Monday, November 7, 2011

Improving the Celestron CG2 tracking mount

When you take something apart and put it back together a few times you start to question just why the designer did some of the things they did. There are probably good reasons (some of which would be out of the designer’s control, like price, production formats, assembly costs, etc). However when you’re not so limited by these things…some of the results really stand out. No hate on the designer, they did make a tracking mount that wasn't all bad for wonderfully cheap.

So allow me to present the problem:

If you have a looksie at this diagram you’ll see that because the main barrel of the tracking mount is cast the tolerances on certain sections are not very good. Even if they were it would still be difficult to get exactly what is needed using soft aluminum.

Rotation in direction A there is the enemy of tracking (well there are other things too, of course, but this is the one I’m poking at). The way the original mount is designed this play is controlled either by tension in direction B, or the tolerances between the axle shaft and the walls of the cast tube it resides in. Since there are no bearings B cannot be very good, and because this is cast A cannot be very good.

What really strikes me about this is that they cast this long tube, which if they had made use of could have helped this issue a great deal. My solution will add a little more weight back in to the fixture but I think it’s worth the sacrifice:

Longer shaft means play has less effect, and tighter tolerances are made by using pre-made bronze sleeve bearings on either end. As much as I’d love to put in precision tapered needle bearings? This is not the place for that. In total it will cost me $7 to change these parts out, and I will need to do a small amount of machine work. Parts are on order…


  1. I have a question, wondering if you might be able to help me out. I bought a Celestron Powerseeker 114 EQ off craiglist and soon found out why they were selling it. My CG-2 mount spins freely on the Declination axis, despite the set screw being tight. Turning the slow motion control cable for declination doesn't seem to move anything. Do you know if there's a clutch or flat of some type that the set screw locks onto?

  2. hmm....there's a couple of places where it connects in.

    The little set screw tightens against part of the worm gear (the non-worm side, "worm wheel according to this: ). If that wasn't engaging for some reason; like maybe if the threading didn't go all the way though that hole or the thumbscrew wasn't long enough (or broke off in there?) then it wouldn't make that contact. So if turning the control cable makes the worm go but not the rest of the axis that would be the place to check out. Just un-thread the set screw all the way and see if you can see all the way through the cast part and to the shiny aluminum, and that it's threaded all the way through

    That set screw/thumbscrew really just drives against the thinner part of the gear, so if it wasn't really getting a good contact it could slide.

    On the other side: if the worm part of the worm gear wasn't making good contact with the worm wheel then turning the cable wouldn't make the worm wheel turn (and therefore wouldn't make the declination turn). There's a couple of nuts you can loosen and adjust the position of the worm relative to the worm gear; they're a little tricky to get to because they're kind of on the back side of things between the rest of the mount and the worm.

    Same is true for where the cable connects to the worm's shaft. There should be a flat cut in to that for the set screw to tighten against.

    Hope that was helpful; for the price these are decent little mounts. Should be great for that 114 reflector. Let me know if you can't find anything wrong; I'll take mine back apart and get some pictures of all the internals so you can see where the interfaces are.

  3. that I'm thinking about it, the declination axis is the one I didn't use. I don't remember it being worm driven exactly...I'll take it apart tonight and have a look. I'll bet it's a swivel-nut kind of thing and is either stripped out or not hooked in like it's supposed to.