Calibration issue

[Santhosh Palani]

My calibration plot looks strange with clustering of points in the Dec axis. When guiding I also see Dec spikes. What could be the underlying reason? I did a longer guiding assistant run to troubleshoot but can't see anything obvious. Thanks for your help.

https://openphdguiding.org/logs/dl/PHD2_logs_J5AX.zip

Best,

Santhosh

[Bruce Waddington]

I would say you have a significant amount of resistance on the Dec axis.  This could come from a significant weight imbalance, cable routing problems, or from static resistance (stiction) on the axis.  The latter is most often caused by a drive system that is too tightly meshed, perhaps in an overly aggressive effort to eliminate backlash.  During guiding, each guide pulse starts the motor for a short time period which essentially pumps a small amount of energy into the system.  Under normal conditions, this energy is immediately translated into a tiny rotation of the axis.  But if stiction is present and the gears can't move freely, the axis may not move at all, in which case that energy is stored elsewhere - in mounting blocks, bearings, wherever.  After some number of consecutive guide pulses in the same direction, the static resistance is finally overcome and the axis suddenly starts to move.  But that also releases the stored energy in the system, and the axis moves according to the size of the last guide pulse plus the amount of stored energy.  You can see this fairly clearly with your calibration graph - nothing happens for a few guide pulses then the axis suddenly moves by an unexpectedly large amount.  This can also explain at least some of the large Dec excursions.  In one case I can see, PHD2 issued a series of Dec guide commands in the same direction for a full minute without any appreciable effect - and then the next guide command, still in the same direction, triggered a large movement of the axis.  Fiddling around with guiding parameters isn't going to help any of this and is more likely to just make things worse - I think you need to pursue improvement of the Dec axis mechanics.

Good luck,

Bruce

[Santhosh Palani]

Thank you for the detailed explanation Bruce. I think you are spot on. I have an EQ6R mount. Based on some YouTube videos, I played around with the outside Dec axis screws yesterday to see if I can eliminate the stiction. Last night, I redid the calibration and got a slightly better graph (please see the one at the time point 22:24:44). I also imaged NGC6960 for 2 hours each on both sides of the meridian. The guiding error is still large. It would be helpful to hear if you have any further suggestions based on this new data. 

https://openphdguiding.org/logs/dl/PHD2_logs_Km74.zip

Best,

Santhosh

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[Bruce Waddington]

I would say there was some improvement but I still see the same behaviors, just not as severe.  I think you still need to revisit the Dec axis behavior.

Good luck,

Bruce

[Chuck Koos]

I think all EQ6R-Pro mounts are stiff in both dec and ra. Mine were. Sky-Watcher tightens the trust bearings way too much.

The dec is super easy to adjust, it takes about 5 minutes. ra is doable but isn't nearly as quick and easy. Watch this guy's video: https://www.youtube.com/watch?v=jZ4LxtE6TzU

[bw_m...@earthlink.net]

Thanks for posting this, Chuck.  It’s always best to hear from people who have experience and insight into the specific kind of mount that is being discussed.

 

Cheers,

Bruce

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[Santhosh Palani]

Thanks Bruce and Chuck. I will give this a try. 

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