[Prev][Next][Index][Thread]

Re: (meteorobs) More on LM estimates and sources of error in ZHRs


John Murrell wrote:
> I seem to remember that compared to looking vertically from a high mountain
> when looking at 60 deg at sea level only about 25 % of the light makes it
> through the atmosphere. I don't have tables of atmospheric extinction -
> has anyone got some more detailed information to confirm or deny this?

John, one figure I've read - from AAVSO documents and the International Comet
Quarterly - is that at sea level, atmospheric extinction at the Zenith often
amounts to less than 0.5 magnitudes! (In fact, the equations for calculating
this seem quite complex... I forwarded an on-line passage below from the ICQ,
by well-known CfA astronomer Dan Green, of light-pollution and CBAT fame.)

If this is true, zenith Limiting Magnitudes at sea-level need not differ by
that much even from Limiting Magnitudes taken from SPACE! Again, it seems
to all be a question of variations in air content which will cause scatter.
Of course, the lower the elevation, the greater the difference: below 20o,
the difference between sealevel and a high mountain site will be at least
a full magnitude or more. At 1o, the difference is nearly *5* magnitudes!

Also, note again that Dan's paper is focused on instrumental photometry: it
does not concern itself with effects of light pollution on the unaided eye,
and how those effects may be reduced or exacerbated by variations in the
reflection, extinction and scattering of light pollution by the air... That
is where I differ with many folks who automatically prefer higher altitude
mainland sites (such as those in the US Southwest), to lower-elevation sites
like that on Long Key FL, which are far from sources of aerosol content, and
have the good fortune to lie beneath rapid, non-turbulent airflows.

Clear skies and many Sporadics folks,
Lew Gramer


==================

From the July 1992 issue of *International Comet Quarterly*,
Vol. 14, pages 55-59.  Copyright 1992
Excerpted at: http://cfa-www.harvarddot edu/~graff/icq/ICQExtinct.html
Forwarded without permission.

   "Hayes and Latham (1975; hereafter HL75) note that there are three sources
of extinction in the earth's atmosphere that must be considered when dealing
with ground-based astronomical photometry:  molecular absorption, Rayleigh
scattering by molecules, and aerosol scattering.  At wavelength lambda = 510
nm, which is the peak spectral response for the rods of the human eye used
in night vision (e.g., Bowen 1984), molecular absorption (which occurs in
spectral lines and bands) is rather negligible (see the graph by Tueg et al.
1977), although for altitudes under 10 deg, ozone can cause extinction > 0.01
magnitude per air mass (HL75).  We adopt Schaefer's (1992) value A(sub)oz =
0.016 magnitudes per air mass for the small ozone component contributing to
atmospheric extinction.
    "Rayleigh scattering by air molecules can be represented by the
following equation (after HL75 for lambda = 510 nm = 0.51 micron):

A(sub)Ray = 0.1451 exp (-h/7.996) magnitude per air mass.                 (2)

    "Aerosol scattering is due to particulates including dust, water droplets,
and manmade pollutants, and the extinction due to this is generally given by
the formula 

A(sub)aer = A(sub)0 lambda**[-alpha(sub)o] exp(-h/H)
               magnitude per air mass,                                    (3)

where the scale height, H, is usually taken as 1.5 km (HL75; however, this may
vary by a factor of 2 on any given night) and lambda is the observed
wavelength (in microns).  The quantity alpha(sub)o varies from site to site;
Tueg et al. (1977) and HL75 find typical values near alpha(sub)o = 0.9, but
we adopt alpha(sub)o = 1.3 after Angstroem (1961) and Schaefer (1992).
Schaefer remarks that the variation in A(sub)0 "is rabid . . . because the
aerosol component varies greatly on all time scales".  Volcanic aerosols, in
particular, are highly variable from site to site and year to year.  For
reasons stated under "Procedure", below, I adopt A(sub)0 = 0.05 as an
average value.  Thus, we will take the extinction due to aerosols for the
human eye as

 A(sub)aer = 0.120 exp(-h/1.5),                                           (4)

so that for elevations near sea level, A(sub)aer is about 0.12 magnitude per
air mass."


To UNSUBSCRIBE from the 'meteorobs' email list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html
Follow-Ups: References: