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Greg. Guiding binning depends on image scale. bin 1 is best on a wide field guide scope, and bin 2 or 3 with internal or OAG at long FLs. bin 1 is no advantage at long FLs, wasted res, so you bin to get brighter/less noise stars. It can make a diff to the point where bin1 is so dim, its unusable and binning is at a given exposure time..Actually, S/N ratio is what counts more whilst guiding, not resolution so much, which is why the cooled SBIG guide cam is so usefull.Blocky stars arnt too much of a problem if you defocus.
Having said that, for standard external guide with a wide field ST80 for instance, I found the QHY5 small pixels and large FOV perfect for the usual 2 sec exposures (as the starfish would be), there was always a bright enough star in view.
Its with internal/OAG that things got ugly.
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