Re: [ATM] Mirror cell design(silicone)

[James P Crombie <jpcrombie@pei.eastlink.ca>]

Mark
  For some more info on silicone on glass, see this article from the 
Steward mirror lab and their problems with silicone.
http://medusa.as.arizona.edu/mlab/lbtglue2.html

Mark Holm wrote:

> 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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>

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