Saturday, January 28, 2017

How I made a better Celestron AVX

I'm in no position to complain, I got my AVX for jellybeans because a surplus shop thought it was broken. But if I had paid full price I would not think well of the performance of the mount right out of the box.

It isn't difficult to achieve subs of a minute or two, but:
  • Glitches happen. I've reloaded my firmware a couple times for the occasional "bootloader error."
  • It's basically impossible to balance, as the mount is so "sticky." Other people call this stiff, but having worked on industrial machinery I know that's bollocks.
  • The motors are clearly struggling. Some of this may be my fault, having tightened everything down to minimize backlash.
  • There's some roughness in RA. I found out why.
So I disassembled the thing completely, did some things, and then I was able to take 15 minute subs. Here's what I found/did...

Step one, remove the dovetail clamp:

First discovery. Think these are assembled in a precision lab? Noooot so much. Yes, that's a rock. Just, randomly dropped in to this thing that's supposed to be accurate to a few arc-seconds. Is it anywhere that would be a problem? No, but it's not a good start.

 I unbolted the DEC assembly from the RA axis, it's just two bolts:

and disassembled the worm gear system. This all looked reasonably good, other than just having way too much grease on it. The grease is what makes the AVX so sticky. You want grease, especially where the worm meets the DEC drive, but they've overdone it quite a bit.

To disassembled the DEC rotating assembly you'll need to loosen two set screws through a small access hole on the back:

And then unscrew the tensioning...thing...from the assembly. I happened to have this tool for taking watches apart that worked well for this. I had to get creative later on though.

 Assembly apart- as you can see there are no bearings in this portion of the mount. It would be nice if they were there, but it's actually not too bad here. The mating surfaces have been turned nicely on a lathe, and the tensioning ring  tightens against a pair of large nylon washers. Would bearings be better? Of course. But it's probably not as bad as many make it out to be.

 Pulling the RA apart was a bit more trouble. I had a bicycle tool that worked well for taking the fancy orange covers off as well as the two tensioning rings:

To remove the assembly you need to loosen these three set screws; they hold a thin metal plate and nylon washer against the altitude adjustment. Once those are loose you can tap the threaded pipe out and free the RA:

Unfortunately my fancy tool wouldn't work on the RA tensioning ring. I had to improvise. Don't tell anyone where you learned this trick, but a couple of allen keys poked in to the holes and a pair of pliers will either save the day or ruin something. Thankfully my day worked out well:

Note: there are THREE set screws on the RA axis to loosen
That's everything apart. And, well, oh dear...

 Ok, one side uses a thrust bearing that gets tension against another machine surface. There are better options for this, sure, but this is probably fine too...

 This, on the other hand, feels like amateur hour. The side closest to the DEC is a regular radial bearing, which of course you shouldn't tension. So their solution was to mash it against another nylon washer. This is a terrible idea. Both the inner and outer rings of that bearing are pressed against the same surface.

This is a textbook use case for an angle bearing, which looks like this:

See? That is meant to be loaded in BOTH directions and still remain stable and solid. At this point I couldn't help but notice the example bearing that I literally grabbed from a box in the lab happened to look like it might just fit...

Ok, crazy lucky: the center part was a matched fit for the rotating portion, and the outer ring was just a little bit small. You can see above that turned a little spacer on the lathe (which was a pain, it's way too thin to reasonably be clamped in the lathe, but I made it work). Ultimately I was able to get a pressed fit on both mating surfaces of the spacer:

by pressed fit, I mean it needed some light bumps from a hammer

If you'd like to duplicate what I did here, the bearing is an L68110. Unfortunately I can't recommend a source, because mine was "that box with random things like bearings in it." But they seem to be a pretty common bearing and go for $5-15 on the internet. 

Other than that, I wiped down every greased up surface with a paper towel so that there was only a thin layer on everything. Once reassembled the mount would move much more freely, and I was able to do a much better job of tensioning each axis to prevent any free play in the system.

I still balance in DEC by setting the telescope assembly (with camera, all bolted to its dovetail) on a round object and teetering it until I find the balance point. I just mark that and slide it back on to the mount, centering the balance point in the dovetail clamp.

But RA couldn't be done like that, and now the RA axis moves very freely. I even went back later and re-tensioned after everything had settled, and was able to get a very tight, but free-spinning axis.

How much difference has this made? I don't have numbers yet as I'm still finding the right PHD settings, but so far things are more stable, it's easier to balance, and there's less play despite everything being much more aggressively tightened.

If you're like me, you really enjoy seeing all the bits of something that was taken apart. Enjoy!
Will this void your warranty? I would assume so. Could you un-do it? Probably. So save the original bearing and nylon washer, just in case...

1 comment:

  1. Thanks for documenting your upgrade, looks interesting.