PHD2 gone wonky on me in guiding

[Mark Ellis]

Ok it seems PHD2 has gone off the deep on on me. Over the last few nights I have been fighting with it. Goes great and that all of a sudden it goes wonky on me and I can't figure out where the issue is. I have even gone and removed it and re-installed it. Run calibration, Guiding assistant. PA is awesome at .09" I have verified the setting in my control software (Green Swamp Server) I have even shut down GSS and restarted EQMod and verified it's settings. I don't use the hand controller, but I did plug it in and ran through the setting to make sure they were good. 

I'm at a total loss here and only know the basics of PHD2. It has don't me good. If someone can help I'd really appreciate it. I just hope my mount isn't what's going wonky on me.

https://openphdguiding.org/logs/dl/PHD2_logs_Vj8s.zip

[Bruce Waddington]

Hi Mark.  I wouldn’t say PHD2 has “gone off the deep end” and, as far as I can tell, all the software is probably working fine.  To steal a phrase from Billy Shakespeare, “The fault, dear Brutus, is not in our stars / But in ourselves”.  J  I assume the problem you’re talking about is this (red is RA, green is Dec):

 

 

In the preceding 5 minutes, you were guiding along fine and then the guide star made an abrupt plunge to the north (red arrow).  It’s important to see that the giant error was in declination because the Dec motor wasn’t even running at the time the excursion started.  Therefore, it isn’t a PHD2 problem and it isn’t a problem with the mount’s drive system – it’s almost certainly caused by an external event.  At the time this happened, you were pointing at Dec = 61 degrees after having spent the earlier part of the evening at much lower Dec positions.  So there are a few possibilities that come to mind:

 

1.       The guide camera/guide scope shifted position as a result of the change in pointing position – this could come from any mechanical interface point in the whole guiding assembly (there will be many of them)

2.      You have a guide cable that is pulling on the guide scope or the main OTA as a result of the pointing position.

3.      The main OTA is substantially out of balance in Dec and the clutch isn’t able to hold things in place reliably

 

Those things are under your control, not anything the software can know about or deal with.  You were also quite close to the meridian at this time although not yet close to crossing it – these pointing positions frequently create problems with cable routing.   I suggest you trouble-shoot this by moving the scope back to this area of the sky (HA about 1 hour east of the meridian, Dec = 61) and look very closely at the mechanics of the cable routing you’ve done and how rigidly the guiding assembly is holding things in place.

 

On another front, it looks like the mount has a huge amount of Dec reversal delay so any kind of direction reversal is going to take a very long time to recover.  Here’s a result of one of your backlash measurements:

 

 

The red line shows the north moves and the green lines shows the south moves.  You can see there is big flat spot in the green line, showing a period when 20 consecutive guide pulses south of 850ms weren’t able to move the Dec axis at all.  This is why the Guiding Assistant keeps telling you that you’re probably going to need to use uni-directional Dec guiding unless you can improve the mechanics.  But again, this could be caused by major imbalance of the OTA in Dec.

 

Good luck,

Bruce

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