Huge oscillations in RA near zenith

[Bob Powell]

Generally very good (<.4") guiding last night.  As the scope approached the zenith and also after the flip, I experienced several episodes of huge rapid oscillations in RA.  I was able to correct by going into looping, re-selecting a star and resuming guiding, but after a bit, it started again.  Almost no wind, no cloud cover, and no cabling snags. Very odd and never have seen before.

Wave150i, GSS, ASI120mm Mini on ZWO 30F5

Appreciate any insights.

Link to logs:  https://openphdguiding.org/logs/dl/PHD2_logs_9eKZ.zip

[Bruce Waddington]

Hi Bob.  I think we need to back up a bit and re-characterize the problem.  First, your guiding is more like 0.6 arc-sec total rms during the periods when you aren't getting these wild RA excursions.  Since you were pointing at Dec=40 degrees, the stats are understating your RMS guiding error by about 25%.  I only bring this up because I think it's important to have well-founded expectations about what the mount can deliver.  Next, I can't see that the huge RA excursions have much to do with approaching the zenith, I think the problem is quite different.  Here's a big-picture look at the RA guiding behavior during a 55 minute period starting at 00:47 - RA is shown in red:

The striking thing here is the obvious periodicity of this wild behavior - it's about every 14 minutes.  Note that this session started while pointing nearly 30 degrees east of the central meridian and ended still pointing about 15 degrees east.  The subsequent guide session remained in the same part of the sky from side-of-pier west but didn't show this behavior even as the scope tracked another 5 degrees closer to the central meridian.  The behavior shown in the graph isn't being caused by guiding although once the problem starts, guide commands may be making it worse.  I think you should take this up with mount manufacturer and see if you can get some help. Another possibility, I suppose, is that you aren't using a counterweight when one is needed or the balance in RA is otherwise out-of-whack.  Obviously, there's no way for us to tell.

Good luck,

Bruce