wondering about the wandering

[David Ridge]

I have been experiencing sub .70rms guiding fairly consistently but on occassion, my mount just refuses to settle down, zigging and zagging for 30-60 seconds it seems. recent calbration and guiding assistant runs successful but for no apparent reason, it just goes into what appears from the guiding graph to be a dithering but during the middle of an exposure, several exposures in a row! ARRRGH!

Thoughts?
https://openphdguiding.org/logs/dl/PHD2_logs_UpgJ.zip

[Bruce Waddington]

This is really starting to feel like Groundhog Day.  All of your current problems occur when the mount has to be bumped and is apparently responding poorly to guide commands.  Your good stretches of guiding happen when the AO is able to control everything without bumping.  A typical guide session looks like this:

In all cases where the wild oscillations are present, the mount was trying to handle bump corrections.  So let's roll the clock back to June when I said this:

"It doesn't seem to me that you're making any progress with this.  I suspect the underlying performance of the mount is so bad that using an AO isn't a viable option.  An AO is not a solution for a bad mount because it has a limited range of motion.  The only way to test this hypothesis is to do extended testing of the mount without the AO.  Continue to use the OAG but use it without the AO.  If you follow the attached procedures for getting a measurement baseline, it should provide more insight into the problems.  You need to do long-enough guiding sessions on both sides of the pier to demonstrate whatever problems the mount has.  After you've created the new profile using the new-profile-wizard, do NOT change any of the default guiding parameters.  Be sure you specify the correct focal length of the system in the profile wizard in order to avoid wasting a lot of time. "  

Did you ever do this?  I can't find any follow-up from you showing the results of the baseline measurements without the AO involved.  I've seen messages about improving balance and other assorted "ah-ha" moments, but I still question whether you've been able to get this 30-year-old Temma mount working properly.  Has it been cleaned and lubricated regularly?  Has it been used frequently during the 30-year period or is it likely the bearings are corroded and not working right?

Regards,

Bruce