Strange tracking behaviour with AZ EQ6-GT

[MikePGPSW]

Over the last few nights I have been able to get out and get some quality time with my setup.  

I have been happy with the AZ-EQ6 for a the 18 months or so that I have owned it, but recently I bought an ATIK 460EX camera and added it to my rig.  When I am imaging I notice the the stars are distorted, it is not coma as it occurs all over the image.  The symptoms are that the stars seem to have spurs on them, and not just one but sometimes 2.  This does not seem to be related to the length of exposure nor to the equipment in the image train, as I have used it with luminance and Ha filters with the same result.  Pointing at different areas of the sky doesn't seem to make any difference.  The effect is not dramatic, but can be seen quite clearly at 100% and is exaggerated at 400% as shown in the attached image.

My first thought was polar alignment, but this has been checked and is good.  My next culprit was (and still is) the guiding setup.  Last night I carefully checked the guiding and sure enough there was a 1 pixel jump every so often.  At first I thought that this was the RA and that what I was seeing was periodic error, then I realised that the adjustments were being applied on the Dec axis as can be seen from the PHD2 screen shots.  Even so the adjustment being made equates to less than a pixel and is corrected within a fraction of a second.  

I am perplexed at what can be causing this, any suggestions would be gratefully received.

Guiding is done with 0.2 second exposures with PHD2 connecting to EQMOD which provides the move commands to the mount.  The EQMOD configuration is below:

Equipment used:

Skywatcher AZ EQ-6 GT

William Optics GTF-81

William Optics 2" extender

Atik 460EX Mono camera with Baader Ha and Baader Luminance filters

William Optics 50mm Guidescope

QHY5-L Mono guide cam

All equipment has been checked for play and the scope is properly balanced on the mount.

[rob0971]

I thought you had to expose for 1-3 secs so you don't chase the seeing.

[Thalestris24]

Yeah, 0.2s  is much too short!

[johnrt]

What is the arcsec/pixel ratio of the guider setup and imaging camera? I would agree 0.2 sec guiding exposures are way too short, 3 sec minimum.

[Owmuchonomy]

As above plus I would switch the Dec guiding off if your PA is good and see what happens then. It may be gunge or swarf in your Dec drive.

[Xplode]

Do you see the same problem if you aren't guiding? 

[MikePGPSW]

Thanks for the replies folks.  I changed my guiding to 4 seconds and saw a marked improvement.  Imaging some distance away from the meridian the guiding is fine, but at the meridian there was some dec drift.  As we have had some good clear(ish) nights I had been able to experiment a little and discovered that my Polar Alignment was not as good as I had thought.  

It turns out that my polariscope needs to be more accurately calibrated.  It is not far off, but obviously enough to make a difference for my tracking and imaging.  Unfortunately I cannot just drift align and leave it as it is a portable system that is stored in the house.  Yet another reason I need to invest in an observatory...

[MikePGPSW]

Do you see the same problem if you aren't guiding? 

Hi Ole,

If I am not guiding I see some drifting which is obviously down to the polar alignment.  I will be working on trying to correct that over the next few days of clear nights to see if I can get a more precise setup.