I understand what you are saying: the point I was trying to make was that photodiodes are pretty insensitive from the viewpoint of the typical amateur trying to make autoguiding work with the usual 8" off-axis pickoff and reading the diodes in realtime mode. A lot of objects don't have bright enough stars nearby to be used like this. The autoguiding CCDs with their variable exposure time and image processing are lightyears ahead of the old approach, IMHO. Also, the old guiding arrangement often had a single knife edge that called for a track rate slightly high or low to keep the star image dragging back against the knife edge. This was bad news when a cloud passed by. The CCD autoguiders don't have this problem.
Mel Bartels