Tony Flanders wrote:
Charming, isn't it, how wildly the experts vary? Let's say that
the dream sky, which can be approached but never equalled on Earth,
is mag 22 per square arcsecond. Knoll/Schaefer places the NELM
for that sky at 6.6, Blackwell/Clark at 7.2, and Ferris at 8.0.
FWIW, under my customary decent rural skies -- surely no better
than mag 21 per square arcsecond, if that -- I have seen stars
to mag 6.8 or 6.9, but I have done no better at all under far
darker and clearer skies out West.

I suspect much of the difference in NELM numbers can be resolved by taking a
closer look at the way the data were obtained. With respect to the 8.0 (+/-
0.5-mag.) number I use, this is based on reliable reports from observers such
as Heber Curtis, Stephen James O'Meara, Brian Skiff and others who've made
repeated NELM observations within that range. These are observers with acute
vision, access to dark skies and experience.

The Blackwell data, which is foundational in Clark's work, is taken from
experiments in which novices were given 15-seconds or less to detect light
stimuli against backgrounds of varying brightnesses. This methodology provides
a clue as to why NELM estimates based on Blackwell's data are relatively
conservative. The Blackwell data could be said to indicate what the average
person would see, while I'm relying on observations made by top observers.

Also, it should be pointed out that there really is no controversy over the
surface brightness of the darkest sites on Earth. That limit is 22.0 MPSA (+/-
0.1-mag.), which has been derived from photometric data taken over decades from
sites all over the planet.

And estimates of NELM under heavy light pollution vary even more,
if possible, although I suspect for somewhat different reasons.

Oh how I long for a cheap, widely available device to give an
objective measure of sky brightness! As things stand, we are
like the people building the tower of Babel, all talking at
cross-purposes to each other.

I'd say Clark, Schaefer, Carlin, Bartels and other have done an excellent job
of speaking in the same language. And they share similar motivations and goals:
to help us better understand how we see under low-light conditions and what our
limits of vision under those conditions are. And they've had some significant
success.

Clark showed us how to talk about the eye as a contrast detector in a
quantifiable manner. Schaefer opened the door for the integration of difficult
to quantify variables, such as observer experience, when predicting limiting
magnitudes. Carlin and Bartels have furthered the evolution of our
understanding in this area by building a bridge between the the theoretical and
amateur communities: Carlin through his analysis and Bartels through his ODM
program.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
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