Astronomy Principles and Practice Fourth Edition.pdf

314 Astronomical optical measurements
Figure 19.5. The variation of stellar magnitude with zenith distance.
Once the value of m has been evaluated for a particular site from a series of observations on
standard stars, corrections for atmospheric extinction can be applied to all other observations according
to the zenith distance at which they are made. As the value of m may have short-term fluctuations
over periods of a few minutes and, as it may vary from night to night according to conditions within the
atmosphere, a series of magnitude determinations needs to include frequent measurements of standard
stars. For photoelectric photometry, this involves the speedy movement of the telescope from star to
star as target and standard objects are observed in turn. For CCD photometry, differential magnitudes
can be obtained more readily by arranging for the target and standard stars to be in the same field of
view and recorded simultaneously on the same frame. It may be noted that there are seasonal or annual
changes in the value of m influenced by volcanic activity that might be thousands of kilometres away.
It is obvious that individual observatories will suffer different amounts of extinction—for
example, by being situated at different altitudes. In order to make magnitude determinations
meaningful, it is necessary practice to reduce the values to above the atmosphere. Lists given in
star catalogues of stellar magnitudes present data corrected in this way.
The need for reduced determinations is further emphasized by considering magnitude
determinations in two colours. Most of the extinction is a result of scattering by the air molecules.
According to Rayleigh’s law (see section 5.3), the efficiency of such scattering is strongly wavelength
dependent and inversely proportional to λ 4 . Thus, it would be expected that the extinction is very much
greater in the blue spectral bands than in the red. This difference markedly shows up in practice; the
typical effect is illustrated in figure 19.6 where magnitude measurements in the B and V bands are
plotted against sec ζ for a particular star. If the colour index B − V of the star is to be determined, it
is obvious that the direct values taken at specific zenith distances will depend on the particular value
of ζ at which the measurements are made. In order for the colour index to be meaningful, the two
magnitude values must be reduced to above the Earth’s atmosphere before the subtraction is performed
(see figure 19.6). For each magnitude system, a particular value of m may be found empirically at
each observing station. Typical values of m are of the order of 0·m6, 0·m3and0·m2fortheU, B and V
bands respectively, this reflecting the behaviour of Rayleigh’s law.
For extremely accurate photometry, improved formulas are necessary for the reduction procedure.