Tom Back's page describes running a bright star out of focus until you get several rings, and comparing their appearance on either side of focus to look for zonal errors in the optics. The number of rings per se isn't important, though Back suggests 10 or more. The description does not indicate that the number of rings one sees is somehow related to optical quality (which is what I inferred from your post). In any case, Back rightly considers the _in-focus_ image to be the "real meat and potatoes of optical performance". It is, after all, the way you normally look through a telescope. \Brian -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.