Applying extinction correction

Extinction correction is applied to an mcat as the last step in the pipeline. It is the last step since we do not correct our standard stars for extinction before using them to go from instrumental to apparent magnitudes. Hence, the input mcat should already be calibrated using standard stars.

The correction can be applied to an mcat by invoking: mcatdir new_mcatdir

new_mcatdir holds the corrected mcat after the program has finished.

The program does the following:
At the location of each object in the mcat (RA,Dec pair) it obtains A(g,r,i) using Schlegel et al. maps and puts it in columns 65, 64, 66. A(r) is written first to maintain the tradition whereby mag_F appears before mag_J. Note that this has not been retained in case of the CCD calibrated mags in columns 161-163. These three calibrated magnitudes are read, corrections applied and written to columns 68 (r), 67 (g) and 69 (i). The values are stored after multiplying them by 1000 to make them integral. The original columns 161-163 are left unchanged. This is done so that we can use either corrected or uncorrected magnitudes easily. Or for being able to apply a different extinction correction.

Procedurally, this is what the pragram does:
Read in legend files from mcat, determine where the calibrated magnitudes are, adjust them by appropriate amounts, find the first blank column, call it ExtF, and put the correction term in it. In the next two blank columns put ExtJ and ExtN. For a standard mcat these are columns 64-66. Write back the mcat (in the newdir).

Additionally, the g mags are corrected with a color term:
g_color_term = -0.373 * (g_o(J) - r_o(F)) + 0.177

Last Outdated: 14 Nov 2000