User reports oscillation following dither

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I have been using Sequence Generator Pro and PHD2 for several months now and 
overall it is a fantastic combination. I'm using an OAG Lodestar via ST4 to a 
Paramount MX and guiding with the mount connection enabled too. Automated 
Meridian flips work fine. I'm using the low pass filter option for DEC at 
default settings and default for RA. I dither my exposures. During the 
calibration process the response to N-S reversals is immediate so I don't think 
I have appreciable DEC backlash.

Here is the problem. I have noticed a Jekyll and Hyde effect.  Over the course 
of every imaging session I can divide my subs into two categories; those with 
flatline tracking, 0.3" RMS or better and those that oscillate, maybe 1.5"-2" 
peak to peak.  Only one in a dozen is oscillating and  most of these frames are 
usable, there is room for some optimization.

I think it is PHD2 because:

The oscillation continues for an entire subframe, until the next exposure / 
dither.
It starts immediately after a dither, not during an exposure (however long)
If I stop the guider and immediately restart it, it fixes the issue.

Things I have tried and appear to make no difference:
Turn fast recenter on and off
Changed DEC guiding parameters - slope in low pass mode and tried resist switch 
Pulse guide / ST4

I have yet to try:
Continue guiding through all SGP processes (focusing / download etc)
Turn dither off
Continually turn the guider on/off to see if I can trigger a bad 'start up'

It seems that the algorithm gets into a cycle that is just sends the mount into 
overcorrection a classic stability issue. Stop/starting the guiding off again 
seems to fix the immediate issue. That and the fact I have never seen it go 
unstable during an exposure - strongly suggests it is not a disturbance (wind)  
or  mount issues.
This image below shows the guider trace - it had been oscillating since the 
dither, I started to change slope values, no difference and then you can see 
where I stopped and restarted the guider in PHD2.

Ideas:  something in the dither / restart routine has a random behavior - or a  
corrupted start up value. Can we have a Maxim - like algorithm - not so 
intelligent - that simply issues a guiding command which is a proportion of the 
last tracking error value?  This would also be useful for diagnostics .

This screen capture occurred at 22:10 in the guide log (enclosed):
Any ideas?
regards
Chris

Original issue reported on code.google.com by br...@boneheads.us on 30 Oct 2014 at 1:48

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I have added this onto the forum but not sure if it should be copied here too.
Here are the two files - overall the difference between good/bad was not as 
striking as it has been on prior occasions but it serves to illustrate the 
point.

One thing I learned from the debug files is that I need to create dark 
libraries for each optical configuration - I'll know for next time.
Image exposures were 8 seconds. I was dithering between frames but autoguiding 
is not paused during download. Image exposures were at about 5:20 minute 
intervals.

A dither occurred at 20:49 and you can see the response became suddenly worse 
and continued -  through subsequent sub exposures / dither. This is new 
behavior and did not occur if I paused the guiding during image download. The 
issue in those cases was limited to a single sub.

I did a stop/start at 21.51 and the guider response went good again.
I noticed the RA was a bit reactive to seeing noise and I changed the settings 
to 60/40 at 21.55 and then to 40/20 which seemed to be ideal with a 0.15 min 
move.
Hope this helps:

https://www.dropbox.com/sh/teee18sxtzpbv1s/AADT1ekJRxM0iliA2mVH7LVia?dl=0

Original comment by ch...@beyondmonochrome.co.uk on 3 Nov 2014 at 12:21

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I haven't had time to look at the code, so this is pure speculation on my part, 
but I can't help wondering if this isn't somehow related to the guide algorithm 
getting confused by the dither.  For example, what if PHD2 was working on 
moving North (i.e. star was too far south) and the dither moved the star North 
so that it was just a tiny bit south, but because of the stored data indicating 
that PHD2 was trying to move north, it now overshoots.

I can't explain how it then overshoots the other way, but...

They question I'm trying to answer is what is different about dithering than 
about the star just moving because of a mount problem (which PHD2 seems to 
handle well), and it seems like the only real difference is that dithering 
moves the motors in an effectively random way.

Original comment by br...@boneheads.us on 3 Nov 2014 at 2:23

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Chris reports the problem solved after using the development build that clears 
the guide algo history after un-pause and dither.
Marking issue as fixed.

Original comment by andy.gal...@gmail.com on 10 Nov 2014 at 6:06

• Changed state: Fixed