Fixing a Pesky CG5 Mount
Rich Anderson
(The CG5 is a German equatorial style mount that Celestron imports from Synta of China - it looks almost identical to the Great Polaris mount made by the Japanese firm Vixen. The CG5 is used on a number of Chinese-made Celestron scopes such as their 150mm achromatic refractor. While you may not own this particular mounting, the article suggests ways to tune up almost any mount of this design.)
I picked up a (Celestron) CG5 mount a while back to hold my Meade 102ED, Genesis SDF and now my Takahashi FS102. Right after I got the mount, I noticed the RA rotation was very stiff. Even with a scope on one side and a counterweight on the other, it still required an effort to turn it. I never had to use the locking clamp. I found the info on the Net at
to re-do the internals of the mount to make them work better and finally did it. I tore it down, cleaned off the various thick greases they used to lubricate the bearing surfaces, cleaned up the surfaces that needed it with 1000 grit sandpaper and water and then re-lubed the mount with light bearing grease that won't change it's viscosity with temperature. Right after I did the first test on it, I found the RA to be just as tight. Turns out the RA shaft where the ring gear rotates is just out of round enough to almost jam. It took a great effort to turn it past the jam spot. I used more 1000-grit sandpaper but it did no good. I figured it would take me forever to do it and the sanding (because it was by hand) would probably not be even and might even be taking off material where it shouldn't have been removed. Then I tried a trick used by machinists. They take some water and Ajax/Comet and make a slurry. They then lap the parts together until the mate is perfect. I coated the bearing surfaces with it and then simply slipped the ring gear on and rotated it. Within about 20 rotations, I could feel the binding effect easing. After no more than 10 minutes, the ring turned properly. It's clear the effect was to take down the material exactly where the binding occurred, which is what is needed. There is still some slack-stiff effect as it's rotated around the shaft, but it's not nearly what it was like. While it's impossible to completely eliminate the out of round problem, this minimized it substantially and now the RA axis turns as it should, with relative ease.
Published in the April 2001 issue of the NightTimes
