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On Sun, 16 Feb 2003, Brian Skiff wrote: > >> This is not possible.... > >> a magnitude of 7.6 for a Vega type star just detectable > >> ...Which becomes magnitude 13.4 for a 4" aperture. > > Alas, this is wrong, too, mainly since it ignores using high > magnifications which makes the background darker. Even the naked-eye limit > from 'true-dark' sites is in the early 8s, around V=8.2 plus or minus > a couple tenths. Doubtful. There have been some reports of people seeing 8th magnitude stars, but for the vast majority of observers, I believe the quoted figure is accurate and not conservative. Those exceptional observers likely had unusually large pupil diameters, rather than more sensitive retinas. My own limit is just about 13.5 with a 4" aperture under dark skies, and hardly ever have I known anyone to go much better than that. > No, I don't believe the mag. 16 with 10cm aperture either. At high > power, something like 150x or so with this aperture, the V limit will be > between 14.5 and 15. Seeing has to be good, of course, to get this faint, > probably 1" fwhm or better. The studies were done under optimum dark-room conditions with targets size <10 arcmins, which would correspond to the best seeing at highest magnifications described. > Maybe Hasegawa was using B magnitudes (which will be numerically > larger than V magnitudes) for this reckoning. On the other hand, Mike's Yes, bad comp stars may well be the explanation again! > from photon-quanta to V magnitudes, or more likely, the definition of what > constitutes threshold detectability. The latter goes down considerably > with experience, typically being in the 5-10 percent rate for the best > (most patient in this case) observers. If you insist on seeing something > 50 percent or even 100 percent of the time viewing it, then sure, the > limit is much brighter. That's not what Hasegawa-san would have been doing, The studies used a 60% detection rate as the threshold of detectability. Yes, fainter objects may be detected at 5-10% rate, but at that level, would you consider it a safe observation? Its hard to reproduce the observed location of an object that appears so infrequently, resulting in a low confidence of correct identification. Personally, if I cannot see my target at least 50% of the time, I wont bother estimating it. Mike Linnolt
Return to the Powerful Daisaku Nogami
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