The Practical Telescope #4
by Dennis Allen
Whenever our club receives a donated telescope, I'll try to restore it to working order. This fall we received a 6" Criterion Telescope. Don't know who owned it, but had to been a club member back in the eighties.
To me, it didn't look in that bad of shape. Nothing wrong with the main mirror, but the secondary needed a recoat. The R.A. motor didn't work, the rings for the finder had couple broken set screws, and the old focuser was junk.
First thing I did was to take off the counterweight. Cleaned up a lot of rust on the set screws and shaft. Then I took apart the R.A. motor. There wasn't anything wrong with the motor itself. Seems someone adjusted the bracket that holds the motor. The bracket was pressing against the worm gear, so nothing could move. Easily adjusted. When I had the motor off, saw the counterweight was way out of balance. Again, easily adjusted. Anyway, put it back together and did a star test. The scope kept a star in the eyepiece for a good half hour, no problem.
The next step was to mount a new focuser. I got a new 1¼" rack-and-pinion focuser from Orion telescopes. I used the old focuser to make a pattern jig out of ¼" plywood. Drilled out the old 1½" hole and mounting holes. I enlarged the jig hole to 1¾" for the new focuser, then secured the jig to the tube with the old mount holes and drilled out the new hole. After removing the jig, I wrapped the new focuser with electrical tape until it's extended tube fit snug in the new hole. Made drilling out the new mount holes easy.
So I got the new focuser mounted. Also drilled out and replaced the busted set screws on the finder rings. Even mounted a new red-dot laser pointer. An ebay company in California was able to salvage and recoat the secondary mirror. All that's left is collimation and a star test. The club should then have another telescope working and available to it's members.
I went out to the observatory Friday night, to do some time-lapse and star test the refurbished 6” Criterion. Clear, cold, but calm and quiet. Tried to run the Criterion, had nothing but problems. The RA motor wouldn’t track and there was far too much wobble in the RA shaft (very difficult to find and center objects). Odd, I thought it was working. Maybe I bumped it out of whack?
The new focuser is a tad taller than the old one. For low magnification eyepieces, there was almost no inside travel. Afterwards I took a sander and shaved about a quarter inch off the top of the focuser. Should give us a little more travel.
As far as the images themselves, the new secondary seems to be working great. Nice images of the dumbbell, the ring, Pleiades and Andromeda. Was able to split Epsilon-Lyra. The four stars of the trapezium looked good, although too low for an attempt to see six stars. I would say the mirror quality of the Criterion is almost as good as the old club 6” scope. Until I get the Criterion mount working, I'll use the tube on another mount.
This winter I finally had a chance to work on the Criterion mount. Pulled the RA drive apart and put it back together. Just did a star test and the RA drive seems to be running good. Just needs to be test driven by someone in the club.
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