[ATM] Ghosts in the Coulter

[vorblesnak@peak.org]

Tube heat.  That is what it came down to, tube heat.

I opened the tube as the sun was going down and did not use it for one
hour.  When I first looked at Venus I had no ghosts but I had some
flaring.  Probably a collimation issue.  I looked at some other objects
and when I came back to Venus, about 20 minutes, I had one small ghost,
just a double image, nearly on top of the main image.  I opened the tube
for another 30 minutes and the ghost went away.

The tube is heavy cardboard, about 3/8ths inches thick.  The bearing box,
(it is a standard DOB in many ways), is made of 3/4 inch plywood and runs
to the bottom of the tube.  The tail gate is attached to the bearing box. 
When the tail gate is closed the bottom of the tube is sealed.  In the
bottom of the tube the wall thickness is doubled, due to the bearing box
and tube wall thicknesses and the voids at the corners between the tube
and the bearing box are sealed at both ends.

The bottom line being, this thing has a lot of mass down by the mirror and
is going to take a couple of hours to cool and perform well.

When I cooled the scope in my earlier attempts I only opened it for about
30 minutes and then assumed it would lose any residual heat to the night
sky.  This was obviously wrong due to the thickness of the tube and the
sealed tube assembly.

Thanks for the many ideas.

David Davis
Toledo, OR 97391






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