[JohnH]

What I am seeing is mostly very small errors/corrections. The attached plots show dx/dy in pixels at a scale of about 3.7" per pixel with an exposure time of 3s.

But are these large spikes normal - would it be seeing related? They do not appear to be periodic, also if I review the log files I see the jump is reported in position and then corrected - normally in a single cycle....

PHD Guide 1.5 pre2 -- 03/14/0722:50:12
Guiding begun
lock 476.0 328.0, star 476.0 328.0
RA angle 1.5, rate 0.0019 aggr 1.0
DEC angle -0.1, rate 0.0035, Dec mode 1
Frame,Time,dx,dy,Theta,RADuration,R ADistance,DECDuration,DECDistance
1,3.324,-0.02,0.01,2.7,0.0,0.0,0.0,0.02
2,6.809,-0.02,0.01,2.6,0.0,0.0,0.0,0.02
3,10.304,-0.03,0.01,2.7,0.0,0.0,0.0,0.03
4,13.789,-0.02,0.02,2.5,0.0,0.0,0.0,0.03
5,17.284,-0.03,0.03,2.4,0.0,0.0,0.0,0.03
6,20.769,-0.03,1.00,1.6,480.6,-0.90,44.4,0.15
7,24.785,-0.03,0.00,3.1,48.0,-0.09,0.0,0.03
8,28.320,-1.00,0.00,3.1,0.0,0.0,0.0,0.99
9,31.805,-0.03,0.01,2.9,0.0,0.0,0.0,0.03
10,35.300,-0.03,0.01,2.7,0.0,0.0,0.0,0.03

I had thought to put my dead band higher - but I cannot set it to 1 pixel and anything less would not alter this behaviour - is it real, is my seeing likely to move a centroid by 4" for 3s or more or is this spurious. Can it be wind loading - I do not have an observatory by my location is pretty sheltered...

If this is not seeing related where to look? It seems odd to me that it is always ~1 pixel and impacts both axes by the same amount - it is almost like a noise issue -perhaps I should see if it is still present if I run all gear off batteries. Perhaps I should consider chokes on the cables going to the mount/cameras?

The impact of the spikes is worse on shorter subframes as the excursions form a larger %age of the exposure time thus my 10 min subs have rounder stars than my 2min subs....attached is a 300% zoom of one of my Canon images at 300s - the stars are distorted in x and y...

Odd/confused (normal state of mind for me!)