Re: [ATM] undercorrecting?

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

tony gondola wrote:
> Those would be interesting tests to run. I'll likely give it a go sometime 
> this week as it would be useful data to have. However the stand test doesn't 
> address the idea of the original post. The theory is that radiational 
> cooling of the front surface will cause a temperature gradient within the 
> glass that results in a significant over correction of the original figure 
> even when the mirror is fully stabilized to ambient.

OK, so I missed the real point there.  Sorry about that.

However, I have star tested quite a few mirrors I have made, and those 
results most definitely do not show a significant degradation of the 
strehl ratio under the real sky.  The correction is generally very 
close to ideal, agreeing with my test results.  So I would say I have 
never seen a significant effect due to radiational cooling of the 
coated front of the mirror.

If you want to check this, star test identical uncoated and coated 
mirrors in the same telescope on the same night.  I've done this with 
mirrors before and after they are coated - I didn't see much difference.

I think what may overlooked here is that the extremely thin aluminum 
coating is very low in mass and doesn't have much heat capacity 
itself, so it is easily kept near air temperature by the heat of the 
air itself.  Keeping the coating near air temperature would of course 
prevent much of a thermal gradient from being set up between the front 
and back of the mirror.

Then there's the question of the thermal properties/effects of the 
overcoating (if the mirror had one), which is typically on the order 
of the same thickness as the aluminum.  If it's a quartz overcoating, 
then it has thermal conductivity and heat capacity similar to glass. 
Has that been considered?

I have seen the effect of a thermal gradient between the front and 
back of a mirror due to the mirror holder: the 4" secondary (1" thick) 
in my 12.5" Cassegrain used to have a fairly closed holder, and the 
telescope would show definite undercorrection.  I thought this might 
be caused by the back of the secondary being warmer, increasing the 
correction of the secondary and decorrecting the system.  Drilling 
some holes in the side of the secondary holder to improve airflow 
seemed to help.  Performance actually seemed best on nights when there 
was a breeze to help push air through the holder.

	Mike Lockwood


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