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

[sayre at foothill.net (Bruce Sayre)]

Jeff Anderson-Lee wrote on 2/18/04:

<<|>1) the mirror's thinnest ratio is 1 to 15
<<|
<<|Thinner is harder, but it's hard to know where to draw the line...

The 1 to 15 ratio only represents my personal experience. That may be
because I get my optics from people who get their blanks from the usual
suspects.

<<|>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...

In principle you're right, but as a practical matter, I think it's
difficult to eliminate lateral slop if the three bottom points are
integrated with the three collimation bolts. Since the bolts drop
through holes in the tube bottom, each hole must be larger in diameter
than the bolt's threads, so there's no guarantee each bolt will rest
with equal weight in each hole. Especially if the threads wear away.
Especially if the holes and bolts aren't exactly and precisely located
and machined with the same center-to-center distances. See those forces
traveling right up the whiffletree to the mirror? 

<<|>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.  

The concern I have here is similar to the above. Typically, two square
metal tubes span the width of the mirror box and hold the collimation
bolts; one holds two and the other holds one. Will they always be
separated exactly by the same amount, so the bolts don't get squeezed?
See those bad forces traveling right up the whiffletree to attack the
mirror? 

<<|>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"?

I wasn't thinking about the small triangles, or "load spreaders" as
they're called by the engineers, where the mirror rests on the RTV pads.
It's the mechanical pivots down a level or two that are subject to
lateral slop.  An 18-point cell has nine of these; a 27-point cell has
twelve. If these move laterally relative to each other because of play,
watch out. See those bad forces ... (Hence, Teflon-lined spherical
bearings.)
 
<<|>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).

What reasons do the sources give? 

Thanks for the comments, Jeff. (One of these days we'll meet. We're
probably within 75 miles of each other.)

Bruce Sayre
P. O. Box 544
Applegate, CA 95703 USA
mailto:sayre@foothill.net
http://www.foothill.net/~sayre