PhotoRed Reductions Routines - First Order (Comp Star)
This method uses the instrumental magnitudes and air mass values from Canopus-like data
in a least squares solution to find the extinction coefficients in magnitudes per unit of
air mass. The data can come Canopus by importing one or more sessions or by using the
Differential photometry wizard in PhotoRed. The former makes sense if you did a long time
series and measured images in Canopus. The latter works is you’re trying to get some
quick FOE values from a few sets of images of the same field taken over a range of air
masses.
All other considerations aside, a star gets brighter as it rises because its light
travels a shorter distance through the atmosphere and so there is less absorption
(extinction). This continues until the star reaches the meridian. After that, the star
dims as gets lower and so its light goes through longer paths through the atmosphere.
Photometric reductions can be based on the instrumental magnitude outside the
earth’s atmosphere, i.e., exoatmospheric magnitudes. The extinction value is applied
via the simple formula
mo = mi - k’X
where mo exoatmospheric magnitude
mi raw instrumental magnitude
k’ extinction value in magnitudes/air mass
X air mass at the time of the observation
PhotoRed finds the value for k’ in each filter by plotting the raw instrumental
magnitude of a comparison star for each observation versus the air mass at the time of the
observation.
The plot above shows an example of the results after running this routine. Of course,
you must be sure that the comparison was not a variable; this can be accomplished very
easily in Canopus.
The biggest problem with this method is that it depends on the sky conditions being
nearly identical throughout the night. If clouds or haze move in, they can cause the stars
to dim artificially and so the solution is skewed. This is why the data is plotted. If
there are "bad" data points, you can remove these from the calculations and get
a better estimate of the true value. If, however, conditions slowly deteriorated through
the night, the results are really useless. The comp star method is not the preferred
method for those working in less than all-night all-sky photometric conditions. The value
you find may not be the correct value because of changes during the night. Furthermore,
that value may not be the one that was "in effect" at the time you took images
of a standard field for finding the transforms and nightly zero points. This can skew your
final results significantly. If at all possible, you should use the Modified Hardie method for first order extinction. |
