From: Mark Holm 
To: tylosaurus{at}sisna.com, atm{at}shore.net
Reply-To: Mark Holm 


The second page of this article:

http://skyandtelescope.com/howto/scopes/article_136_1.asp

shows just what you want to do.  The fan should blow in, so that the high
velocity stream from the fan blows across the face of the mirror.  I know
some people think that sounds all wrong, but the experimental evidence is
very clear:
it works.

This Sky & Telescope article has a more thorough discussion:

Alder, Alan, Thermal Management in Newtonian Reflectors, Sky &
Telescope, Vol. 103, No. 1, January 2002, pages 132 - 136


You can make your own cell for an 8-inch mirror quite easily.  It will be
as good as, or better!!!, than most commercial cells.  Use the silicone
blob method
and put the blobs on a circle of radius 1.6 inch.  There is an S&T
article that shows just how to make this type of cell (except it isn't
clear about the 1.6 inch dimension).  I haven't got the reference right
now, but I can dig it up.

You need to be able to do the following:

Cut two circles out of 1/2 or 3/4 plywood.

Drill 3 holes reasonably square and clean.  (Actually it is 6 holes but for
the first drilling you stack the wood, so you only drill 3 times.  Then you
unstack it and drill out 3 of the holes to a larger diameter.

Turn a wrench to thread three bolts or pieces of threaded rod into one of
the plywood circles.

Use a throwaway brush to paint some Polyurethane varnish.

Squirt three blobs of silicone from a tube of sealant or caulk (get the clear kind).

Pretty easy.

Believe it or not, this cell will be, for an 8-inch mirror better than most
commercial cells on the market and equal to the best.  When you get into
larger mirrors, more sophisticated cells are needed, but for an 8-inch this
simple thing works very well.

You can cut some of the wood away, or drill some extra, larger, holes in it
to provide better rear ventilation, but with the fan and vent discussed
earlier, that may well not be necessary.

Mark Holm
mdholm{at}telerama.com

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