CCD Data Taking Modes and Flatfielding Problems

Abstract

We briefly review some problems in the flatfielding of CCD images, in the context of three data-taking modes: stare, short scan, and drift scan. The principal sources of flatfielding imperfections are: (1) mismatch in the spectra of the astronomical sources of interest and the flatfleld illumination; (2) a variety of low-level additive errors; and (3) nonlinearities of the CCD response. Residual flatfielding errors are probably the limiting factor in high-precision astronomical photometry. Flatfielding accuracies of the order of 1 – 2% per pixel are commonly achieved; with some effort, accuracies of ~ 0.1% can be reached; higher accuracies require a substantial effort, or improvements in the quality of CCD chips. In general, scanning data taking modes outperform the “standard” stare mode.