Collimating accessory for the C102HD Refractor
(and other small scopes)
by William T Geeritsen Jr
izar@gateway.net 2/20/99The three drawings describe a push-pull collimation accessory originally designed for a Celestron C102HD refractor. These, and other, small, relatively inexpensive refractors have the objective lens cell (usually plastic) attached the tube (metal or plastic) with three screws spaced at 120degrees around the tube. There is, for all practical purposes, no collimation possible for these scopes as the manufacturer supplies them.
As someone who is never satisfied, I felt an overriding need to collimate my C102HD, especially after checking it with a Cheshire site tube and a couple of star tests. It was close, but that only counts with hand grenades and horseshoes to someone like myself.
A trip to the hobby shop yielded 1/4" birch (5-ply) plywood and 1/16" birch plywood, which makes up the body of the whole thing. A trip to the hardware store and auto shop yielded the other parts needed. I already had glue. I used CyanoAcrylate medium viscosity glue and accelerator to attach the 1/16 flange to the 1/4" rings. This was done with the ring/flange in place on the tube and cell. Be careful to not get glue on the tube or cell. I suggest spot gluing (lightly), removal from the tube and cell and finish up the gluing off the scope.
Note the gap in the flange, this allows the hose clamp to tighten the flange to the tube and cell without damage and allows removal later.
The bolt assembly is straightforward. The nylon washers reduce friction, the lock nut forces the cell to move rather than the bolt and the wavy washer keeps the bolts tight against the tube flange but moveable. The nut crazy glued in place near the head of the bolt is used for the actual collimation adjustment.
Install by tightening the lock nut until the wavy washer is about 1/2 compressed, install the spring and then the cell flange and cell flange hose clamp, tighten the cell flange hose clamp. Turn the bolts (all three in rotation) until the springs are 1/2 compressed. Slide the tube flange over the tube until the rear of the cell itself just about covers the original three mounting holes in the tube. This overlap will effectively seal the end of the tube. If a better seal is needed, duct tape can be attached around the edges of the 1/4" rings, not pretty but it will keep dust out. Tighten the tube flange hose clamp. The cell should now be mounted solidly and supported by the three-bolt/spring assemblies, but completely free of the tube. Turning the three bolts will move the objective relative to the centerline of the tube. This is exactly the same as a three point, spring-loaded Newtonian mirror mount.
Rough, but very close, collimation is done on the workbench or tripod which ever is the most convenient. Place the lens cap over the lens hood to keep out stray light. Remove the star diagonal and insert the Cheshire site tube in the focuser drawtube. Arrange for a small light to shine in the side opening. I use a military, crookneck flashlight standing on the worktable. Look through the peephole and rotate the Cheshire until there is a bright reflection visible near the center of the view. Unless you are very lucky, you will see two or more overlapping rings of light, maybe even completely separated rings of light. The purpose of this operation is to bring these reflections as close as possible to resembling a single reflection by adjusting the three screws as you look through the peephole.
My arms are not long enough to reach the screws and the Cheshire at the same time. So, I made one more accessory as follows. Find a socket which fits the nut cemented to the bolt near the bolt head. For me it was a 3/32" socket. Next buy a 36" length of 1/2"diameter wood dowel and cut one end square to fit the square hole in the socket. Cement the socket to the dowel. Now you have a tool which will stay on the adjustment bolt and allow you t turn it as you watch through the peephole in the Cheshire.
These adjustments will take some time and frustration, but once you figure out what is going on, it will become clear to you. You may never reach a perfect single reflected image. The idea is to get the best nearly single image as you can. No matter what happens, your collimation will be better than the factory collimation and you can always readjust and star test with ease.
To paraphrase an old TV (Outer Limits) series, "You are now in control of your scope".