I remember this being brought up, and it is the most objective visual measurement of seeing conditions, however just like with magnification limits on seeing, estimates on double star separation is also limited to the resolution power of the telescope. With that said, and I'm sure there is a simple equation for this, what would be the smallest aperture for determining sub-arcsecond seeing? Also of note--and dropping back to the various scales--in my previous post I planned on comparing the seeing forecast model at: http://www.cmc.ec.gc.ca/cmc/htmls/seeing_e.html with the night's actual seeing. The model predicted a 4 out of 5 on their scale, and for the most part they were accurate with seeing riding 3-4. On the 1-10 Pickering scale, between 11pm and midnight the seeing rated a ~6 and sometimes a 7. I was able to observe Jupiter with little softness at 400x, and briefly at times 560x. -FRANK -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.