TheSkySearchers

Hi all,

Today my AVX is going under the wrench.

I have enjoyed this mount very much for the past three or four years.
DEC backlash Has always been problematic, measured at 13000ms with PHD2.
Usually though, my RA error is more than the DEC error. So, the total RMS error in PHD2 running the Explore Scientific ED102 scope was quite manageable at 0.7-0.8 arcsec.

Now that I am running the much lighter Sharpstar 61, the ability of the mount to correct for DEC reversals and mainly dithers in DEC has really exaggerated the issue. Depending on the intensity of the dither, it might take PHD2 30 to 50 corrections to get DEC back on track.

Since I would never get any imaging done at that rate, my dither recovery settings in NINA are pretty liberal.
My saving grace is that my image scale with the scope is 2.8 and even with the increased DEC error, my stars look really (amazingly) nice.

I’ve tried unbalancing the scope deliberately in DEC and see no difference in guiding. Still the huge DEC backlash and tons of PHD2 corrections.

I’ve had enough and today I’m going in to try to reduce backlash at the DEC worm gear.

I know they are lots of AVX users out there and many of you experience the same issues in DEC.

Has anyone tried adjusting the DEC worm?
If so, did you run into any issues?

Results?

Please take lots of pictures as this problem plagues every AVX owner.

I consider anyone who decides to "adjust" the inner workings of their mount to be very brave!

Good luck,

+1 on the pictures, Jim. I tried adjusting the worm on my ASGT, which is similar to the AVX, about three years back. IIRC correctly there is a fitting that has three hex nuts to adjust the worm gear to the teeth on the DEC cylinder. The two outer ones are for loosening and tightening the entire assembly, the center one does the actual adjustment. Backing the center screw out makes the mesh tighter, and turning it in makes the mesh looser.

The trick is to tighten it to remove some of the backlash, but not to make it so tight that the gear binds at any point in the rotation. My procedure was to loosen the two outer screws, make a SMALL adjustment to the center screw, then tighten the outer screws again. I then ran the dec hand controller through as much of a 360 degree rotation around the dec axis to check for binding.

I know I was successful in making the dec slewing louder, and PHD2 reported improvement in DEC RMS error in use, down to where I usually average in the .3 arcsec range (before adjustment I was usually in the .8-1.2 range). I still can't get the guiding assistant to measure dec backlash successfully, and dither still takes a long time to recover, but dither time-out doesn't seem to affect dec guiding or my images in any way. It's like PHD2 long pulses don't work correctly to recover from large errors, but short guiding corrections are quite effective. Also I haven't had to redo my calibration in 2 1/2 years!

Good luck with the adjustments, please share results (and any links you have found to help guide you through the process!).

Bob

Have you tried the backlash compensation? Take a look at how that works, it is quite impressive! I know some people advise against it because if the settings are not right it can cause chatter, and these optimal settings may be position dependent (though I haven't noticed it). The advantage of the AVX backlash compensation is that it is many times faster than PHD2 because PHD2 cannot command it faster than the (very slow) guide speed.

Another idea - since you probably have a ton of PEC to deal with already - is to have DEC be slightly off so corrections are only in one direction and you don't get all that chatter.

What I have done for my G11 is implement spring loaded worms. The G11 is nice for that because it can all be opened up. Not sure how that would be done with an AVX.

Yes I have adjusted the worm. The mechanism is quite simple and easy to tell when it's too tight or too loose. I don't think a lot can be gained there unless it's way off.

SkyHiker wrote: ↑Fri Oct 15, 2021 4:28 am Have you tried the backlash compensation? Take a look at how that works, it is quite impressive! I know some people advise against it because if the settings are not right it can cause chatter, and these optimal settings may be position dependent (though I haven't noticed it). The advantage of the AVX backlash compensation is that it is many times faster than PHD2 because PHD2 cannot command it faster than the (very slow) guide speed.

