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(IAAC) Limiting Magnitude and altitude (was Re: [amastro] M81 with the unaided eye)



(Before I wade into the oft-debated and -conflated topic of Limiting Mags,
I wanted to report that I've sighted M81 naked-eye on just a few occasions.
For me, the problem is usually not dark skies - but that my dark-sky sites
are specifically chosen to give the darkest and clearest possible SOUTHERN
horizons! After all, no site (I've ever seen) is truly uniformly dark, all
around the horizon. So maybe, just maybe, that makes my relative paucity of
naked-eye M81 sightings an aberration? Anyway, on to LMs... :>)
Stephen O'Meara delivered a wonderful talk about this subject at the Winter
Star Party last month... According to research O'Meara cited (I don't recall
specific references, maybe other WSP attendees might?), the "ultimate" human
limit for low-light detection - presumably within a high-response spectral 
region, and with some non-trivial "integration" time of a few seconds - is
six photons! With various assumptions this could put an "ultimate" limiting
magnitude somewhere well into the 9s... However, as Brian points out, any
practical limit is more likely to depend on the sky than the eye - in other
words, on the eye's ability to distinguish a faint point source amid diffuse
glow, rather than to detect that point source against "total blackness".
And of course, some of the numbers which are often cited on this topic may
or may not derive from truly "double-blind" (i.e., star-count or unprepared
sketch) methods of estimating LM, which are likely to be the most stringent.
By the way, recent respondants on this thread are always careful to note
elevations when relating measured LMs... In his talk, Stephen O'Meara also
stressed the importance of reducing airmass to deepening your LM. (And of
course, the gaiting importance of elevation on LM has certainly become what
I could call "received wisdom" among astronomers - amateur and pro.)
However, the only fully referenced and well-cited theoretical paper that I
have ever seen on the range of factors which can influence visual Limiting
Magnitude (Hayes & Latham, 1975ApJ...197...593H) indicates that even right
at sea-level, total reduction in *Zenithal* Limiting Magnitude due to the
atmosphere can amount to *less than 0.5* magnitude! (Naturally, the effect
of elevation on LM increases nonlinearly as zenith angle increases.)
Assuming this to be true, zenithal LMs at sea-level need not *necessarily*
differ overly much from LMs taken in SPACE - let alone at high altitudes.
This was first brought to my attention by Dan Green (CfA, NELPAG), and is
discussed more fully in the archive of the 'meteorobs' mailing list at:
    http://www.tiac.net/users/lewkaren/meteorobs/msg17101.html
With an informative follow-up at:
    http://www.tiac.net/users/lewkaren/meteorobs/msg17104.html
Again, this assaults the "common sense" of astronomers ("Hey, really dark
skies can ONLY be achived at gasping-high altitudes!" ;>). And yet, when
I've taken an open-minded approach and actually tried MEASUREMING (via
IMO star-count LMs - yet another, even more stringent method), this does
seem to bear up quite nicely to observational test.
Anyway, as always, thoughts/questions/brickbats from others are welcomed. :)
Lew Gramer
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