[AZ-Observing] Re: Alt Extinction was: Mag sequence
- From: "Jones, Jack (AZ75)" <jack.jones@xxxxxxxxxxxxx>
- To: "'az-observing@xxxxxxxxxxxxx'" <az-observing@xxxxxxxxxxxxx>
- Date: Thu, 18 Oct 2001 15:44:46 -0700
>Russ wrote:
> I might like to try that magnitude sequence in my instrument at
varying altitudes of an object to see what the horizon extinction
really is at some of the sites we use. I have found that to be
more variable than I first thought from looking at it so far. The air
here in the Valley this week looks like it could stop a Kilowatt
laser beam. :-(
>RC
Russ:
I can't find the various aperture mag limit chart but I did find the alt.
mag extinction chart and Brian's challenge to do one.
Jack Jones
Here we go again: there is no simple answer to AJ's question about
altitude and sky-quality versus telescopic views, since so many factors come
into play. Just a year ago there was some discussion about this on the
az-observing list, and it came up in October in the 'amastro' list. Below
is
my somewhat modified az-observing response. More to AJ's and Stan's point
about object elevation (as opposed to altitude of the observing site), the
main effect at moderate zenith distances is the increase in sky brightness
and
worse seeing, and only at low elevations does the actual extinction being to
become important. Because your eye works so strongly as a contrast
detector,
the sky brightess/seeing effects work to make things _much_ worse for
threshold viewing even when you're looking at only say 45 deg from the
zenith.
\Brian
=========================
version: 19 November 1999
Tom Polakis and I have recently [Oct 1998] been corresponding about
trying
to attach some numbers to the question of visual magnitude limits versus
altitude. Given that transparency goes up as you go higher, but also that
visual acuity goes down with altitude from simple lack of oxygen, the
question
is then what is the optimum altitude where you get the best transparency but
lose neglgibly from oxygen deprivation. I've argued for many years, based
on
observing at a few true-dark sites at various altitudes, that the optimum is
not sharply defined, but lies somewhere in the 5000 to 8000 foot range, and
probably toward the high end of that range, especially if, like me, you live
there! (A contour corresponding to 7200 feet runs under my cottage on Mars
Hill.) The question of what happens when you come up from Phoenix, say, at
1000 feet, and start viewing within a few hours at 7000 feet is another deal
altogether.
Tom was hoping to find some data regarding visual threshold sensitivity
versus altitude, but hadn't had any luck on the Web (this does not mean it
doesn't exist). This won't be the only factor, since the sky brightness
drop
you get at high altitude (less stuff in the air above you, so less scattered
light from stars, zodiacal light, etc.) is probably the main effect in
giving
a better magnitude limit. A plot in an old aeronautical manual shown to me
by Gene Lucas suggests the loss in threshold sensitivity begins immediately
above sea level, but since visual deep-sky observing is obviously better at
a
fair elevation, other factors must much more than compensate for this,
assuming
it's accurate. As a silly time-wasting project, I suggested that one could
spend several nights travelling Arizona, going from one dark site to
another,
incrementally increasing elevation each night, ranging from just a few
hundred
feet up to 10 or 11,000 feet. The idea would be to observe a series of test
objects at each place with the same telescope/observer(s) to see how the
results varied. The stable photometric weather of the last two weeks
[Sep/Oct
1999] would have been ideal for this junket.
Taking another approach to the problem, below are some numbers I found
showing visual extinction versus altitude. The "extinction" is something
professional astronomers measure when they do photometric observations in
order
to correct the brightness observed to outside the atmosphere, so no matter
where you observe from, results can be compared directly. The number
answers
the question "how much fainter is this star because of the atmosphere
compared
to observing from orbit?" The units are magnitudes per airmass, the zenith
being airmass of 1.0, and the airmass increases (for reasonable zenith
angles)
as the secant of the angle; thus 60 zenith distance = 2.0 airmasses. The
two
columns for extinction in the table below show that due to gaseous air plus
ozone (second column) and typical values for dry sites (third column). The
"extra" stuff in the third column comes from aerosols such as humidity,
dust,
pollens, pollution, etc. The second column shows in effect the absolute
minimum that the extinction can be.
