Sunday, March 10, 2013

Hypertuning my CGEM mount

Because there is still drift when I'm imaging, I decided to hypertune my mount - based on Ed Thomas kit and instructions. The kit comes with a DVD from Ed Thomas where he walks one through the process. He is great in explaining bits and some tricks - especially how to disassemble the mount.

I started with taking the mount apart and everything went pretty well. At one of the ball bearings, the rubber cap was slightly misaligned and I could feel resistance when I turned it. I wiggled the rubber cap in place, but to be on the safe side, I ordered a new one.

But then I came to the point where needed to remove the worm gears from the Dec and Ra housing. The rings on the side of the spur gear were impossible to remove. After trying (and almost ruining one) for some time, I decided to leave them in. I could remove the worm gear anyway - hopefully that wouldn't make it difficult to tune it later as I won't be able to move the worm gear left and right.

The next problem was to remove the spur gear from the Dec worm gear. The set screws are apparently almost glued in. And one of them, I stripped! Luckily they were standard 4mm M4x0.7 screws (cone). So, I ordered a new one, waited to make sure that it really fits and then extracted the stripped one. I also ordered a set of allen wrenches that Ed Thomas has in the video. The set that is in his kit is pretty good - but for these glued in screws you need A LOT of torque.

So, I cleaned all the parts and then tuned the two ring gears. From Ed's video, I expected that I will spend an entire day sanding them down. But after an hour, they both fit really well into their housing. Polished and cleaned them (again).

Next, I screwed the motors back in and then started to measure the RA axis. I was amazed how easy it was to get all the ball bearings back into the ring gears after removing all that freaking grease that Celestron put into it. Putting the worm gear back into the housing was fairly easy - except for sliding the spur gear onto the worm gear. It somehow got stuck and I had to carefully hammer it into it. But once that was done, the rest was fairly easy. Then I started with the DEC axis, but could only go so far until I get the worm gear and spur gear back from Ed.

I used the break put all the new knobs from the knob upgrade kit (except the clutch lever knobs).

And finally, I received the disassembled RA worm gear from Ed Thomas. Boy, am I relived that it's pretty much undamaged. Can't wait to continue the tuning ... but it snowed in the Sierra's, so this weekend we are back up to Northstar for some snowboarding ...

Finally, I could reassemble the mount. This was surprisingly easy. The only setback was that the RA axis did not turn easy enough and I had to sand down the ring gear a little bit more (and of course disassemble and degrease everything again). But after that, the axis turned much easier. I then attached the new clutch nobs and put everything together.

Final result:
  • I sanded down the ring gears until they are less snug. In fact, now they have a tiny bit of play when I put them into the housing without any lubricant. So, I was hesitant to do more. The don't seem to have any play when I put the lubricant in there.Both axis turn now much easier then before. But the don't spin like they do in Ed Thomas' video.
  • Adjusting the motor location and worm gear backlash was quite a challenge (I went forth and back several times). I could not get either axis into a state where a) there is no backlash, and b) the motor sounds "good" (compared to the sound in your video).
    • The DEC axis has no backlash at all, the RA axis very little.
    • Both motors make a little bit of the "wha-wha" sound. If I loosen the gears, the sound goes away, but then I can feel more backlash.
    • When I turn the motors now, there is some kind of faint clicking sound. I looked everywhere but could not find a source for it.
    • When I alternate press the up/down or left/right button very fast, I can hear some clicking sound. Which I think comes from the two spur gears having just a little bit of play. But when I move them closer together, the motor locks up.
The mount definitively "feels" much better already. But I'm not sure if I should try to tune this more (and how). So, I checked with Ed Thomas.
  • The ring gears are probably OK. As long as the axis turn easy enough that I can balance the scope properly.
  • The CGEM mount will always have a little bit of backlash - it's just not manufactured better.
  • The "wha-wha" sound is not bad for the mount. I basically have to choose between less backlash and less sound...
  • The clicking noise when the motors turn is weird - will have to investigate that.
  • But the clicking noise when I change directions is normal for the CGEM mount. The spur gears will always have a little bit of play.
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I could not figure out where the clicking comes from. It's most pronounced when slewing really fast on the dec axis. I decided to give it a try. And my first experience is just awesome:
  • It's easier to balance the scope - way easier
  • My guiding results are much better. The correction that PHD needs to make are WAY smaller (and I haven't even done backlash compensation or PEC training)
Very hopeful!

My summary:
  • Yes, Hypertuning is tricky and cost some (actually not a lot) $$$
  • In my case it took a while and was quite a lot of work (and stress) - but it was totally worth it. Not just the end result of having a much better mount, but also the better understanding on how my mount works and how I can tune it.
  • Finally, I'm somewhat disappointed how hard Celestron makesit to do any of this work with their mounts by (quite literally) glueing them together. I'd be thrilled if Celestron would sell the mount in pieces and then lets us put it together.
And finally: Ed Thomas rocks! The CD's that he ships are really good and explain a lot. But then he gave me a lot of help and guidance through the process.