Another idea - since you probably have a ton of PEC to deal with already - is to have DEC be slightly off so corrections are only in one direction and you don't get all that chatter.

What I have done for my G11 is implement spring loaded worms. The G11 is nice for that because it can all be opened up. Not sure how that would be done with an AVX.

Yes I have adjusted the worm. The mechanism is quite simple and easy to tell when it's too tight or too loose. I don't think a lot can be gained there unless it's way off.

Like Henk, I support use of backlash compensation. I found that it was at least as effective (if not more) as trying to tune out the backlash mechanically. I know it is not recommended by PHD2, but I find it reassuring to hear that little "click" when DEC reverses direction while guiding. That was the change I made that has allowed me to keep re-using my PHD2 calibration data for all this time.

Bob

Hi everyone and thanks a lot for the responses. Thanks Bob @bobharmony jaytee @JayTee and Henk @SkyHiker

This endeavour ended up being a big fail.

Sorry for this long ramble, but I just wanted to share this experience. Well, since it turned out poorly, I really didn’t want to share, but need to.

I have been having issues with the mount for some time (really always). The issues were not as critical early on because I had bigger fish to fry learning AP.

DEC was always an issue, but RA was much more problematic. DEC never did recover well after a dither or a reversal, but it was usually only slightly amount out and even though it took many, many corrections, the DEC error in PHD2 was always lower than the RA error.
In other words, I could live with it even though is did have some impact on my stars (due mainly to the RA error).

Recently, DEC error and recovery were unacceptable. Guiding total rms error was running 1.5-2.0 arcmin/sec where it had been running 0.7-0.8 a year ago. Most of the increase was related to DEC now. Still had the same non-periodic spikes in RA.

My last night out imaging, I had reduced the dither px to the minimum (2 px at the imaging sensor) and I still had the big DEC errors because the mount would not recover from the dithers. The data was useless since it was full of pattern noise.

I have always been pretty handy, so I had to do something and started by removing the worm gear assembly, it looked pretty simple.

I have always had a really hard time balancing the small scope (Sharpstar 61) in DEC because the scope is light and there was so much friction in the DEC axis. I had always read that there was way too much grease inside which contributed to the axis stiction, so I removed the DEC cylinder.
It came apart quite easily. Two machined journals on the cylinder that mated up to the opposite counterparts in the barrel. No bearings. Tons of grease.
I cleaned it up with mineral spirits, dried, greased liberally, wiped it off, and greased very lightly. Big mistake.
I slipped the cylinder into the barrel and snugged up the nut on the bottom. I re-mounted the saddle.
The assembly spun better than before, except there was a bit more friction in one part of the revolution. I kept spinning the cylinder thinking that the grease needed to be distributed better. I probably spun the cylinder 20-30 times and it started to bind up. The more that I tried to free the cylinder, the worse the binding got until it was completely immovable. It was so bound up that I had to use extraordinary measures to free the cylinder from the barrel. The binding/freeing up of the two components caused extensive galling to the cylinder and barrel journals.

So, the mount is pretty much junk at this point. Huge bummer.

It took extensive sanding/polishing of the journals to get the assembly back together. Of course, hand sanding machined journals is never a good idea, but I had little to lose at this point.
Upon re-assembly, there was much more friction than before and kind of gritty feeling. Not smooth. Not surprised, but together so far.

I am hoping that I can at least get it usable enough for visual, but I would be very surprised if that worked out. The whole DEC rotating assembly is not smooth at all.

I am now where I was originally planning to start. DEC worm gear adjustment.
Just messing with the screws is really a touchy and tedious procedure. Kind of sloppy! It seems really kind of hit-and-miss whether I can get it any better than it was before, especially now with the damage to the DEC assembly.

For anyone interested in the AVX DEC worm gear adjustment, this is the link that I will be using. https://www.cloudynights.com/topic/7019 ... stment-20/

I do plan to actually mount a scope to this poor beat up AVX and see if it will at least slew and perform visual operations.

