#14260 Jan 24, 2006
a little while back I posted in a few places that I was building a
new truss tube 18" classical cassegrain for imaging. At Roland's
request I am posting this report here.
I am now making first tests. Friday night was my first testing with a
camera and I was blessed with great seeing but broken clouds. There
was a little bit of sky between the clouds. It remained clear enough
to get focused, calibrated and so on and I even was able to take two
90 second exposures of the Trapezium region before the clouds put a
stop to it.
Unfortunately I had several internal reflection problems and a few
other minor issues to sort out. Sunday I got clear skies finally but
the seeing was just terrible. But I mananged to take a Horsehead shot
and saw that my bead blasted and anodized baffles were causing
reflections. Yesterday I flocked the baffles and shot the Horsehead
again, this time under even worse seeing than you can imagine. I was
getting best focus at about 6.5 arc-seconds. Everything was twinkling
like searchlights. By comparison, Friday night was sub 3 arc-seconds
and steady stars.
Well the image came out fine considering the seeing: the reflections
are gone and the stars have good shape but they are really fat.
So now I am waiting for clear skies, good seeing and light wind or
I mentioned the wind: on Sunday night I had 10-15mph gusts on top of
the bad seeing and the scope still tracked well. The truss tube
design apparently has very little cross section in the wind. The
scope weighs 101# on the mount and am using 6 of the AP 18#
counterweights to balance it. It is remarkably rigid and has very
little mass on the nose end: most of it is concentrated at the rear
just like you would like for it to be.
I am building a near identical copy of this scope that has a few
improvements that should save some weight. Maybe it will be logging
in around 85 to 90 pounds.
The before and after flocking pix are on this page. Just scroll down
to the first and second horsehead shots. The flocking made quite a