Last night I did a rough focus and polar aligned to within .7 arc seconds on both axis.
I then slewed to Arcturus to achieve precise focus using a tri bahtinov mask.
Next I slewed to a guide star near the meridian on the same side of the meridian as my intended target (M13), calibration went smoothly.
After acquiring the target I took a test shot and started imaging on autorun.
Everything OK until after around 40 subs. Guiding went from under .3 to .5 arc seconds on the DEC axis until suddenly the error shot up to over 3 arc minutes of error! Sometimes this also happened on RA axis as well. Sky conditions were superb, no wind, no water vapor, Bortle 2, same as night before when guiding was stable all night. After that guiding continued to go from good to terrible for the remainder of the session. I tried adjusting aggression but it didn't help.
Guiding with the ZWO ASI290 mini and the Skywatcher Evoguide 50ED.
Might someone know what could cause this to happen? Guiding usually goes well for hours...
Thanks,
Jerry