I will update with finding/results. And, will be looking for a new mount.

Jim, sorry to hear that! I had mine bind up horribly like yours. I sent it to Celestron for repair. While the service was not super fast I got it back in a few weeks. It was repaired well, and the cost was quite reasonable. It rotated much better than usual so they my have given it some extra TLC. If your mount has not been totally junked, I suggest you try that.

SkyHiker wrote: ↑Sat Oct 16, 2021 12:14 am Jim, sorry to hear that! I had mine bind up horribly like yours. I sent it to Celestron for repair. While the service was not super fast I got it back in a few weeks. It was repaired well, and the cost was quite reasonable. It rotated much better than usual so they my have given it some extra TLC. If your mount has not been totally junked, I suggest you try that.

That is very helpful Henk and thank you very much for sharing that with me.

I don't think that it is totally junked as I have gotten it back together. But, it is in a lot rougher shape than it was when I attempted repairs/adjustments. Plus, really, the mount had significant issues when it was running well and has degraded a good bit recently.

I "somewhat" have the go ahead to replace it since I rendered it "damaged", so I will probably go for a new mount rather than spend the money on shipping/repairs on this mount that was troublesome anyway. I don't plan to spend a bunch on a new mount, but I plan to get a more reliable mount like maybe a HEQ5 or ioptron gem28 (see thread viewtopic.php?f=24&t=21203).

Thanks again Henk. I really appreciate you sharing your experiences.

Jim, I'm so sorry this happened. Like you, my AVX is relegated to only visual use.

JayTee wrote: ↑Sat Oct 16, 2021 2:17 am Jim, I'm so sorry this happened. Like you, my AVX is relegated to only visual use.

Thanks JT. I had to try. Things had gotten just too sloppy.

I did get many nice images with it though!

Lots of good learnings. Hopefully, the next mount that I choose will do a better job.

I am sorry to hear you had so much trouble, Jim. It makes me kind of relieved I never did the grease cleanup on the ASGT.

Hopefully the new mount will give you many years of service once you have it in hand!

Bob

I know this is out of your price range, but I just took delivery of an iOptron CEM 70. In the coming days I'll be posting the unpacking and rejoicing from having this new mount.

As it turns out, maybe the AVX is a good training and learning for AP equatorial mount.

JayTee wrote: ↑Sat Oct 16, 2021 7:09 pm I know this is out of your price range, but I just took delivery of an iOptron CEM 70. In the coming days I'll be posting the unpacking and rejoicing from having this new mount.

As it turns out, maybe the AVX is a good training and learning for AP equatorial mount.

Thanks JT!

Yeah, I am looking seriously at the Skywatcher HEQ5 and the Ioptron GEM28. The HEQ5 is tried and true and tough. The Ioptron has some differences in clutch usage and mounting from the HEQ5, but from what I have been reading, the guiding performance is outstanding.

I will probably pull the trigger early next week and I am trying to get feedback from anyone that has either of these two mounts.

Looking forward to hearing from you about your new CEM 70! It certainly has been a long time coming!

For me, I don't plan on a bigger scope than the ED102 and the GEM28 is perfect. Especially with its 10 lb mount head weight! I'm getting older JT!

Juno16 wrote: ↑Sat Oct 16, 2021 7:39 pm I'm getting older JT!

You and me both. We are the same age but I feel that my mileage has been hard miles. I can't even use a straight through finder scope anymore. My neck just doesn't do that anymore.

I have recently started using my AVX again with the ASI2600MC on the Redcat51. I actually like it a lot for that work. I can lift the whole mount with telescope and all around so setting up is super easy. I used to just autoguide the AVX remotely but I now also run it with Ekos which lets me plate solve, polar align, image and autoguide via the serial port. So my AVX is getting a second life and I enjoy it a lot with the Redcat. If you get the GEM28 there's probably no reason to have the AVX repaired. I presume you have ordered it by now, please keep us posted.