RE: [ATM] Mirror cell design

[Mark Holm <mdholm@telerama.com>]

Just in case you did not notice, Richard Schwartz rather enjoys writing 
the opposite of what he means, giving generous clues that he is doing so 
by including outrageous statements.  It is a little oddity of his 
personality.  Once you are used to it, you find that he usually has 
useful things to say.  Richard's style could be a bit confusing for 
someone encountering it for the first time, especially someone for whom 
English is a second language.

In fact, a lot of telescope mirrors have been successfully mounted with 
silicone adhesive.  The big advantage is that it frees one from having 
to support the edge.  The big disadvantage is we don't have a good tool 
yet for quantitatively predicting whether one will get trouble when the 
cell is not horizontal.  Enough people, including professional opticians 
and astronomers, use this method that it can be recommended as probably 
no more likely to cause trouble than other methods of edge support.

It is certainly possible, and perhaps even prudent, to build some 
mechanical edge and front restraints into your cell as well as trusting 
the silicone.  Make these not quite touch the mirror in normal use.  
Their purpose is to provide a failsafe in case the silicone should let 
loose.

The best silicone easily available is probably clear aquarium sealant 
or, perhaps in second place, clear bathtub caulk.  You want clear, not 
so much because pigment might be harmful, but because pigment might hide 
the dilution of silicone with cheaper polymers.  That would probably 
make the product less adhesive to glass.  It is hard to make the sealant 
or caulk reasonably transparent unless it is fairly pure silicone.  Good 
silicone sealant or caulk will be more expensive than other sealants or 
caulks.  In the US, there are some brands that mix silicone with cheaper 
polymers.  Read the labels carefully to avoid these products.  Ask here 
on the list if in doubt.

Also, you want fresh silicone.  Don't use a tube of caulk you have left 
over from a remodeling project five years ago.  Buy new, and try to buy 
from a store where the stock looks fresh.  If the container has a thick 
layer of dust on it, look for another source.  Old silicone sometimes 
loses it's ability to cure properly.

Regarding the back support:  1. Do not use the proportions listed in 
Kriege and Berry.  Instead, send your mirror dimensions to this list, 
and someone will use Plop to calculate proportions for you. (or you can 
download and use Plop yourself.  Google on Lewis Plop.)  We need: glass 
type, mirror diameter, mirror thickness (at the edge), mirror focal 
length, planned diagonal mirror diameter (an estimate is good enough for 
the diagonal mirror diameter).  2. If you are going to try gluing the 
mirror to the supports with silicone, use thicker screws than K & B 
recommend.  The screws will be supporting the weight of the mirror in 
bending as well as compression and you will want the extra stiffness.  
Also, you will want to eliminate as much sideways play as possible from 
the mechanism.

Mark Holm
mdholm@telerama.com

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