Telescope Reviews: What has Become of the Cassegrain’s Popularity?

I've had experience with 2 Cassegrain scopes. One was a Star Instruments 10" f15 classical Cass. It was fitted in a fiberglass tube, solid back plate, typical Cave or Parks Newtonian style components. It was a bear to get collimated and didn't cool well. Principal problem with collimating Classical Cass and RCs is that the centering of the secondary optical axis to the primary mirror's has to be within a few thousands of an inch. Typical secondary holders can tilt the mirror in a way that shifts the axis.

So that scope went away. A cople of years ago I got an old set of 3B Optical 10.25" Dall Kirkham optics based on an f4 primary. I made a wood open frame tube with adjustable trusses to work the spacing and then recently remounted in a lined aluminum tube with fans, etc. This scope is working at about f17 with the focuser and diagonal I'm using and is very insensitive to misalignment of the secondary. Since the secondary is spherical there is no optical axis per se, and I never have on axis coma or astigmatism that can't be collimated out by adjusting the primary.

Also, the secondary is on an AstroSystems holder and I can set the focus range wherever I need to move it by moving the secondary fore/aft via the threaded rod and nuts on either side of the sider hub and then when set, I use a Feathertouch 2015BCR I have on the back end. Movement of the secondary is only needed to change to accommodate a camera or binoviewer and is pretty delicate to get set right. But the focusing in use is with typical Newt style focuser on the rear cell.

Lastly, I'll second Dick Parker and Tim53's observations on how good these long f ratio scopes are for visual deep sky. They aren't limited to planets. And visually I never notice coma, it would show in imaging but visually even with a 50mm Axiom or 40mm Pentax XW, the field is visibly sharp to the edge. I've had fantastic views of even larger objects like M8, M42, and M31. I just need to pan the scope around to see all of them, but the views are stunningly sharp and contrasty. Center of M42 around the trapezium looks like cotton candy its so filamented, and the E and F stars just snap even at 90x or thereabouts.

I'm very partial to DKs based on experience with this scope, as the tuning of the collimation isn't so daunting as the classical cass or RC and it works superbly well visually.

Dave

Edited by davidmcgo (02/03/13 05:44 PM)