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PhotoRed Reductions Routines - Comps Standard Magnitudes

PRCompsStandardMags.GIF (44632 bytes)

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 = mi - (k’m * X) + Tm * CI + Zm
where 	M 	converted standard magnitude
	mi 	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.

 

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