[ATM] Thoughts about mirror cell design from a stubborn RTV-head

[jonah at eecs.berkeley.edu (Jeff Anderson-Lee)]

Bruce Sayre wrote:

>[...] But I'm making these assumptions: 
>
>1) the mirror's thinnest ratio is 1 to 15
>
Thinner is harder, but it's hard to know where to draw the line.  I have 
a 16x7/8 inch mirror blank in the queue at which point I will be able to 
do some hands on testing of various mounting strategies.  However, for 
the rate of progress on my other projects it's likely to be a few years. |-P

>2) the three bottom whiffletree points are not also used for collimation
>
Whether or not you use the 3 bottom points for collimation is immaterial 
to the support, but making the making the collimation points wider does 
make collimation less finicky.

>3) the whiffletree is compressed, or as thin as possible
>
Definitely helps!

>4) the whiffletree rests on a rigid, flex-free metal structure
>
See (2) above.  "Rigid" is relative.  Since there are (only) three 
support points, the issues are (a) refocusing and (b) collimation.  
Unless you go super thin are either of these likely to be a real issue?

>5) the whiffletree's flexible pivots are incapable of lateral movement
>
Can this be accomplished without added complexity or stiction.  If all 
support points are RTV'd to the mirror this is a moot issue anyways.  Or 
do you mean excessive "slop" or "play" rather than "rotation"?

>6) support locations are determined with Lewis' PLOP
>
Good strategy, but not completely necessary.  Other placements will work 
(just possibly not as well ;-) ).

>7) the silicone pads are 1.5mm to 2mm thick
>  
>
Sources suggest 2 to 3 US quarters for RTV pad thickness (or roughly 
1/8in to 3/16in or 3mm to 4mm).

>Successful mirror cells made with slings for lateral support probably
>observe the first six constraints. But slings probably have to be
>well-executed too -- something RTV-heads don't have to know anything
>about!
>
That's one of the chief reasons for RTV'ing in my books.

Jeff Anderson-Lee
Sacramento, CA