Specifications

AUTOSTAR CCD PHOTOMETRY 67
APPENDIX E
Determining BVRI Color Coefficients
Introduction
Different photometric equipment and telescope combinations can
produce different responses to the same star. For accurate data and
data that can be combined with that taken by other observers with
other equipment, it is important to use a calibrated photometric
system. This means knowing the color transformation coefficients
of the system and at least an average set of extinction coefficients
for the observatory. In fact, to determine the color transformation
coefficients, the extinction coefficients must be known.
Determination of the extinction coefficients can be done during the
same night that the data for the color transformation coefficients
are determined (see Appendix D).
Air Mass
Air Mass is the thickness of the Earth’s atmosphere that star light
has passed through. The Air Mass value increases as a function of
the angle of the star downward from the Zenith (straight overhead).
Computing the Air Mass value is complex; this is described in
detail in Appendix B. The Air Mass must be known when
determining both the atmospheric extinction and color
transformation coefficients. To standardize your data, you must
determine your system’s color transformation coefficients and use
those coefficient when reducing the data.
Having determined the extinction coefficients that account for
atmospheric effects, the next step is to transform the data to the
standard UBVRI system. This is done by observing a set of
standard stars with a wide range of known colors, applying the
extinction corrections, and then determining the coefficients using
the following equations.