>> estimates on double star separation is also limited to the >> resolution power of the telescope. If the seeing is better than the resolving power of the telescope, do you really care what the actual value is?! Dollars-to-doughnuts you won't ever see such seeing if your telescope is larger than about 8-inches aperture. Your 10-inch, Frank, might actually show pairs at substantially under 0".5 (which would be the Rayleigh limit for the aperture. But most folks' delivered image quality is a lot less, so they'll be measuring seeing + image-fuzz caused by not-quite-right optics. But that's what you have to deal with anyway, so might as well include it in the measurement. In relatively small apertures, I would certainly include bulk image motion in the seeing measurement, not just instantaneous image size. Around Flagstaff and elsewhere I've observed, often a star will show a well-defined Airy disc that is shifting around by some fraction of its own size. I'd try to estimate that and add it to the total. In larger apertures usually you won't see any Airy disc at all since it is so small compared to the seeing blur size. That's where just finding pairs that are merged/not merged gives a good estimate of the seeing. \Brian -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.