(IAAC) Fwd re: Limiting Magnitude and altitude

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Lew Gramer <owner-netastrocatalog@jovian.com>

------- Forwarded Message

Date: Fri, 16 Mar 2001 00:52:38 -1000
To: meteorobs@jovian.com, amastro@yahoogroups.com
From: Jim Bedient <bedient@hawaii.rr.com>
Subject: Re: (meteorobs) Limiting Magnitude and altitude (was Re:
  [amastro] M81 with the  unaided eye)
Cc: Internet Amateur Astronomers Catalog - Discussion
    Meteor Observing Mailing List <meteorobs@jovian.com>
In-Reply-To: <200103160025.AA13182@trillian.dev.latrade.com>

At 07:25 PM 3/15/01 -0500, Lew Gramer wrote:
>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".

Probably true... there are some other studies that bear this out.  I.S.
Bowen published a paper titled "Limiting Visual Magnitiude" (Publications
of the Astronomical Society of the Pacific, Vol. 59, No. 350, p.253) in
1947, in which he references an experiment, the results of which were
published by Heber Curtis (On the Limits of Unaided Vision, Lick
Observatory Bulletin No. 38, p. 67-69) in 1903, wherein an obserever could
see eighth magnitude stars when strategically placed screens blocked the
night-sky background.  Henry Norris Russell's paper "The Minimum Radiation
Visually Perceptible" (Astrophysical Journal, vol. 45, p.60, 1917) further
describes Curtis' finding that a star of magnitude 8.9 was "glimpsed at
intervals, very doubtful"; magnitude 8.3 was "seen with difficulty".
Russell goes on with his own experiments, observing the light of bright
stars shining through a small gap into a darkened room, and concludes the
minimum visible star is magnitude 8.5.  All of this was found to be
dependent on eliminating the bright sky background.

>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.

As you point out above, the effect of the atmosphere is non-linear with
increasing zenith distance... so the appearance of the sky above the
atmosphere is radically different, since the airmass at the zenith (z=0
deg) and the "horizon" (z=90 deg.) is essentially the same, zero, so much
fainter stars will be visible closer to the horizon.  That's the benefit of
getting above as much of the atmosphere as possible, the area of useable
sky increases greatly.

>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.

Observatories are built atop mountains because dark skies are only one of
the criteria.  Getting above as much of the atmosphere as possible helps
eliminate the deleterious effects of the atmosphere on the stability of the
sky.  Seeing is perhaps as critical as darkness.  Also, less atmosphere
above reduces the amount of water vapor, which obscures infrared and
submillimeter radiation.  Mauna Kea is Mauna Kea because it's above the
tropical inversion layer, which isolates the upper atmosphere from the
lower moist maritime air and ensures that the summit skies are pure, dry,
and free from atmospheric pollutants; thus it's suitable for submillimeter
and infrared observing.

I maintain that for the visual observer, at least some of the advantage
gained by increasing altitude is lost to the insidious effect of reduced
oxygenation of the brain and retinas, particularly after a few hours at
altitude.  The sky atop Mauna Kea, or Haleakala, though lovely indeed, just
doesn't look that much darker to my bare eyes, which exist at sea level 99%
of the time.  Very mild exercise, like walking up a small knoll or a flight
of stairs, gives me the impression that someone turned the stars out.
Using supplemental oxygen occasionally will help.

Just my $0.02, from my O2 deprived brain,

Jim B.

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