Re: [ATM] basic questions about cooling fans

[Thomas A Simmons <tasimmons@raytheon.com>]

Mark Holm's response to Marco says (in part)

>>First, there are two excellent articles in the most recent issues of Sky
& Telescope on the use of fans for cooling telescope mirrors.  Look in
the May and June 2004 issues.<<

I was wondering how long it would take before someone commented
on Bryan Greer's articles.

>>Very often, a layer of air near the mirror becomes warmed when the
mirror is warmer than the outside air.  This layer is called a boundary
layer.  If the boundary layer had a nice uniform and optically correct
form, it would not harm the image, but that is never true.  In fact, the
boundary layer often causes the most damaging image degradation of
anything in the light path.  Mixing up this boundary layer by blowing a
fan on it can improve the image very rapidly, even when temperature
differences are still present.<<

Nice summary, Mark.

>>4 -and about the upcoming Venus transit: is a cooling fan especially a
must
>>have???
>>
>
>I don't have enough experience with solar telescopes to know.  I do know
>that a white painted telescope is best, and it will usually be best to
>observe over a grass field, or perhaps a lake, but certainly not over
>pavement or buildings or dark plowed fields.

Sky and Telescope had an article IIRC Feb 2003 by Harold Suiter which
showed a solar telescope by a friend of his. Perhaps Marco won't have much
time
to make major changes to his telescope?

Perhaps the most important item to focus on (due to the short time to
prepare) is
keeping the heat on the mirror from building up in the first place, and
someone has
already mentioned Baader Solar Film. Make sure the filter CAN NOT
accidentally come off.

IMO, there isn't so much of a >mirror< boundary layer problem as this will
be over
whatever you are using for a Solar filter.

Tom
Tucson, AZ

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