PhotoRed Reductions Routines - Color Index
In order for the differential reduction formulae to work in PhotoRed, you must know the
standard, not instrumental, color index of the comparisons and target. The color
differences between each comparison and the target factors into converting the raw
instrumental magnitude in a given filter to the standard band.
Whats handy about the PhotoRed approach is that you need only one to three images
in each filter of the target field. If you have more than one image, PhotoRed averages the
values of the set to generate a single value. You need a minimum of three to compute a
true standard deviation.
When you get those images depends on how you did the transforms, more specifically, if
you used first order extinction terms in the reduction process. If you set the FOE values
to zero, then the assumption was that the target field and the reference field were 1)
close to one another and 2) that the fields were not too low to the horizon such that
extinction differences were exaggerated by high air mass values. If the two fields are
within 5° of one another and you shoot them when they are at least 45° high, the
assumptions will be valid.
If you used good assumed or actual FOE values, then the only requirement is that you
not shoot the fields too low to the horizon, though you can go lower than 45°.
Another handy feature within PhotoRed is that you do not have to take and measure
images in Canopus to find the color index values, as was required in early versions of the
software, which would leave a number of very small sessions used for a limited purpose. In
PhotoRed, you can run one of two photometry wizards that produces data in the same style
as if you had imported data from Canopus.
If you did or will do a time series session in Canopus with the intent of importing the
data for reduction to standard magnitudes, then you must be certain when measuring the
images in PhotoRed for color index values that you use the exact set of comparisons. This
means not only the same stars but in the same order, i.e., Comparison1 in Canopus must be
Comparison1 in PhotoRed.
Those doing limited work, e.g., keeping track of variables where precise photometry has
not been done, can use this feature to find the color index of the comparisons and target
so that they can used PhotoRed-only data to find the standard magnitudes for their
targets. Again, it is no longer necessary to measure images in Canopus if your only
purpose is to get just enough images to find the standard magnitude of a target and not
worry about a prolonged time-series.