Re: [ATM] Temperature-changes

[Mark Holm <mdholm@telerama.com>]

Soda lime glass (Plate)  8.36 x 10-6/°C
Pyrex 7740               3.25 x 10-6/°C
BK7                      7.0 x 10-6/°C


Temperature changes have two effects.

1.  When the mirror is not the same temperature as the surrounding air, 
it heats or cools the air near it.  Usually the mirror is warmer than 
the air, so the air is heated.  The heated air has a different 
refractive index than air at ambient temperature, thus it bends light. 
The heated air layer is never uniform, so the bendig ends up being non 
uniform and significantly degrades images.  Blowing the layer of warm 
air away with a fan can nearly eliminate this effect, even while the 
mirror is still warm.

2.  When the mirror is not the same temperature as the air, the air 
heats or cools the mirror.  The layers of the mirror closer to the 
surface heat or cool more rapidly than the inner portions.  This causes 
the warmer or colder portions of the mirror to expand or contract with 
respect to the rest.  Since the glass is not expanding and contracting 
uniformly, stresses are set up that deform the mirror shape slightly. 
Low expansion glass helps with this problem.


Mark Holm
mdholm@telerama.com

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