Nocturnal Measurements of Atmospheric
Composition at an Astronomical Observatory
Successful determination of the radiometric characteristics of celestial
objects depends on accurately accounting for how those characteristics
are altered by the Earth’s atmosphere. The theme of the proposed work is
that what is normally trash to an astronomer may be treasure to an atmospheric
scientist. For example, the determination of the absolute
magnitude of a star from ground-based measurements requires that the optical
depth of the atmosphere be factored out. Rather than throwing this factor
away, we will study it to see what can be learned about the nocturnal behavior
of atmospheric `interlopers’ that both hinder observational astronomy and
play a significant role in Earth’s climatology. As a 2nd example,
three-color photometric observations are necessary for astronomers to correct
for the reddening of starlight by the Earth’s atmosphere. This measurement
should also allow the atmospheric scientist to (1) determine the Rayleigh
component of the total optical depth and (2) compare this direct measurement
with standard atmospheric models which are routinely used in daytime aerosol
optical depth retrievals. In addition, the band-pass characteristics
of the color filters (channels) used on the SFASU photometer may well allow
retrieval of column O3
and NOX
in much the same way as is done in daylight with multichannel radiometers.
A
3rd example: Good photometric observations depend on the degree
to which one can assure that atmospheric conditions are static during viewing.
Thin cirrus makes the photometry quite “lumpy,” hence star light is typically
sampled every few ms and the results summed into 1 second integrations,
giving 100 samples each second to be used to compute a standard error.
If the error exceeds a threshold (currently 2%) it is flagged. Also,
each reading is compared to the previous reading on the same star, and
if these differ by more than 4%, another flag is placed in the record.
This procedure not only allows one to flag thin cirrus events in the aerosol
optical depth retrievals, but also may allow one to quantify cirrus coverage
at night.
Photometric observations will be made with the 41” telescope at Stephen
F. Austin State University (SFASU), in Nacogdoches, TX (http://www.physics.sfasu.edu/observatory/obs.htm).
The observatory-based measurements we will propose to initiate are as follows:
(1) Identify suitable stars, (2) measure brightness as functions
of slant path, (3) use standard models to calculate molecular optical depth,
(4) use color filters to provide an independent measure of molecular optical
depth, (5) test atmospheric homogeneity by sampling stars of approximately
the same elevation but varying azimuth, (6) characterize thin cirrus clouds,
(7) retrieval of night time NO2
and O3
concentrations with three-color photometry.