Weird dithering (or a mount problem)?

[vineyard]

Hello,

I noticed a strange thing last night.  Had the AP rig running.  Two hours on Fireworks Galaxy went fine until it went behind some buildings.  And so switched across to IC 410, and the guiding just seemed to go to pieces.  It would start fine, but then the RMS would suddenly become horrific and the whole image would shift w trails.  I gave up after 49 lights - frustrating b/c the night looked pretty clear - but while looking at Blink this morning, noticed that the trailed images were lights number 4,13,23,30,31,32,38,39,49.  Not quite a regular cycle but a few cycles of 9/10 dithers in there?

Yet the same mount had just done almost 2h on another target the same night w/o any such problems.

The only difference I can think of is that the Fireworks Galaxy was the scope pointing west of meridian, IC410 was the scope pointing east of meridian.

Has anyone ever had this before?  Is it a dithering problem, or does the mount need servicing or could it have been a balance issue - or something entirely different?

(Its an HEQ5Pro belted, & I was using EKOS internal guider with an ASI120MM mini on a ZWO mini guide scope. The main scope was a 4" frac f8.6 but reduced/flattened  0.79x to f6.8).

Cheers,

Vin

[PeterCPC]

When you switch from West to East you need to start the PHD guiding from scratch.

[vineyard]

Thanks @PeterCPC - I did that a few times.  (EKOS automatically had to recalibrate & restart the guider anyway when it switched to the new target of IC410, but I also forced recalibration of auto guiding a few times manually when I noticed this runaway-RMS, but this kept happening).  Perplexing.

[PeterCPC]

Sometimes balance can be off after a flip - I always balance for both. Have you checked balance pointing East?

[vineyard]

Hmm, good Q.  I thought I had checked both E & W on the RA axis but maybe not well enough.  And it was RA that seemed to go wandering on the guide graph.  Thanks - will try & be more careful next time - fingers crossed!