Here you see the typical autoguiding accuracy I get in reasonably good seeing and a light breeze.

Astroart plots the star centroid over time and keeps the history on the screen for a great visual
indication of how the autoguiding is going throughout the night. The combination of the highly
responsive Tak NJP mount and the Astroart4 autoguiding interface makes for accurate autoguiding
even in average conditions. The numbered rings are in pixel radii and as you can see during this
session the guide star has not wandered out side the 1 pix circle. Each pixel is about 2" square so
a dy/dx average error plot of 0.20/0.16pix is well and truly sub arc sec guiding, in really good
seeing and still air average guide errors drop to small fractions of an arc sec!

See here: http://www.pbase.com/strongmanmike2002/image/141334127/original

The best I have achieved is dy/dx errors = 0.10/0.08pix over a 1hour imaging period in very good
seeing this equates to average guide errors of less than 0.2 arc sec!