It's the guide star stupid!!

[MartinB]

Main scope focussed, guide scope focussed. A few mediocre little stars on the guide chip so adjust the rings and up pops a beauty. Very bright. Set the tracking using 1 sec exposure. Off goes the guiding hmmm not so great. RMS tracking error in dec of around 0.2 pixels (about 1 arc second compared with usual 0.06-8). Guiding looks good for a little while then tracking error wanders off up to 0.4 pixels - yuk! That would be no good if I was imaging at 2000mm F/L. Tweak the aggressiveness - makes no difference at all. Set a minimum move of 0.1 pixel - no joy. Offset the balance to the east more, just the same. Swear and curse - that helped.

Went inside and had a glass of wine. Ahaa! The guide star is saturated so my centroid is up the spout, Maxim can't tell where the centre of the star is. Changed to a diddy little star out near the edge. Bingo! Tracking error in both axes down to an RMS of 0.06pixels or about 0.25 arc seconds. That'll do.

Learning point for me - brightest isn't best, check the levels.

[Grant]

See - wine always has the answer

[centroid]

Yep, been there and done that Martin, so you are not alone.

The problem is, that unless you give it some serious thought, it isn't all that obvious as the source of the problem.

Dave

[daz]

Wine is the international measurement of a problem!

That was only a 1-glass problem.

Imagine getting something really tricky!!!

[jcm]

I found an immediate improvement when I changed my guide cam to the SXVM7 ( camera I was not using at the time). Not only could you guide on faint stars the download is very quick being USB 2.

John