Re: ATM Expansion calc. Deformation in mirrors as a function of thickness variation to delta temp

[Krajci Tom J Maj 27OSS/IN <Tom.Krajci@cannon.af.mil>]

From: Bill Gillespie <bgillespie@pcrs.net>

> Andy Saulietis wrote:

>> If the glass is isotropic (CTE same in all directions) then
>> changing temp will not change the relative shape of the
>> mirror,

>Please help me out here guys,

>In my fragile mind ......If someone set a curling iron on my fore head
>with a choice of having  

>a) a single sock between me and the live iron .....or 

>b) a layer of 3 socks between me and the live iron.....

>I think I would prefer b! The transfer of the heat would occur more
>slowly.

That's fine for protecting your forehead, but this is not a good analogy for
optimizing telescope thermal performance.  (In the forehead analogy you are
insulating your forehead to maximize temp differences...in the telescope
case you are trying to get temp differences between ambient air and
telescope to be as close to zero as possible.)

>So .... Given the material is an isotropic substance - non the less -
>when it has differing areas of mass density - heat transfer rate occurs
>relative to the material transferring its mass.

>In other words - of particular concern in thinner faster mirrors - I was
>thinking the different thickness areas of the mirror would meet ambient
>temperature at different rates (relative to their thickness) ...would
>they not? - the thicker portions being further away from ambient longer
>- - and the thinner portion arriving at ambient sooner?

>Would this not cause the surface front distortion that everyone is
>concerned with regarding higher CTE materials like plate glass and even
>Pyrex?

Yes, while there are temp differences within the mirror (outer faces and
corners are cooler than the inner regions) then the figure of the mirror
will be distorted.

However, a thinner mirror has less material to cool...so all other things
being equal...it will cool faster than a thicker blank...and give you good
images sooner than a thicker blank.  (Also, the warm mirror produces tube
currents, in addition to the distorted figure from a non-uniform temp
profile in the mirror.)

My advice still stands:  Strive for the lowest thermal mass, the highest
thermal conductivity, and the best use of active cooling (fans) so that your
telescope has the "fastest thermal response time" possible.  That way, less
time will be spent having the image spoiled by a warped figure or tube
currents.

(This is similar to the advice for building the telescope
mount/tube....lowest mass/angular inertia, highest stiffness, and the
highest damping possible...this gives the shortest recovery time after a
bump or wind gust.)

Tom Krajci