[ATM] Cooling of a primary Mirror

["Russell Jocoy" <russjocoy@hotmail.com>]

     To the list,

         After reading John Sherman's response on the effect of cooling on 
glass and the effect of
the alunimum coating, I must say I agree.  The Aluminum coating on glass is 
so thin that it has
almost a null effect on heating or cooling. Think about it, your mirror is 
figured to millionth's of
an inch and the coating is so thin it does not effect the image..... Do you 
think that this ultra thin
layer could affect the Billions of molecules of glass behind it?? Not unless 
you put it in a microwave
oven... John made the statement of the rough ground "back" of a mirror 
absorbing and radiating
more than the front... This should be true, as he stated, there is more area 
on a rough surface
than a smooth surface (paint smooth pine and paint rough sawn cedar, and see 
which surface
takes more paint to cover).
    This brings up a controversy that was debated a few years ago on this 
list... Should the back
of a primary mirror be polished???  Could this lead to more uniform 
cooling??  There have been
statements that polishing the backside of the mirror relieves stress?? Is 
all of this just bullcrap??
    I have seen large mirrors at star parties that were back polished, and 
not just one or two....
             Just some food for thought....     Russ Jocoy


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