# PhotoRed Reductions Routines - Transforms
This routine uses raw instrumental magnitudes of stars in a single standard field and
fits them into a least squares solution using the catalog standard magnitudes to derive
the values required to convert the instrumental magnitudes into standard magnitudes.
See Finding First Order Extinction and Transforms in PhotoRed about how the transforms
and first order extinction routines interact.
The Henden and Kaitchuck book covers the theory behind deriving transforms in
considerable detail and has excellent worked examples. In brief, for the V filter, one
uses the general formula
V – vo = a(CI) + ZP
where V catalog V standard magnitude
vo exoatmospheric magnitude
a transform value
CI color index
ZP zero point offset
Note that the term "vo" may or may not include first (and second) order
extinction corrections. You can set the FOE values to zero, assumed values, or determine
the nightly FOE values using the First Order methods. When you run this routine, you are
asked which catalog color index value to use, i.e., B-V, V-R, or V-I. Make sure there are
catalog values for the selected filters. Base your selection on which filters you’ll
use to take images of the target field, both for finding the color indices of the
comparisons and target as well as finding the standard magnitudes of those objects.
You must use the same color index throughout the process. You must also have
observations in the filters indicated by the Color Index you select. Do not use B-V to
find the transforms when all your images were in V and R.
The left had side is used as the Y-axis data and the CI value for the X-axis. When
placed into a least-squares solution, the plot resembles the one shown above.
As with most routines in PhotoRed, this routine finds transforms for single colors and
not color indices, e.g., it finds transforms for V and R, not V and V-R. This allows a
more direct conversion to standard magnitudes when the bulk of observations are in one
color. |