Celestron advanced VX - Suggested parameters to improve backlash

[Marco Malziotti]

Good morning
My setup includes a Celestron Advanced VX mount that I control from my PC using the mount's control panel and AScom drivers.
For guiding I use a 120mm scope and a camera with 3.75 micrometer pixels.

I ran the PHD2 Guiding Assistant twice: once pointing a star to the east (see the graph from 22:26:46 to 22:38)
and once pointing a star to the west (from 22:47:36 to 22:58).
For both tests, I used the following parameters in PHD2 "Advanced settings":

Global tab:
- Dither Settings: mode = Random / Scale = 1.0

Guiding tab:
- Assume Dec orthogonal to RA = disable
- Use Declination Compensation = enable
- Guide speed n.nn x sidereal = 0.9

Algoritm tab:
- Backlash Compensation = disable
- Declination guide mode = Auto
I guess I'll need to enable "Backlash Compensation", can you tell me what values I should enter in the fields:
- amount
- min
- max
Besides these three parameters, do you recommend changing any other parameters to improve the declination backlash?

I attach the two PHD2 files, some screenshots of the settings screen and the driving assistant.

Thank you and clear skies.

Marco Malziotti

[Bruce Waddington]

We can't really tell you anything useful from this data.  All of your GA sessions and all but one of your guiding sessions were done with a calibration that was done 7 months ago.  It's very clear that the guiding camera assembly has been rotated by a large amount since that time, so none of the data are usable before the new calibration you did at 23:15 on Sept 18.  The guiding session using the new calibration was only 12 minutes long but even that shows your mount has a fairly huge amount of Declination backlash.  I suggest you start over following the attached procedure, being sure to use the Calibration Assistant to do your calibrations and then collect enough data that we can help you see what's wrong.  It would be advisable to do all this without imaging and dithering, just run PHD2 manually to complete the procedure.  Under normal circumstances, the Guiding Assistant will make recommendations for PHD2 Dec backlash compensation settings but only if the total amount of Dec reversal delay is less than 2-3 seconds.  If that isn't the case, it will recommend that you use uni-directional Dec guiding or find a mechanical solution to the problem.

Going forward, please use the documented procedures for uploading your PHD2 logs rather than attaching the files to your forum messages.

https://openphdguiding.org/getting-help/

Regards,

Bruce

[Marco Malziotti]

I'll try again. I followed the instructions requested, and I'll ask the same question as in the previous post (see X's email dated 27/9/2025 - I have not changed the settings),

"Besides these three parameters, do you recommend changing any other parameters to improve the declination backlash?"

This time attaching the logs as required by the procedure. Thanks, and clear skies.

Marco Malziotti
https://openphdguiding.org/logs/dl/PHD2_logs_amVd.zip

[Bruce Waddington]

Hi Marco.  We're still working with limited data here so we're limited on much advice we can give you.  The reason I say that is as follows:

1.  You didn't use the Calibration Assistant to do your calibrations and you were calibrating in a sub-optimal part of the sky.  Perhaps you have sky obstructions at your site but is it really the case that you can't get closer to the celestial equator than Dec = 57?  You can and should use the CA for all your calibrations even if you have limited sky visibility.  You can tell the CA to slew to whatever location you can use, just get as close to Dec=0 as possible.

2.  Once you finished with the Guiding Assistant, you didn't do long enough sessions for us to get a clear view of your RA performance.  You did one at 21:19 that lasted for 15 minutes but only a minute of that had usable guide stars.  The next guiding session lasted only 6 minutes which is too short.

Setting that aside, the calibration didn't produce any alert conditions and looks pretty reasonable.  The GA backlash test you did produced a result that looked like this:

The red data points show all the north moves, the green points show the response when the direction was changed to south.  You can see the axis basically didn't respond at all despite getting 27 consecutive guide pulses of 1000 ms south.  

This points to a large amount of backlash on the Dec axis, too large to control effectively with PHD2 Dec backlash compensation.  You need to make some mechanical adjustments to improve this, something that has been done by hundreds of Celestron mount users over the years.  The GA measurement is probably an over-estimate because it forces the gear train to be fully engaged in the north direction - but it's still a big problem.

Earlier in the night's session, you had several incidents where the connection to the guide camera was lost, then recovered.  This is another problem you will need to sort out - it's typically caused by a poor USB cable or connection.  Although you seem to be focused on the Declination backlash, the short 6-minute guiding session at the end indicates the RA tracking errors pose a larger limitation to your results than Dec.  But it will take more consistent and longer testing on both sides of the pier to get a better view of this.  It looks like your RA system may have a periodic error of up to 40 arc-sec peak-peak, but again we need more data to get a better estimate.

Regards,

Bruce

[Marco Malziotti]

A few questions before trying for the third time. What does "GA measurement" mean? Where do I see the number of usable guide stars? How long does the guide session have to be? Does it have to be entirely dither-free? Thanks.

Marco Malziotti

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