>...
>In Jack Newton's latest book he mentions that his 25" is
>supported by bubble pack material -- you know, the kind you like
>to pop! Has anybody else done anything along these lines? It
>sounds so deviously simple. What's the catch?
The catch is that although the bubble pack masks the small variations in the surface of the mirror cell bottom board, it doesn't compensate for gross distortion of the board. If you have a cell made from two thicknesses of 3/4 inch plywood it will sag at high pointing altitude because it has a heavy mirror pressing down on it, causing the mirror to sag right along with it. Another problem with bubble pack is that it insulates the back of the mirror so it can't cool off effectively.
A couple of months ago I posted a description of a pneumatic flotation cell I made for my 20 inch mirror. It circumvents these problems, and you might look that up.
Aart M. Olsen aart@uiuc.edu 217-333-7467 College of Veterinary Medicine Univ of Illinois at Urbana-Champaign