Aluminum Class. Cassegrain truss project

Has anybody here built their own cassegrain truss for optics in the 16" range? I think I could handle the truss tubes and maybe the central "box", but I would like some help finding sources for the Aluminum ring at the secondary end, and the main mirror cell assembly. Besides AB Engineering, are there any other scope makers that do this? Also, has anybody tried making their own electric secondary focus system from scratch?

Thanks,

Bob Schnitzer
Grenville Base Observatory

Hello:
I have a 12.5" cassegrain that is currently in a truss tube that I modified from a truss dobsonian. I plan to put this scope in a solid aluminum tube to save weight and to mount it equatorially. The mirror cell is a pretty standard 12.5" cell, nothing special about it. If you're buying one make sure it'll work for an equatorially mounted scope, most mirror cells made for large dobsonians won't work. My top end ring is 3/4" plywood, about 2" wide. The truss poles are 3/4" square aluminum tubing and attach with aluminum angle and bolts. I didn't have to do much to modify the dob mirror box to hold the casegrain mirror. Although this set up was meant to be a short experiment, I have found it to be stiff and overall quite useful. The trickiest thing for me was to get the secondary mirror location right since changing the position of the secondary will greatly effect the location of the focus. In my case, both of the mirrors are fixed and I have a regular focuser. To do this, I pointed the telescope at a collimated beam of light, then put washers between the secondary holder and spider until the focus was at the top of the diagonal (with the focuser in the middle of its range of motion).

Cheers
Mike Connelley

A local imager built an 18" cassegrain. Here's a link to his website where he explains it.

http://www.narrowbandimaging.com/cc18_page.htm

I built a 16” truss RC and can comment a little. The RC is very critical of primary - secondary spacing. Some designs are more forgiving. You can see the spherical distortion in the star test as you change the spacing; I keep mine with in 1 mm.
Some tips:
Mirror sets are made with the back focus held constant. So to adjust the spacing set the distance from the center of the primary back to the focus plane to the back focus distance. Adjust the spacing to bring focus at infinity. I slide a tape measure up the focus tube to the primary's edge set the back focus then adjust the spacing until I have focus. As you can tell I don't believe in moving the secondary to focus. There was an article in Sky and Telescope a year or so ago that has a chart of the spacing change vs. spherical distortion.

If you must focus by moving the secondary! To get the focus change at the eyepiece, multiply the power of the secondary times the change in spacing. My RC has a secondary power of 2.5 so moving the spacing 1mm will move the focus 2.5mm.

Take a big sheet of paper and draw the tube assembly full scale. One thing you will notice is the size of the secondary is controlled by baffling. If the secondary is to small stray light will pass by the secondary through the hole in the primary and into the eyepiece. This is why most cassegrains have 38% to 40% primary to secondary ratios. The larger the primary the smaller the secondary, unfortunately things don't change much until about 40”. There has been a long time argument as to which is worse stray light or large secondary. You can take a really big sheet of paper and see why the Hubble works so well.

The secondary must be centered with the primary. When I built my secondary support I added a small hole in the center. Before I install the secondary I shoot a laser up the focuser through the hole. Of course this assumes that the primary is centered and the focuser is parallel. Nothings easy!

By using the truss design I was able to keep my tube assembly to 60lbs. Not bad for a 16”!

Good luck. It's a fun project!