Re: [ATM] Vacuuming a Foil

[Douglas S Angle <Douglas.S.Angle@can.dupont.com>]

> Why not span a thin reflective foil over the rim of a basket and then 
put
a
> slight vacuum beneath. This would create a paraboloic shape.
> This idea sounded familiar to me. Maybe I have read it once on the
> ATM-pages.

The book Unusual Telescopes shows an interferogram of a mirror made this 
way.  If I recall, there were problems near the edge where the mount 
pinched the film, but the center looked pretty good.  I considered trying 
this approach, expecting to mask off an inch or so of the edge. I think 
I've also read of attempts to control zones by heating coils in the 
vaccuum chamber - a little localized heating will change the figure 
slightly. 

Films typically have several percent variation in thickness, which will 
result in different amount of stretch, and so figure errors.  You will 
want to start with a high quality mylar - maybe Baader film?

-Doug


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