Re: ATM Another cooling fan question

["Albert Highe" <ahighe@ix.netcom.com>]

Although I haven't cut holes in my scope yet, I have had quite a few
experiences observing through telescopes where a fan blows across the mirror
face. In summary, it works.

For example, Tom Noe has been using fans that blow across the mirror face in
his Teleport telescopes for a few years. To my knowledge, its the only
commercial scope that does so. I've had the opportunity to observe the
performance at RTMC a couple of years ago. Images stabilize within about 15
seconds. You can see star images bloat after turning off the fans. Tom uses
a plenum to direct airflow. In my opinion, this isn't as effective as
allowing the fan to blow directly across the face. Anything that diverts the
air flow reduces the efficiency.

Also, I live close to Alan Adler and I've observed with him numerous times.
While looking at close double stars, for example, I could see stars become
sharper and become much better resolved within seconds after turning on the
fan. Conversely, the stars would swell, closing their separation, once the
fan was turned off.

You also don't need to cut the holes to convince yourself. Alan demonstrated
the effect in my open 12.5" ultralight. While viewing close double stars, I
could see the same improvement while Alan just fanned the front surface of
the mirror using a piece of cardboard. Try this at a star party with any of
the open truss scopes where the mirror is relatively exposed. Of course,
you'll need reasonable atmospheric seeing to observe the improvement.

The bottom line is that blowing air across the front surface is, by far, the
best way to improve your images.

The lore says to cool the primary. And this makes sense. If the primary is
the same temperature as the air, there will be no warm layer of
air at the front surface. However, the air temperature almost always is
changing through the night. For thick mirrors, it is almost impossible for
the mirror temperature to keep pace, even when blowing air on the back. Its
no wonder that you hear people reporting that "seeing" got better late into
the night. When the telescope has been sitting in 90 degree heat all day it
takes many hours for the telescope to equilibrate with the night air, which
often doesn't stop changing until after midnight.

But if the layer of warm air at the front surface is the problem, why cool
the back?  Heat has to travel all the way through the mirror. In fact, you
don't need to cool the entire mirror. You just need to remove the layer of
warm air at the front. So blowing air across the front surface produces an
immediate effect.

Blowing air across the front also eventually cools the mirror, just as it
would if it were blowing across the back. So the choice is to get
immediate benefit from blowing air across the front or wait hours from
blowing air across the back.

Albert

----- Original Message -----
From: "Joe Curran" <jcurran@asymtek.com>
To: <atm@shore.net>
Sent: Wednesday, December 12, 2001 1:58 PM
Subject: ATM Another cooling fan question


>
> Has anyone cut new holes in their mirror boxes/tubes to try out Alan
Adler's
> technique of mirror cooling across the face of the mirror?  Even with the
> detail in the S&T article and the author's obvious confidence, I'm still
not
> quite ready to start cutting new holes without broader acceptance from
this
> group.
>
> Or am I to be the squealing pig?
>