# PhotoRed Reductions Routines - Comps Standard Magnitudes
The Standard Magnitudes – Comps routine converts imported Canopus data into
standard magnitudes. This routine can also work on data measured in PhotoRed. The latter
is handy for those not doing extended time-series work but just getting standard magnitude
values on some target fields, e.g., working a set of long period variables.
You need observations in only filter for this routine. However, you must first get
standard magnitude color index values for the comparisons. That routine requires
observations in two standard filters but only a small number.
The *basic* formula for converting instrumental magnitudes is
M = m_{i} - (k’m * X) + Tm * CI + Zm
where M converted standard magnitude
m_{i} raw instrumental magnitude
k’m first order extinction
X air mass
Tm transform for the single color (not a color index transform)
CI * standard* color index of the object
Zm zero point offset
Note that the *standard* and not *instrumental* color index is used. This is
because PhotoRed is finding the standard magnitude in one color and the previously found
transforms for this purpose were based on knowing the standard and not instrumental color
index. This is a departure from the more traditional approach that finds transforms for
all colors except V in terms of color index, e.g., B-V, by having instrumental values in B
and V, i.e., b-v. The approach in PhotoRed eliminates the need of having observations in
more than one color, assuming that either you find the standard color index values for the
comparisons or provide them manually based on previous work.
## First Order Extinction Considerations
Note that the first order extinction value is included in the formula. This allows for
when you use an assumed or true value. **It’s critical that the FOE values be the
same that you used when finding the transforms.** The reason, in short, is that the
nightly zero points are affected by the FOE values. It can be shown that all other factors
being the same, the only difference in the transform for a given filter with different
values of FOE is to shift the nightly zero point. If you used 0 for the FOE values because
the target and reference fields are close together, then you must use 0 when you run this
routine, otherwise the reduced magnitudes will be wrong by the FOE value.
Once the conversion is done, PhotoRed plots the values. Assuming you have at least two,
preferably three measurements, PhotoRed also reports the standard deviation of the
averaged values for each comparison. If you have only one observation, the s.d. is set to
99.999 since the s.d. in this case is undefined. |