Re: [APML] Thermal Time Constants

[Aplanatic@aol.com]

Hi Jack:

> Is it safe to assume that the thermocouple was imbeded in heat conducting
>  material and that the entire cylinder from which the glass was removed was
>  isolated from the air flow?

The TC was attached to the wall of the core with a drop of superglue and the 
fan was turned off during the temperature measurement.  Recall that the TC 
was about 1" deep inside a thin (0.080") cut in the glass.  It was in very 
good thermal equilibrium.
  
>  After uncovering my 16" scope, I point a  16" pedistal fan at the back of
>  the mirror and it takes at least 4 hours for the mirror to come to
>  equilibrium from the normally elevated temp used to prevent dew.    I use
>  the figure of the mirror to determine equilibrium.  It is grossly
>  undercorrected when first uncovered and takes about 4 hours to return to
>  normal.

That's interesting.  As the outer edge of the mirror cools wrt the rest of 
the mirror, one would think that the edge would turn down and the mirror 
should have an overall overcorrected appearance.  Obviously, that's not 
what's happening in your case.  Four hours seems like a very long time for 
the OTA to settle.
  
>  How does 17 minutes "thermal time constant" fit into that real world
>  situation?

Three or four time constants should be enough.  For this 13.1" diameter, 2" 
thick mirror, a fan should thermalize it when in the OTA in less than one 
hour.  Without a fan, it might take three hours or longer.

Dave Rowe

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