Re: (ATM) Thermal Distortion of Figure
Clay Spence x3039 (cds@peanut.sarnoff.com)
Mon, 14 Aug 1995 09:56:41 +0500
In thermal equilibrium the mirror is simply scaled up or down in size
from some other temperature. A perfect paraboloid will remain
perfect. It is when it is not in equilibrium that you have trouble.
When different parts of the mirror are at different temperatures, the
different parts all want to be their natural sizes at that
temperature. They can't be, so there are stresses inside the glass,
and it distorts. Even a flat with perfectly parallel sides will
probably become non-flat when its temperature is changing.
Imagine such a flat with the temperature falling. The outside will try
to shrink, while the inside will try to remain at the same size. The
inside gets compressed, the outside is stretched. If it were an
infinite slab, it would remain flat. But the edge tries to contract.
Probably the edge turns down. The exact shape is very difficult to
calculate, even if you knew the exact time course of the internal
temperature at all points in the mirror. When you are just changing
the external temperature in some way (and we rarely know how that is
happening), it becomes even harder.
Clay Spence
cspence@sarnoff.com