450mm f/12.6 classical cass now being tested

Jan 24, 2006



#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




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