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STARGAZING by Tony Flanders
RTMC 2010: the 42-inch CDK
Fully assembled, the scope is quite impressive, but nowhere near as daunting as a classic 42-inch f/4 Dobsonian reflector, which would stand some 14 feet tall and require a huge ladder to reach the eyepiece. In fact, that was one of the team's design goals:
They achieved these goals using a variant of the Nasmyth design, which is itself a variant of the Cassegrain design. A Cassegrain focuses the light using a concave primary mirror and reduces the angle of the light cone with a convex secondary mirror, which reflects the light out through a hole in the primary. The Nasmyth design adds a flat tertiary mirror to reflect the light out the side of the tube — a more convenient location in many ways. A classic Nasmyth directs the light out through the altitude axis, so that the eyepiece remains at the same height regardless of where in the sky the telescope is pointed.
The 42-inch CDK team modified this design in many ways. The hyperboloidal secondary mirror of a Cassegrain is notoriously difficult to figure, so they used an ellipsoidal primary mirror and a spherical secondary: the Dall-Kirkham design. But a simple Dall-Kirkham suffers from severe coma, so they had to add a corrector lens between the tertiary mirror and the eyepiece. Finally, the altitude bearing in their scope is a little low, so they moved the tertiary mirror and the eyepiece up the tube, just above the big altitude bearings.
The end result is spectacularly successful. The 42-inch f/6.1 scope delivers just over 200X at "low" power, with a 31-mm Nagler eyepiece. Huge lines formed to look through the telescope, but they thinned out around midnight, allowing me many fine views of galaxies. I have never seen the dust lane in the edge-on spiral galaxy NGC 4565 so wide and well defined.
I've viewed through plenty of big Dobs with mirrors of comparable size, but only while standing at the top of a tall and rather scary ladder. Being able to view through 42 inches of aperture with my feet planted solidly on the ground was a new and thoroughly enjoyable experience for me. I hope this design catches on.
Posted by Tony Flanders, May 25, 2010