Sapp8

Jim Sapp built this 8 inch F16 cassegrain reflector. This is a Dall-Kirkham design, which has an ellipsoidal primary and a spherical secondary.

The scope and mount are entirely homemade, with the possible exception of the optics, which were bought used. After recoating, these have proved to be of excellent quality. Jim casts and machines many of his own parts. This scope was build at fairly low cost, using many surplus parts and scrap aluminum.

The mount consists of a very stable wooden tripod with a german equatorial head on top. The tripod is well done with a nice finish, even though Jim claims to dislike woodwork and tends to stick to metalwork. The german head is made from machined 2" pipe tees with ball bearings and stainless steel shafts. This is attached to a 45 degree pipe fitting, which rests on a tiltable platform for latitude adjustment. Many components are of a common size, and are interchangeable. For instance, all the knobs are large diameter knurled aluminum for easy manipulation in the cold and dark. The polar shaft is bored for a polar alignment scope. Dumbbell weights are used for counterbalance.

The declination drive uses a 360 tooth ring gear and worm, with a handle which always points to the rear of the scope. Using a 360 tooth gear gives you 1 degree of movement for a full rotation of the knob - handy for celestial navigating. The RA drive is a conventional style clock drive. This mount is also used for Jim's 6" refractor.

The telescope is made of aluminum tubing, extra long at the front to act as a dewshield and light baffle. There are many beautifully machined parts.

The focuser is a crayford type for 1.25" eyepieces. It uses 4 small ball bearings on the top, and a spring mounted, brass bushed roller on the bottom. It slips into the back of the scope like an eyepiece, and can be rotated. A drawtube rides in the focuser (not shown in the photograph) and holds the eyepiece. There is no focusing image shift, since the primary mirror is not being moved. To create the focuser, Jim first drilled the holes for the ball bearings in an oversize aluminum block (see focuser drawing). He then cut off the corners of the block and drilled the other holes. The brass bushings are simply rods with holes drilled in them for the roller rod. The roller rod (the one with the knobs) rides in an oversize hole in the aluminum block. This allows the rod to move up and down to put pressure against the drawtube.

You can use this idea even if you don't have a lathe. A simpler crayford focuser can be made using only a drill, a file, and a tap. For this one, you mount the ball bearings on a piece of aluminum angle, and use Jim's roller rod idea in a small block of aluminum. Mount these on a backplate with a large hole drilled in it for the light cone to enter.

The mirror is not held in the conventional cassegrain manner. Instead, it uses an 18 point flotation cell, with the mirror isolated from the central light baffle. This removes the mirror distortion caused by the normal central mounting method of clamping the mirror to a slider on the baffle. The 18 point cell may sound like overkill, but when you're machining your own parts, there's only a few cents worth of difference in the cost, so why not make it excellent instead of just good?

Jim uses a lathe, drill press, and hand file for most of this work. He does not have a milling machine (yet). Milling is accomplished on the lathe when possible, or with the file. He casts his aluminum parts using a small, charcoal-fired homemade foundry.