G-11 Tracking Problem

I'm having a tracking error on my Losmandy G-11. Anybody have any ideas?

Attached is a graph of a four or so minute run on as tracked by DSLR Focus. I have my G-11, a C9.25 at F6.3 (1345 mm effective FL). The units are pixels on my Canon D20. As you can see, Delta X (lower graph)is relatively stable, with just a little drift (probably polar alignment off). But Y has a periodic error on a 78 second cycle. A very pretty Sine Wave. (By the way--this confirms the visual. At F10, with a 12 mm eyepiece, the star moves at more than the double-crosshair box in the center. First up, then down, then back up, then down, etc.

I have unbalanced the mount. I have swapped the dec motor for the RA motor. I have changed cables. I have changed electronic control box. I have changed power supplies. I have removed my Digital encoder. I have even changed from the upgraded worm gear that was in there to the earlier worm gear--and the abberation is still there.

This is obviously a periodic error--but certainly not the kind we may expect( roughly four minute cycle). This is not only very large but on a much shorter cycle than one would expect.

Any ideas?

Alex

Attached Image:

>>Y has a periodic error on a 78 second cycle. A very pretty Sine Wave. >>

It appears that you have a bad gearbox on your drive motor. The gears are very tiny inside this gearbox, and if they are not meshed properly or if they are not manufactured with a proper tooth profile, you will get a cyclical error that is many times faster than the worm periodic error. Unfortunately, there is nothing that you can adjust to fix this.

RC

Mine has a very similar error with an 80 sec period.
Just from the period I would guess it is in the
stepper motors as Roland indicates. Mine are Hurst
motors. I also swapped the RA and Dec motors, and one
motor was much better than the other, but neither
is perfect.

You might try the newer Swiss motors that Losmandy
now uses. Maybe they are better, but I do not know.

Alex,

I encountered a similar problem with my CI-700. The spur gears are fixed on the motor shaft with allen-head set screws. As these screws are tightened, they tend to push the shaft away from the centerline of the gear causing runout. In my case, the gears would just bind up and stop moving for a beat or two causing a dropout in the guiding motion. I was able to respace the gear mesh so it wouldn't bind up when the high spots of the gears met up. The gear system in the G11 is a little different but this gives you something to look at. Also, you might try posting this on the Yahoo Scopemounts group:

http://groups.yahoo.com/group/ScopeMounts/

Very helpful bunch

Well, I found another motor. I had to solder the leads back to the connectors, but it worked. Had the same problem as the others, though.

I also swapped the RA and DEC worms. Same results. (I also swapped the rest of the worm hardware-bearings, little nylon transmission connector)

So far I have tried three motors, three RA Worms, two sets of electronics. I've taken the Digital Setting Circle Encoders off.

The results look visually the same.

If I manage to go out to the club site this weekend, I will try a couple of motors that I know were working fine when my friend swapped them out to upgrade to Gemini. If that does not do it, it has to be in the ring gear, or shaft, or bearings, or where?

ALex