[JohnG (John)]

An autoguider will correct for most errors providing you are using the correct software with that autoguider.

I use an ST-4 attached to a William Optics ZS-80 (f/6 - 480mm) and have no problem guiding my Tak at f/8 - 820mm or with Focal REducer f/5.98 - 610mm, see attached image.

The ST-4 will track using either CCDSoft or as a stand alone unit with sub pixel accuracy. When setup and guiding you are using a 9 pixel by 9 pixel box to guide in and on average, on a still night my guiding errors are usually no more than + or - 0.3, in laymans terms this equates to the following:
1 ST-4 tracking unit equals .2 of a pixel or 3 microns. So an ST-4 unit movement of 4 equals .8 of a pixel, well within any film resolution.

You have to make sure you do a decent calibration, once that is done, my setup will track all night on the one star.

JohnG