Re: ATM 18 Point Flotation Cell

[Mel Bartels <mbartels@efn.org>]

>>> Logically, I would think the support points should be equally spaced 

I thought so for a good period of time, but the key is how does the
glass sag between the support points?  If you look at the mirror and the
support points' locations you can see that there is not an obvious
solution. About 10 yrs ago I cut out a disc from thin paper and put it
on a flotation support and the sagging was not even close to being even
between the points.

The best answer lies with finite element analysis.  I am told that even
Taki relied on Chandler's cell program to select support points for his
FEA analysis in Sky and Tel, so Taki's analysis is not necessarily the
best support locations for minimum sagging.

Richard Schwartz recently sent me his FEA of a 30" f/4 mirror of 2" edge
thickness and 27 pt support, and he was able to improve the sagging from
1/4 wave to 1/24 wave principally by moving the outermost support ring
outwards by .4".

I wish someone had the time (unfortunately I am buried with many
projects) to do a series of FEA for common mirrors sizes.

-- 
Clear skies, Mel Bartels    Programmer/Analyst, amateur astronomer
Eugene, Oregon, USA         homepage: http://www.efn.org/~mbartels
mailto:mbartels@efn.org     atm, atm-digest list-owner
Motorize A Dob: http://zebu.uoregon.edu/~mbartels/altaz/altaz.html