(IAAC) Re: Limiting Magnitude and altitude
Thanks to everyone who has so far shared their experiences as well as their
knowledge, on this topic. I find it a very interesting thread - not to mention
very apropos for both deep-sky and meteors - and I hope others do as well?
Don Pensack of 'amastro' wrote:
>From that, I came to the independent conclusion that reduction in airborne
>aerosols more directly relates to transparency than does absence of
>scattered light in the atmosphere.
Don, I believe this is similar to the conclusion I've gradually had to reach,
and which I hinted at in my first post on this thread. In fact, the average
cross section and mean free path between aerosols (under which for convenience
I'd include water condensates) seems to me a critical determining factor in the
limiting magnitude. (AND visibility of low-surface brightness extended sources?)
Now the question becomes: "What are the determining factors of those particle
sizes and concentrations?" Clearly for dust and other non-hydrometeors, it is
a "simple" matter(!) of sampling the air. However, for water condensates, I'd
expect it becomes a mighty tricky problem: to derive values for both sizes AND
concentrations of water droplets, forming in ALL important segments of the air
mass to be considered? And is near-ground humidity a factor? Perhaps, but with
the above in mind, I wonder just how explanatory a factor it really is!
And is ground elevation an important factor in the problem? Of course it must
be, but again it's likely to affect contrast in what I expect are non-obvious,
largely non-determinant ways! But more on that contentious idea below...
Last but not least, it still seems certain that we have to take scattered light
into account. In fact, it seems likely there must be second-order terms in this
problem! In other words, a constant intensity of incident light will cause more
or less net degradation in contrast, depending (at least partly) on the current
concentration of scatter-efficient aerosols in the whole volume of air mass
incident between the light source and the observer...
(An off-topic aside: This last problem reminds me of the equations derived for
intensity of zodiacal light sources, by the way! Only the "aerosols" in that
case are actually sub-gram, sub-millimeter interplanetary dust specks. :>)
>If deep-sky is your game, there is no substitute for altitude.
This last statement, I have NOT come to agree with... Again, as you suggest,
there IS clearly some correlation between low air mass and high contrast. And
this is admittedly critical for high zenith angles. (At altitudes below 40
degrees, the effects of even "pure" airmass scatter and absorption begin to
ramp up fast: below 5 or so degrees, even with no significant particulates
present, point sources will still lose 5 magnitudes, or even more!)
Nonetheless, the airmass relationship alone can't fully explain contrast, nor
does it seem likely the correlation implies any direct causation. Instead, it
seems likeliest that higher altitudes only mean lower AVERAGE concentrations of
aerosols. (And the same would likely be true for mean free paths, though maybe
not for aerosol scattering efficiencies at visual wavelengths?) And so we are
still left with the question of how much DEVIATION there may be in a conclusion
which is based purely on averages... In other words, lower concentrations at
high elevation ON AVERAGE need not imply poor contrast at all low elevations!
Surprising? Maybe, but I haven't yet figured out what flaws might derail this
train of thought. Maybe folks more convinced of a STRICT relationship between
elevation and contrast can help point out where the above thinking goes awry?
In the end, I suspect any amateur with a limited taste for the theory (and its
open questions) may simply be forced to leave behind cherished preconceptions,
however widely accepted they may be, and however often they may be espoused by
respected sources. And with those preconceptions - about elevation, etc. - left
firmly behind, the amateur is left to do what they do best: to actually MEASURE
Limiting Magnitudes (and/or whatever metrics they can dream up for low contrast
extended objects), at many zenith angles, and at many sites - having the widest
possible variety of possible contributing conditions, including site elevation,
humidity, patterns of air flow, proximity of pollution and artificial light
sources, etc., etc. At any rate, this sounds like a fun task to pursue! :)
And what will the amateur find? Well, all I know is what I have seen myself,
with my own eyes... Namely, that altitude is only one of the factors affecting
both contrast and ZLM - and not necessarily the most critical one.
If you love the mountains, go to the mountains. But if you love the sea, then
don't let unsubstantiated statements by others dissuade you from also loving
the deep-sky - or meteor observing and the naked-eye sky, for that matter! :)
Clear (and contrasty!) skies to all,
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