-- Aperture Correction Tasks Installed in the PHOTCAL Package

Aperture Correction Tasks Installed in the PHOTCAL Package

Two new tasks for computing aperture corrections, mkapfile and apfile, have been released with IRAF 2.10.3BETA. mkapfile and apfile compute the magnitude correction required to place the instrumental magnitudes of program stars, which were measured through a small aperture, on the same system as those of the standard stars, which were measured through a large aperture, using the growth-curve method (Stetson, P. B. 1990, PASP, 102, 932). This algorithm computes the growth-curve for each image by combining a best fit analytic model growth-curve with the observed growth-curve, using weights which favor the observed curve at small radii where the observational errors are small, and the model curve at large radii where the observational errors are large. The adopted curve is integrated between the desired aperture radii to produce the aperture correction for each image.

mkapfile and apfile can be run in non-interactive or interactive mode. In non-interactive mode the parameters are set at task startup time, and the fits for each image are computed sequentially. In interactive mode, the user can interact with the growth-curve fitting process by examining plots of the model fit, residuals versus aperture radius, magnitude in the first aperture, or x and y coordinates, and aperture correction as a function of radius. The user can also change the number of model parameters to be fit and their default values, delete and undelete points with the cursor, and refit and reexamine the fits until satisfied with the results.

mkapfile and apfile are installed in the photcal package, and write aperture correction files that can be read directly by the existing tasks in photcal, mknobsfile/mkobsfile/obsfile. The two tasks differ only in the type of input they expect. mkapfile reads the aperture photometry files produced by the IRAF apphot/daophot packages, whereas apfile reads aperture photometry files in a simple text file format created by the user.

Users who have not yet upgraded to IRAF 2.10.3 BETA, or who are running IRAF on machines for which IRAF 2.10.3BETA is not yet available, can acquire the new tasks by retrieving and installing the external package digiphotx from the IRAF network archive.

Lindsey Davis

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