Tests of photometry: the case of the NGC 3370 ACS field

ABSTRACT

A critical analysis and comparison of different methods for obtaining point spread function (PSF) photometry are carried out. Deep ACS observations of NGC 3370 were reduced using four distinct approaches. These reductions explore a number of methodological differences: software packages (DAOPHOT and DOLPHOT), input images (individual and stacked frames), PSF models (synthetic and empirical), and aperture correction methods (automatic and manual). A comparison of the photometry leads to the following results: (1) Photometric incompleteness between individual reductions shows only a minimal difference (<10 per cent). (2) Statistical errors are 20 per cent to 30 per cent smaller for DAOPHOT runs on stacked frames than DOLPHOT runs on individual frames. (3) Statistical errors assigned directly by the photometry codes are 25 per cent to 50 per cent smaller than the errors measured from artificial star tests. (4) Systematic errors are magnitude dependent and become larger at the faint end, at the level of σs ∼ 0.1 mag. (5) The automatic aperture correction routines in DOLPHOT result in a significant systematic error (σs ∼ 0.05 mag). (6) Individual reductions agree well at the 0.02-mag level when the systematic errors are properly corrected through artificial star tests. The reasonable agreement between the reductions leads to important implications that (i) the reduction-dependent errors can be reduced to a 1 per cent level in the luminosity distance scale, and (ii) the stacked frame photometry can be a good means to study non-variable stars in external galaxies.