (I think 6”) and a larger membrane mirror (I think its 42”) based on special synthetic film from a company called “3M”.

Both mirrors are I believe diffraction limited and still under testing and evaluation. The focal ratio can be altered by vacuum and electronic means upon the synthetic 3M film to induce fast focal systems such as F4 for wide field viewing to longer F ratios for planetary viewing etc. It’s quite impressive and as far as I know the ELT (Extremely Large Telescope) lead scientist was very interested in this technology indeed, I think Eamonn may have presented his research to the ELT team already.

The large membrane mirror and supporting structure is very light weight compared to a glass equivalent size and a hell of a lot cheaper to manufacture, it would be great to have such mirrors (i may have a problem getting the 42” into the shed) for the wider astronomical community in Ireland (understatement intended)!

Clear skies
Mike

I developed a new membrane mirror which is capable of imaging in the near infrared for astronomical imaging from near infrared and above. The development of the membrane mirror has the potential for order-of-magnitude aperture size increase or weight reduction for large aperture ground based telescopes.

The membrane mirror is reproducible at a fraction of the time and cost as compared to glass equivalent size. It also requires a considerable mass and cost reduction of the telescope mount as compared to conventional size equivalent mounts.

The membrane mirror has overcome the problems of near edge distortion when stretched that has been a limiting factor until now. Further more, the surface problem of having an oblate spheroid has been overcome now by deepening the centre thereby creating a parabola through active central distortion of the figure.

Because there is no active system to produce pattern surface distortion, the ratio of thickness and size has resulted in a stable surface with diffraction-limited results in the near nfrared. Research is continuing on to develop an upgradeable mirror with visible range diffraction-limited results.

A unique interesting feature of the mirror for astronomy is optional variable focal or N focal lengths. Because it is used as a direct imager focussing can also be accomplished though precision vacuum control. This unique adaptability of the mirror creates multiple telescope applications.

For example an Off axis Gregorian. The primary mirror is a light-weighted mirror made with tuneable radius of curvature according to new technology concept of controlling a membrane by air pressure.
Both primary and the secondary mirrors are of elliptical shape, which allows to compensate spherical aberration and coma. In principle, the figure error of the primary mirror can be compensated by modifying shape of the secondary.

The advantage of such off-axis Gregorian telescope is the following, There is no central obscuration in the system, which makes possible to obtain images of high contrast. The concave shape of the secondary is easier for optical testing compared to the convex secondary in Cassegrain systems. Refocusing of the system (moving the focal plane from the nominal position) can be done by changing curvature of the primary and varying the distance between the mirrors.

Initailly, a 6 inch diameter mirror telescope resulted in good performance which led onto a 48 inch diameter mirror. Astronomical images were taken of the Moon and the planet Jupiter with excellent results.

The cost of producing a membrane mirror below 18 inches diameter is actually not economical. Mirrors 18 inches in diameter and above are really cheap to make. For example, if one was producing 48 inch diameter mirrors the cost is surprisingly low at about €6000. This cost would probably come down if one was making a batch of them.

There is a published paper on this technology if any body is interested.

Eamonn A

Eamon,

I'm fascinated that you have taken this research forward and would very much like to read your paper. I am especially interested to see how you have overcome edge-zone distortions and how you compensate for atmospheric changes in real-time. Have you had any of your prototype mirrors tested to characterise surface quality and deformation rates dut to atmospheric and temperature changes?

Have you (has anyone) investigated the possibility of using similar thin-fily technology to produce lenses or correctors? I imagine that different gasses between two membranes could assist in altering the refractive index of the system. Multiple membranes could be used to separate gasses with different RIs to produce doublets and triplets.

Intrigued!

Phil.

If you use a material for the mirror which is fairly rigid and hard-to-deform then it would require a lot more vacuum force to “suck” it into a paraboloid shape.

This would eliminate the much feebler deformations induced by changes in atmospheric pressure.

The shape should “stay put” under tension.

The “deformation properties” of the material would need to be studied carefully however.

Only an “ideal” substance would deform to a perfectly paraboloid shape.

Peter.

Link to info on spinning molten glass to make mirrors,and the "guru" Roger Angel:
www.universityscience.ie/pages/scimat_Mirror_Mirror.php

Peter.