You can avoid the subjective nature of the various arbitrary scales by going with estimates of the image blur-size in arcseconds. The list of similarly-bright and roughly-equal double stars available here: ftp://ftp.lowell.edu/pub/bas/dbls.fil ...was built with exactly this purpose in mind. The idea is to look at a few of these that are as near the meridian as possible and find (or interpolate) the separation at which the two star images are just touching. A compilation of estimates I've made at various places is at the same place: ftp://ftp.lowell.edu/pub/bas/seeing.vis ...which is drrawn from an 'amastro' post from a few years ago. \Brian -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.