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. Thanks for documenting your upgrade, looks interesting.

  2. Dang, I made a comment, and apparently it didn't post. Okay, I'll try again.

    First, thank you VERY MUCH for doing this, and documenting it for other AVX users. So many people hate the mount, but if taken care of, it's not a bad mount. Your procedure helps it immensely.

    Okay, a couple of things I saw in here.

    Tensioners. For the tensioners in both R.A. and Dec, plus the two on either side of the Altitude adjustment, I backed off the Allen screws to the point where I could Loctite them. Use BLUE Loctite! This allows me to ease off the tensioners so that things moved freely. The problem you had with your R.A. tensioner is that you may have missed the three Allen screws around it. There is a hole on the side, but only one, like on the Dec Axis, and you need to rotate the R.A. axis around to get to all three.
    Your 'bicycle' tool probably would have worked there as well if the Allen screws had been loosened. Instead of your tool, I simply took a pair of long nosed needle nose pliers, and put the tips into the tensioner and backed it off. Same thing to reinstall, after getting it to the point to tighten it.

    You didn't make ANY mention of lubrication. Yes, there is a ton of grease inside of this thing, except on my R.A. axis, which there was almost none. I use a white Lithium Grease. It sprays on, and seems to work well

    Regarding the tensioners, I did not put them back on as tightly as they were on my AVX. They are snug, but not tight. This allows for easier movement in both R.A. and Dec.

    The ball bearing on the R.A. axis seems to have two plates, one on either side. Initially, I thought it was a part of the bearing, but now I'm not sure The other bearing on the R.A. axis I was unable to remove, so I simply sprayed some Lithium grease on and under it in the hopes of getting it on there.

    All in all, I'm very pleased at how this came out. It took me about three hours to complete, and now, with my small and lightweight William Optics ZenithStar 71, I can actually balance that! In fact, I'm going to have to attempt to put a 14" Celestron Vixen plate on there to allow me to get it far enough forward to balance it! I have not been able to do this before! R.A. also balances a LOT easier now.

    So, again, thank you on behalf of all of the AVX owners out there. I'm so pleased with this that I've posted the link over on Cloudy Nights in both the Celestron/NexStar forum and the Beginner and Intermediate Imagers forum.

  3. Very useful tutorial, and neat work, overall :) Thanks for sharing!

    What can you tell us about the resulting tracking precision of the mount after this operation? What is the difference in PE - tracking only (non-guided, non PEC-ed)?

    Also, what are the dimensions for those big ball bearings for the RA shaft? The one you mention, with that code, seems to have 35mm ID and 59mm OD. Judging by the extra ring you machined for the cone bearing, the casing ID, where the OD of the bearing should fit, must be @ 62mm - and i saw that there are 35x62x18mm cone bearings. If i buy one of these, would it fit nicely?
    Or, can i use a thin shim over the inner ring of the original ball bearing, wide enough just to stay away from the outer ring of the ball bearing? I know the two types of bearings take the load differently, but still.. it might just work better than the original solution (which isn't a very "happy" one).
    I'm considering this solution, with the shim, instead of a cone bearing, so i won't create a gape in the mount casing at reassembly. And yes, i think i need to tune my AVX too, if i want to improve its capabilities. With a +/-27" PE for a brand new 1000$ mount, i think it can and should perform a little better than just that, imho.. And PEC didn't do any miracles yet - at best it managed to cut that error in half.

    Thank you and clear skies!

  4. Nice AVX teardown.....essentially the only one in existence!

    While I didn't find any rocks, there were chips and a small chunk missing on the DEC housings mating surface. So I patched and polished all the surfaces then cut some 1/64" PTFE shims to replace the nylon. Most recently I drilled and tapped the counterweight hole at 3/4-10 and added a 1x18" counterweight bar. The factory thread had become worn resulting in quite the wobble. The list goes on....

    I to suffered from periodic bootloader errors and eventually discovered the RJ6 pins in the controller jack were depressed, causing intermittent contact. I went through all of the jacks and pulled the pins up and haven't had a problem since.


  5. Now that youve had this a while how is tracking, guiding, overall satisfaction? Curious minds like to know :-)

  6. Informative and interesting Blog! Beautifully written, as usual, I like the post. Thank you so much for nice sharing with us. Keep posting!
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