Jerking motion and oscillation issues with DDM85XL-A

[Keeble]

We have an ASA400 RC telescope, 40 cm F/8, with a DDM85XL-A mount. We have had several issues with tracking with the mount. One issue is a kind of jerking motion with the telescope, where the telescope will seemingly jump positions, resulting in double-lobed stars. Another issue, potentially related, is that in specific positions the telescope will start to rapidly oscillate. This happens when the telescope is looking close to the north celestial pole, oriented over the mount. We've tried rebalancing the telescope and have gotten a good balance in RA, but are limited in the Dec axis. The counterweight wants to be higher up than is allowed before it hits the base of the telescope, so there is a mis-balance there we can't seem to get rid of. The PID loops are below 0.5 arcsec and look ok.

Anyone seen anything like this? I found a post here that talks about a similar oscillation problem, and another post here that talked about the jerking motion, but sadly no solutions.

[robertp]

Hi,

have you tried to fine-tune the servo-settings at one of the specific positions where you get the oscillation?

It might also be helpfull if you could post a servo-log file from your ..\ProgramData\ASA\Autoslew\ClServoLogs directory from a session where you experienced the problem.

Best regards,

Robert

 

Edited November 15 by robertp

[Keeble]

On 11/15/2023 at 11:12 AM, robertp said:

Hi,

have you tried to fine-tune the servo-settings at one of the specific positions where you get the oscillation?

It might also be helpfull if you could post a servo-log file from your ..\ProgramData\ASA\Autoslew\ClServoLogs directory from a session where you experienced the problem.

Best regards,

Robert

 

We have adjusted the servo settings where we've seen these oscillations, and were able to eliminate the ones we saw. But we have continued to see this effect. Perhaps we pushed the oscillations to another location?

The servo log for one of our recent lobed star issues is attached. The telescope was also not tracking well and we had to re-slew to the target star, as it was drifting out of frame, in the direction of the lobed shape. The log is attached as 2023_11_8...txt. We started seeing the stars appear as lobes and the telescope drifting at 22:30, then eventually re-slewed to the target around 22:43. The logs don't seem to show anything out of the ordinary during this time period. The tracking was better after we re-slewed.

I don't have instances of the oscillating behavior during observations marked, unfortunately. The image I attached of the circles was taken when we were adjusting the PID loop so there is not much information in the log, but I'm including that here too. This log is 2023_7_26...txt. The circles image was taken at 00:31. I don't remember much about the rest of the observing session that day, so I can't say what was going on before or after that.

Let me know if I should include anything else!

2023_11_8_17h10m.txt 2023_7_26_20h2m.txt

[Keeble]

Any thoughts on this? I am losing data and would like to try some solutions.

[robertp]

Hi,

sorry for answering so late, I have been quite busy the last few days.

I had a first look at your log-files and did not find a real clue to what the problem might be. In the log from 2023_11_8 the average pos-errors while tracking look ok and the currents are not indicating any problems. The only thing that looks suspicious is the following:

If you look at the mag-angle of the Dec-axis (axis2)  after the slew at 18:38 you can see that at the beginning the mag-angle is 231 degrees. At 18:42 it changes to 232 degrees, at 18:49 to 233 degrees and at 21:56 back to 232 degrees. I would expect the mag-angle for the Dec-axis not to change during tracking (at least as long as you do have a decent polar alignment, an up-to-date pointing file and no MLPT or guiding activated).  I verified that in my log-files and always see no deviation of the Dec mag-angle.

Can you post a screenshot of your advanced pointing control screen?

In the log from 2023_7_26 I can see that between 20:00 and midnight before you did the tuning the position-errors were very high and fluctuated quite strong. While tracking, I seldom see a pos-error bigger than 0.06arcsec. The currents (especially of RA) are often very high, sometimes close to 5 amp, so it seems that the mount struggled hard to compensate the pos-errors. Aftter the tuning, the situation got better, but still rather high pos-errors. As this looks way better in the log from 2023_11_8, I guess that this issue was partly resolved.

One last question: In your first post, you mentioned that you could not fully balance the scope in Dec due to not being able to move the counterweight further up. Is that a special counterweight sitting at the scope? A photo of you setup might also be helpfull. Achieving the best possible balance is really helpfull with our direct drive mounts, so it might make sense to try to improve that.

Best regards,

Robert

Edited Tuesday at 05:49 PM by robertp

[Keeble]

No worries!

Do the log files track manual movement of the telescope? I suspect that the observer was manually slewing the telescope using the NSEW buttons on the Autoslew window, to try and align the telescope with our target. Could that be the cause of the changing Dec mag-angle? I will take a screenshot of the advanced pointing control screen and post it here.

When I was tuning the motors and balancing the telescope, I do recall that the motor signal would get very high when passing through these "resonance" positions. I saw this on the balance window - the yellow line would go off the screen, which was set at 7 A. It would also produce an audible hum. I believe in this specific instance the spike in amps would occur when balancing the RA axis. Adjusting the RA counterweight was able to reduce this effect, and I was able to get the telescope balanced in RA.

Photos of our setup are attached. The camera is not attached. It is about 1.5 kg, and attaches to the bottom of the telescope. The best balance I could get in Dec is shown in the attached file. Moving the counterweight up the shaft was not able to bring the two red lines closer, but it did push both more to the left (lower amps). I assume this is better, as the motor has to work less. The counterweight is flush against the bottom of the telescope and can't be moved any higher. Should I look at trying to add more weight?

[robertp]

That's a beautiful setup! The photos really help to understand your  issue with balancing. In my opinion, the inbalance is very high given the fact that the graph is set to 7ams full scale. It is very likely that improving the balance will help to get rid of jerky motions and oscillation.

What I don't understand: If you would have to move the weight higher, then the camera-end of the telescope is too heavy. So adding more weight to that shaft would increase the inbalance. So I see the following options:

* Move the whole telescope a little "up" in the saddle plate - I guess that 1 or 2 cm would be enough to achieve a great balance.

* Take off the DEC-counterweight completely or use a much smaller one.

I would recommend that you attach the camera and everything you normaly use in an imaging session (all the cables and stuff), rotate the scope to a horizontal position (scope and the RA-counterweight shaft horizontally), put the small DEC-counterweight in the middle of the small shaft and then balance DEC roughly by moving the whole scope in the saddle plate and then do the fine-balancing with the DEC-counterweight.

Let me know if I have a wrong understanding of the situation,

Best regards,

Robert

 

Edited 23 hours ago by robertp

[robertp]

One more comment: When balancing the axis, the goal is to make sure that the two red lines meet at the same spot - meaning that the maximum power indicated by the red lines is identical when moving the axis in both directions. It is not the goal to move the red lines as far to the left as possible. I would guess that if you have the dec-axis well balanced, the mount will draw about 2.5 to 3 amps when slewing which is absolutely fine given the high load and should not be a problem at all. During tracking, the current will probably be much lower.

 

[Keeble]

We can definitely look at moving the telescope to balance it. Do you have any manuals or diagrams that would show us a procedure for that, or which screws to adjust?

[robertp]

I guess the connection between the telescope and the mount looks like on the image below (or similar).

If you loosen the screws marked blue slightly, you should be able to slide the telescope within the saddle plate. I would suggest that you do it with at least 2 persons, move the scope so that the counterweight is pointing down and the telescope is in a horizontal position and be very careful not to loosen the screws too much, otherwise you risk the scope to create very expensive noises.

Be sure to really know what you are doing, otherwise get the support from somebody who is familiar with tasks like that or contact ASA.

If your setup is not similar to the one in the image below, please post some pictures showing the mount/telescope from all sides.