[APML] Thermal Time Constants

[Aplanatic@aol.com]

Dear List:

We all know that glass stores a lot of heat and needs active cooling to bring the OTA into thermal equilibrium.  I've done a little experiment with a 13.1" diameter primary to help quantify the problem.

I partially cored the 2" thick glass blank leaving the plug attached to the front of the mirror with a thin layer of uncut glass.  The core is 4" in diameter.  I inserted a thermocouple probe into the thin cylidrical region where the glass has been removed.  This allows an accurate measurement of the temperature of the center of the glass.  The primary was polished out and figured to a sphere and thermally induced currents were observed using a standard Foucault tester with a 0.003" slit (for good contrast).  During testing, the mirror was supported on a stand where the back and front of the mirror were both exposed to ambient air.

I heated the mirror in a bathtube full of warm water for 15 minutes, put it on the test stand, inserted the thermocouple and took measurements.

With no forced air cooling the mirror had a thermal time constant (to the center of the mirror) of 75 to 80 minutes.  In one test the mirror started with a center temperature 7.5 C above ambient and was still 1 degree C above ambient after 130 minutes.  Thermal currents were still faintly visible in the Foucault tester after 3 hours.

I then positioned a 3" square 12V "muffin" fan 8" from the center of the back of the mirror and performed the same experiment.  The moving air was directed toward the back of the mirror.  With the fan on the thermal time constant reduced to 17 minutes, and after 45 minutes there were no visible thermal currents as viewed by the tester.  The fan was turned off during Foucault testing.

The message should be clear.  A full thickness mirror will take a very long time to come to equilibrium without moving air.  Even a small fan makes a big difference.  No surprise.  If your OTA does not have a fan, may I suggest that you add one if possible.

Dave Rowe




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