Does the attached 'Pattern' of spikes suggest anything?

[gary]

Hi Folks:

The linked guiding log below includes a run of about 4 hrs from the night of 11/08/2025. The data is from an imaging run on a CDK 12.5 ota mounted on a Paramount MX+ with a 210mm guidescope and ASI 178mm guide camera ( I know, not ideal). A Calibration run and Guiding Assistance run both preceded this guided imaging session. After the first several minutes a repeating pattern of small Dec spikes paired, nearly simultaneously, with much larger RA spikes is observed. 

I realize the separate guidescope/camera is not ideal but it has performed adequately in the recent past with totals regularly within a range of 0.40-0.55" rms and without these repeated and paired spikes. Of late guiding has faltered (I've posted on this recently) and we are still trying to determine why the down-graded performance. The mount mechanics was just serviced and a new PEC training was performed - showing 2.0" peak to peak. I am about to redo a proper 300 point TPoint model and I hope that helps but I'm hoping someone here might have previously seen, and might be able to explain, the cause of this pattern of spikes. 

Thank you for whatever advice you could offer.

https://openphdguiding.org/logs/dl/PHD2_logs_y8X4.zip

[Bruce Waddington]

Inasmuch as the spikes occur in Dec, I would say it's unlikely to be coming from the mount - the Dec motor would normally be idle at these points unless you have a bad pointing model that's generating unnecessary Dec corrections.  Other than that, I would be looking at what's happening with all the gear that's riding on the mount.  The roughly 2-minute intervals between Dec spikes might correlate with imaging camera exposures, filter changes, automatic focuser movements, things like that - anything that might create a vibration or mechanical impulse that would be seen as a guide star excursion.  I don't know if this is a permanent set-up - if it is, the polar alignment error seems a bit large, > 5 arc-min.  If you have reason to think this is not accurate, then you will need to find an alternative explanation for the amount of Dec drift seen during the GA run.  Perhaps there is a single thing such as a cable routing problem that is contributing to both the spikes and the Dec drift.

Good luck,

Bruce