Specifications

AUTOSTAR CCD PHOTOMETRY 55
Appendix C
Determining Standard Star Data
Observing Standard Stars in M67 (NGC2682)
To determine color transformation coefficients, the first thing that
must be done is to image some standard stars whose BVRI
magnitudes are known. During the winter and spring, the open
cluster M67 (NGC 2682) provides a good grouping for this
purpose. It is best to plan your observing session so that you can
observe M67 twice over a long time period during the evening.
One observation should be with M67 far from the meridian, either
before or after the meridian crossing; and one observation with
M67 close to or on the meridian. For best results several images
should be taken spaced somewhat evenly between the first and last
observation. Precise spacing is not needed, however. These
observations will provide data over a large air mass range for
determining the atmospheric extinction.
The observation at or close to the meridian will also be the one
used for the color transformation coefficient determination. This is
because there is always an error in the extinction determination,
and extinction effects are minimized close to the meridian.
For wintertime observations, it is usually best to get an image early
in the evening to the East and then one on the meridian. Later in
the springtime, you will need to take the first one close to the
meridian and the second one further West. Ideally, the Air Mass
difference should at least 1.5.
Be watchful for clouds, hazeness, or fog that can upset the
readings. The night should be clear. The Moon will not present a
problem, unless it is close to M67. Since the sky background is
subtracted from the star + sky data, a bright sky is compensated
for.