Whats the Maddest scope you've ever seen?

A semi-rigid but deformable substrate (like glass or perspex) still needs its front surface to be optically smooth - at least 1/4wave surface quality - to be of any use. What makes a stretchy thin-film surface attractive is that when you pull a vacuum behind it the front surface irregularities become less of a problem - they get stretched out. Working at IR wavelengths makes the tolerances a little less exacting.

Another method of making ultra-light mirrors was perfected by Peter Chen in the US. Chen polished and figured a negative glass mould and then applied a special gell-coat followed by layers of directional Kevlar. Chen's great contribution was to work out and characterise how the Kevlar and resins behaved while they cured and shrunk to their final shape, and ended up with a perfect figure of revolution. He was able to form many mirrors this way before having to re-polish the negative mould. There is a great photo of his 7 year old daughter holding up a 40" mirror with just one hand (through the central hole!) Chen was developing the technology for possible Moon-based observatories for NASA.

Phil.

Phil,

Send me your email regarding sending the paper.

Since that paper was published in 2005, I've had good progress using other polyimide materials coated, that demonstrates the primary requirements for precision membrane optics can now be met, of course one assumes one has addressed the controlling technology necessary for figure control within the membrane. This was achieved in 2004 see paper.

Analysis at full aperture of 15 cm (6 inch) membrane mirror has demonstrated a figure error of 4 microns RMS. A central area coated with magnetic material with a magnetic actuator placed behind the membrane makes it possible to achieve optical quality ~1/4 wave flatness. Further tests have revealed that the Hencky curve can be eliminated and an acceptable parabolic curve is achievable according to recent tests. In light of these milestones a larger membrane mirror has been made in the visible region.

A 75 cm (30 in) aperture membrane mirror was made using the above controllable technology, which shows promising results. The autofocus can be controlled by a barometric auto corrector (in process) or a wave front sensor (WFS).

NOTE: Air-side of membrane typically has a superior finish than the Angel casting process for large miorrors. The casting process for a 3m mirror takes 18 months as against 1 month for a membrane mirror. The cost is a 1/100th and the weight is 1/1000th
Early tests of another new polyimide of 4 inch diameter achieved a thickness uniformity ~1/20 wave rms. Extreme temperatures show good tollerance,also no cool down time is neccessary in hot weather.

Results were recently presented to the European ELT Project with positive reaction. In particular for the secondary mirror of the ELT. According to one delegate Alexander Gongorov one of the lead optical engineers, this solves the major problem of making off axis paraboloids for the primary, because the membranes in an array have the added facility to become off axis paraboloids.

Eamonn A