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Re: (IAAC) Re: Limiting Magnitude and altitude



Hi all
Just got these and here are a few of my experiences:
I agree the cleanest air is a plus for limiting magnitude.  I've seen the
faintest stars in winter after heavy storms (least airborne dust) at high
altitude when the ground was covered with lots of snow, the air very cold
(less water vapor) and the air finally stabilized.  Altitude helps at the
zenith mostly in that if there is any turbulence, it is minimized by the
thinner air above, so more often than not you will see fainter stars at
altitude, even if the air is not rock steady.  Very dark skies contribute to
improving the overall appearance of the sky, including zodiacal light,
nebulae, galaxies, and other extended objects.  It takes a dark sky to see
M81, for example.  I've never tried using a hood or mask to limit the amount
of sky seen, but it seems like it should help achieve maximum retinal
sensitivity.  Also, acclimation to altitudes above about 6000 feet over many
days is a must.
Cheers
Dan Duriscoe
----- Original Message -----
From: "Lew Gramer" <dedalus@latrade.com>
To: "Pro-Am Astronomy List" <amastro@yahoogroups.com>
Cc: "Internet Amateur Astronomers Catalog - Discussion"
<netastrocatalog-announce@jovian.com>; "Meteor Observing Mailing List"
<meteorobs@jovian.com>; "Lew Gramer (me)" <dedalus@alum.mit.edu>
Sent: Tuesday, March 20, 2001 11:53 AM
Subject: (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,
> Lew Gramer
>
>
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