11.2 Aperture Photometry

To establish the transformation between instrumental magnitudes and standard magnitudes, we need to do photometry on the standard stars. Since the galaxy surface photometry is done as aperture photometry, we also need to do aperture photometry on the standard stars, as opposed to for example point spread function fitting.

The larger the aperture, the more of the flux from the star is within the aperture. But the larger the aperture, the larger is the error from the sky subtraction, and the more cosmic ray events will be within the aperture. Thus, the optimal aperture is one which is large enough to enclose most of the flux, but otherwise is as small as possible.

The correction for the finite size of the aperture is called the aperture correction. It is computed from a growth curve, i.e. a plot of magnitude within a given aperture versus aperture size. The aperture correction is simply the magnitude difference between the asymptotic magnitude and the magnitude at the given aperture.

The most straightforward way to determine the aperture correction is to measure it directly from a number of growth curves. A more advanced method is the DAOgrow algorithm (Stetson 1990), implemented in the task mkapfile in digiphotx.photcalx. It fits a five parameter (with usually only 3 of them free) stellar profile model to the growth curves for one or more stars in one or more images. It then computes the aperture correction from a given aperture to the largest aperture.

One of the five parameters in the model, R_i, represent the seeing radius for the individual images. The other four parameters are global for all the images. For more details, see Stetson (1990).