RE: [ATM] Mirror cell advice

[Mark Holm <mdholm@telerama.com>]

Jerry Redell wrote:

>Darran asked:
>
>It's a 10inch f6.4 but is only 1 inch think. Being as it's on the thin
>side, will this require a floatation cell, or can I get away with a
>Berry style wooden disk and three blobs of silicon adhesive?
>
>  
>
>My response:
>Everything I have seen says you need at least 9 points of support for a 10
>inch mirror.
>


I have done a lot of work with Plop in order to be able to answer 
exactly this kind of question.

For a 10 inch by 1 inch, you want a 6-point cell. (actually a 4-point 
design would be sufficient.)  Skip the 9-point idea.  Plop shows that 
9-point cells are not really a good idea.  (That statement is backed up 
with Plop experience from more than one ATM.)

Send the following data, and I will run a Plop optimized layout for your 
mirror.

outside diameter (Might as well measure it accurately since you are 
getting a custom design)
thickness at edge (Measure to nearest millimeter.  Thickness is the most 
critical variable)
glass type (Pyrex, plate, BVC, etc.  different glass types have 
different physical properties.)
focal length or focal ratio.  (Not a tremendously critical variable, but 
might as well be close)
planned diagonal minor axis (diameter)  (Plop takes into account the 
fact that the center of the mriror will be shadowed.)

I know that 6-point cells seem somewhat unconventional, and that 9-point 
cells are blessed with the patina of age.  The simple fact is, before 
the work of Toshimi Taki (Plate), Richard Lewis (Plop) and others, atm's 
had no reasonably accurate design tool for flotation cell layout.  
Before these tools, cell design was on very shaky ground.  Most of the 
lore, including what is contained in most of the books, is simply made 
obsolete by what has been accomplished with Plop.  Many atm's have used 
Plop designed cells.  It has shown it's utility by producing good 
support for mirrors much thinner than anything atm's would have dared to 
do in the old days.

By today's standards, a 10 inch by 1 inch mirror is not at all extreme.  
Ask John Dudgeon who has a 30 inch by 1 inch on a Plop designed cell.  
(That really is pushing the limits.)

Mark Holm
mdholm@telerama.com
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