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

TL;DR

This paper critically compares different PSF photometry methods on ACS data of NGC 3370, revealing minimal photometric incompleteness, error differences, and systematic biases, with implications for precise distance measurements.

Contribution

It provides a comprehensive comparison of photometry techniques, highlighting the effects of methodological choices on accuracy and systematic errors in extragalactic stellar photometry.

Findings

Photometric incompleteness varies less than 10% between methods.

Errors are smaller for stacked frames compared to individual frames.

Systematic errors increase at faint magnitudes, reaching ~0.1 mag.

Abstract

A critical analysis and comparison of different methods for obtaining point spread function (PSF) photometry are carried out. Deep ACS observations of NGC3370 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%). 2) Statistical errors are 20% to 30% smaller for DAOPHOT runs on stacked frames than DOLPHOT runs on individual frames. 3) Statistical errors assigned directly by the photometry codes are 25% to 50% 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 $\sigma_s\sim0.1$ mag. 5) The automatic aperture correction routines in DOLPHOT result in a significant systematic error ($\sigma_s \sim 0.05$ 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% 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.