VAPHOT - A Package for Precision Differential Aperture Photometry

The aperture photometry package ‘vaphot’ was developed to perform reliable and precise time-series photometry of uncrowded fields. This package works within the IRAF environment and is built upon the standard aperture photometry task ‘phot’ from IRAF. The design goal of ‘vaphot’ was a simple-to-use photometry task with relatively few input parameters that performs precise aperture photometry, using optimum sized apertures. The derivation of optimum sized apertures is outlined; the calculation of these sizes is performed by a subroutine ‘apcalc’ within the ‘vaphot’ package. We also outline the derivation of signal-tonoise values for differential photometry, when the flux reference is constituted by an ensemble of reference stars. A comparison of vaphot against normal aperture photometry shows, that the sizing of apertures with the psf of the CCD images greatly isolates against changes in the size of the psf during an observing run. The use of apertures sized individually to each star on the field does not greatly improve photometry against the use of similar-sized apertures. However, the use of photometry with previously determined optimum aperture sizes ensures that this photometry will be of the best quality that can be obtained.