altitude visual extinction (magnitudes/airmass)
(feet) gas total
0 0.13 0.26
1000 0.13 0.23
2000 0.12 0.21
3000 0.12 0.19
4000 0.12 0.17
5000 0.11 0.16
6000 0.11 0.15
7000 0.11 0.14
8000 0.10 0.13
9000 0.10 0.12
10000 0.10 0.11
14000 0.09 0.09
source: Merle Walker, 1986, "Characteristics of Optimum Sites", in
"Identification, Optimization, and Protection of Optical Telescope
Sites", Millis et al. eds.
Bear in mind that the effect of extinction on visual observations is
_not_
only the nominal dimming, which is hardly detectable by eye, but the
increase
in scattered light in the sky, and thus the lowering of contrast, which is
nearly all that matters for visual observation. Note also that in areas
such
as the polluted, urbanized eastern seaboard of the US, or the midwestern US
in
summer (and probably much of continental Europe is the same), typical
exintction values are actually measured at around 0.35, not 0.26 as the
table
shows. Skies in such areas are especially bright because what light
pollution
there is is scattered much more than it would in a clean environment.
For comparison, we frequently actually measure extinction of 0.11-0.12
in Flagstaff in winter especially, but also much of the year outside the
monsoon, i.e. the atmosphere has _zero_ aerosols. (We got these winter-like
numbers also last June [1998] for two weeks running, and again this past
week,
giving us onyx-blue daytime sky right down to the horizon.) [Ditto for June
and October 1999!] Our annual mean is typically 0.15, similar to what's
shown
for our altitude in the table. This is typical also for places like Kitt
Peak
and other sites in the Southwest at similar elevation. Similarly, when Tom
and
I were observing at Las Campanas in Chile, a grad student who was there
later
reported that her observations showed extinction of 0.15-0.16 during our
first
week. Tom can confirm that the general appearance of the sky and limiting
magnitude there was very similar to that of the US Southwest at similar
elevation (7500 feet).
For a long time I've considered an extinction of 0.20 to be the max for
any kind of decent deep-sky observing. This mainly comes from experience
during episodes of volcanic enhancements, a la El Chichon and Pinatubo, when
we had long periods with extinction over 0.25, up to 0.5 briefly. As the
extinction gradually fell down to the ordinary level, it was easy to find
when
deep-sky observing got good again. At an extinction of about 0.25, one
loses
about three-quarters of a magnitude naked-eye (compared to normal 0.15
extinction), far greater than you'd expect just from the extinction
increase,
so obviously the sky brightness increase is a big factor. Under such
conditions the sky looks cloudy to me---not enough stars. One has only to
look
at observing reports on the sci.astro.amateur newsgroup to see that most
folks,
living at sea level somewhere "back East" (or Houston), ordinarily have sky
conditions far worse than this.
Anyway, this rough number (max extinction 0.20) suggests "best" viewing
sites are never lower than about 3000 feet. Then if you consider that our
seasonal peak mean extinction in Flagstaff---just before the Monsoon---tops
out
around 0.18, or 0.04 higher than the mean for 7000 feet listed in the table,
one should consider only sites above about 5000 feet to avoid problems.
This
jibes with my qualitative assessments from places such as Palomar (4500
feet),
Prude Ranch (TSP, 5000 feet), Kitt Peak, Anderson Mesa, and Mount Hopkins
(all
about 7000 feet), and Las Campanas (7500 feet). One year I observed some of
the same objects with the same telescope at Anderson Mesa within a few days
of
being at TSP, and found the high-power telescopic limiting magnitude to be
a few tenths of a magnitude better at Anderson Mesa, reasonably attributable
just to the somewhat higher altitude and lower scattered skylight.
Transparency of course gets better as you go up, but the diminishing
returns from physiological effects still has to be examined. But this 5000-
foot figure sets a fair lower limit to optimal observing sites for visual
astronomy. For imaging of course, you want to go as high as possible.
All for now.
\Brian
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