ATM Aluminum Dew

[Jack Schmidling <arf@mc.net>]

>>From: Sue&Alan French <72724.2270@CompuServe.COM>

>>The only Newtonian's I've ever seen with dewing problems havealuminum tubes.

This certainly is not my experience.  My new 10" is aluminum and the
old 8" was fiberglass and they both require dew heaters in the primary
and secondary.  I wouldn't bet the farm on which is better but neither
are dew free and I would not discourage anyone from using aluminum in
a serious telescope.

Speaking of dew, here is an interesting problem for someone...

When I take my 16" Dob out, I leave it out tiil the moon returns.  I 
cover it with a Desert Storm cover after use but every morning I must 
uncover it to get rid of the dew on the primary.

I mounted two 15w light bulbs in two of the holes in the mirror cell and 
have a muffin fan in the third hole.  If I turn on the lights when I 
cover it there is no dew the next day and I use the fan to cool it down 
for use.

Clearly, the mirror cools down to below the daytime dewpoint and when
I open it to the day time air, it dews up.  No problem.  Lights take 
care of that now.

However, when not in use, I store it in an unheated building with open
windows where the temperature and air/humidity is essentially the same
as outside.  The mirror goes through the same temperature cycle, is
exposed to the same humidity but never dews up.  Well, almost never.  The
last time I brought it in (in the afternoon) it dewed up the next day
but never again.

How come?

js 


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