SBIG A0-7 for ST7/7E/8/8E (others?), excellent condition.
This is sold as their "adaptive optics" unit, but
arguably is instead a very fast guider. Works to
correct some atmospheric effects, and can remove
residual mechanical tracking errors.
Using the AO-7 will not make bad images into good
ones, nor correct for bad equipment or technique. However,
it will reduce stellar radii, and allow you the get the best
possible imaging from your camera/scope/weather combination.
Depending on your focal length and drive, it can remove
periodic error from the drive components as well.
Includes SBIG's Hartmann mask software.
$700 postpaid or COD to the inner 48.