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Re: (meteorobs) More on LM estimates and sources of error in ZHRs
SORRY about the length of this one, folks! Maybe you all can save this up, and
read it during the SLOWEST periods on our list this Winter... ;> And BTW, some
readers may argue this thread is OFF-TOPIC for meteorobs: I won't disagree, but
keep in mind that for visual meteor observers, the eye is our "scientific
instrument"! And for some like me, one of the most interesting things ABOUT
meteor observing is the way it can point up hard questions about human visual
perception, its biology, quirks and limits.
>But I don't believe a human could detect mag 8.9, an order of
>magnitude fainter than the normal limit, which is why I wanted
>to study the "double-blind" studies in detail.
The claim is certainly worth examining, Mike: I don't have references on these
experiments ready to hand, so I can only suggest a brief search using DejaNews
or one of the other News+Web search engines. I believe the one I'd heard about
in most detail was conducted at succeeding Texas Star Parties starting in 1993?
Following it up may involve accessing one of the more obscure amateur magazines,
like "Amateur Astronomy" or even the beloved old "Deep Sky".
On the argument that most people only detect 6.5 magnitude stars, and thus that
mag 6.5 represents any sort of physical limit to human photon sensitivity: again
this still seems a case of assuming the conclusion in the argument. The truth is
the vast majority of people do not allow themselves to fully dark adapt, and if
they do, their low-light vision may be impaired by a host of factors, some of
them certainly psychological as well as physical?
The fact that avid deep-sky observers (I'm one) can "learn to see better" with
time - often coming to detect STRUCTURE in objects whose very EXISTENCE they
would have missed before training themselves - is an indication that people do
not "normally" ever even approach their own visual limits.
In the same way, stellar objects are often ONLY detectable with "concentrated
vision" (use of extra-foveal vision in one static eye-position for an extended
period, in effect "integrating" with the eye). This seems to say that the way
images are formed by the complex interaction of eye, nervous system and higher
brain function is very poorly understood - as are its limits.
Anyway, Mike, if you do follow up this question, I'll be very interested to hear
what you find - especially if you uncover any references which directly relate
theoretical limits of human vision to stellar magnitudes in any way.
BTW, I agree with George Gliba's comment that measuring LMs under the WHOLE SKY
will significantly affect results. Even at the darkest site, the combined light
of airglow, starlight, ecliptical light and other natural sources must be enough
to reduce rhodopsin levels in the eye measurably. It would be interesting some
time to measure HOW MUCH this affects LMs - vs. measuring your LM while looking
down a narrow tube, as George suggested! :)
====
Of course, we all know the above has little to do with LMs as they are measured
for meteor observing: When estimating your LM for meteor observing, YOU SHOULD
NOT PUSH YOUR VISUAL LIMITS!! In fact, doing so is a "no-no": please just focus
instead on measuring the faintest stars you EASILY see with averted vision...
(These are presumed by this method to represent the faintest meteors your eyes
will be able to detect under the current observing conditions.)
My criteria for "EASY" averted vision are, that I can see a star repeatedly,
more than half the time I'm in that area. Also, thanks to some advice from my
fellow observers on this list, I take as LITTLE time as I can when I do each IMO
Star Count, to avoid unintended concentration on fainter stars.
In practice, I seem to never log meteors fainter than about 0.5 magnitude better
than my LM, using this method... And no matter the LM, I generally log a fairly
smooth, unimodal distribution of magnitudes, at least for Sporadics. Magnitude
distributions generally have a fatter left (brighter) leg for me, with often a
sharp drop-off among the faintest couple of magnitude classes.
Of course, my hours of observing in any given year are barely large enough to be
statistically significant - maybe 40 to 50 the past couple of years... What do
other, more *experienced* observers find are the relationships between their LMs
and their Sporadic magnitude distributions, on average?
Lew
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