Re: ATM open/closed tube?

["Bob May" <bobmay@nethere.com>]

Anybody that does a lot of testing quickly understands where those waves of
heat come from.  You have a choice when testing, either do it in a quiet
enviroment and take care to eliminate all sources of heat waves circulating
from the light's path or to homogonize the air (working outdoors in a breeze
often does the job nicely but you then run into the problem during the
daytime of seeing anything) in the enviroment and put up with some small
sized rapid variations of the lightpath while things aren't quite equalized.
The fans in tubes often get to an apparent stability (true stability comes a
bit later when things get truly equalized in temp) of the image quickly by
doing this mixing of the air and keep it getting better by conduction of the
heat off of the various offending surfaces and rapidly mixing it with cooler
air.  Watching the stars stabilize when a fan is turned on will indicate the
rapid oscillation of the light which quickly gets finer and finer as it gets
more rapid.  Much slower is any corrections that happen as the glass gets
into it's proper shape for usage as an optical surface in the scope.
Bob May
http://nav.to/bobmay
bobmay@nethere.com