Strange Declination Behaviour with EQM35 Mount

[NigelHt]

I've recently started astrophotography, and after beginning with a DSLR and rotator haveprogressed to an EQM-35 GoTo mount and Zenithstar 73 telescope, with Ascom control and guiding with PHD2. While I've got some decent results, I've consistently had problems with guiding, even though I've tried adjusting the mount as described in online videos to remove backlash, and unbalancing the telescope. Initially there was obvious backlash that I could feel; I've now eliminated that, although I notice that the stiffness varies with declination (something I've seen reported by others), and it's almost impossible to avoid backlash at some declination angles without making the mount far too stiff at others.

While I'm aware that this type of mount is always prone to backlash issues, what I'm seeing is still different to what I'd expect. If I drive the mount, say, North (either manually or as part of calibration or guiding), stop so that everything comes completely to rest, and then drive the mount South, what I see is that it continues to move North for a while before beginning to move South as expected; the behaviour is exactly the same if I start moving South then switch to moving North. The attached image is part of a typical calibration from PHD2 Log Viewer to show what I mean. As you can imagine, this completely messes up guiding unless I offset the polar alignment and guide in one direction only, or align very accurately and switch off declination guiding altogether.

If this was simply backlash, I'd expect the mount not to move at all initially after switching direction rather than to move the wrong way, and I can't think of a mechanism to cause this behaviour. I'd be grateful if anyone can explain this, please, and give me some advice on how to cure it.

[michael8554]

Does your mount have its own Dec Backlash Compensation, and is it switched on ?

Michael

[NigelHt]

I believe there is an option for this, but I've not used it as this is what PHD2 advises.

Nigel

[michael8554]

Hi Nigel

Upload your GuideLog and DebugLog to the PHD2 Help Forum.

Instructions in your PHD2 Help menu

Be careful to select files with the correct dates in the filename.

https://groups.google.com/g/open-phd-guiding

Michael

[NigelHt]

I could do that, but the problem isn't with PHD2; if I manually drive the mount north, stop, south I see exactly the same "wrong way" movement.

Nigel

[michael8554]

If the Calibration is going wrong for reasons as yet unknown, then the PHD2 DebugLog might point to what's going wrong with the mount.

Michael