PhotoRed Reduction Routines
Most of the routines in PhotoRed are based directly on the algorithms used by Henden
and Kaitchuck. The differential approach to standard magnitudes is based on a paper by
Richard Miles in the Journal of the British Astronomical Association. Where possible, the
data and results from those algorithms were used in Canopus to verify the correctness of
the results.
Groups
Say you want to get one or more V and R images for a field and treat them as if they
were taken at the same time, e.g., you want to derive a V-R measurement for that moment.
With a single camera, that’s not possible. PhotoRed somehow needs to know that the
images are to be treated as if they were taken simultaneously. This is accomplished by
assigning a group number to a set of images. In the example, the V and R images taken at
approximately same time would have the same group number. When PhotoRed runs through its
routines, it can group the images as if you had a multi-channel camera.
Usually, you don’t take just one image through each filter. Instead you take two or
three with the intent that the values be averaged. This reduces scatter in the data.
Combined with the grouping just mentioned, PhotoRed can automatically do the averaging for
you.
For example, say you take a quick succession of three images in each of three filters of a
standard field. In PhotoRed, you would assign the same group number to all nine images so
that PhotoRed knew that the data for the nine images was to be “lumped
together.” Then, when running through the routines, PhotoRed sums the magnitudes for
each star in the three images for a given filter and uses the average value as the
magnitude for that star in that filter for that group. It is not required that you have
the same number of images for each filter, though having at least two helps reduce data
scatter. Getting more than three images in each filter is probably not warranted in most
cases.
The concept of groups is an important one to understand when using PhotoRed. In the
simplest terms, all observations of a given object in a given filter that have the same
group number will be averaged to find a single magnitude for that filter and air mass
value. Therefore, it makes little or no sense to put observations made many minutes or
even hours apart into the same group.
The Color Index (CI) Value
For the V filter, the CI value is often B-V, i.e., the catalog B magnitude minus the
catalog V value. However, in PhotoRed the solution can be based also on V-R and V-I. Some
of the routines specifically ask which catalog color index to use when running the
calculations. Later on, when you run the routines that find the standard magnitudes of
comparisons and target, you must tell PhotoRed which CI was used in the previous methods.
What’s critical when you convert Canopus instrumental magnitudes to standard
magnitudes is that you use the same filters that were used for determining the transforms.
It just won’t do to find a slope for B-V magnitudes and apply them to images taken in
V and R.
As mentioned before, the C filter is treated as its own band but uses V magnitudes from
the catalogs (plus one other for transforms and CI determinations). In short, think of the
C filter as a substitute V filter with a higher pass through but the reductions for with
will not be as good in terms of matching the standard system as using a true V filter.
