The photometry of undersampled point-spread functions

An undersampled point-spread function (PSF) may interact with the microstru cture of a solid-state detector such that the total flux detected can depen d sensitively on where the PSF center falls within a pixel. Such intrapixel sensitivity variations will not be corrected by flat-field calibration and may limit the accuracy of stellar photometry conducted with undersampled i mages, as are typical for Hubble Space Telescope observations. The total fl ux in a stellar image can vary by up to 0.03 mag in F555W WFC images depend ing on how it is sampled, for example. For NIC3, these variations are espec ially strong, up to 0.39 mag, strongly limiting its use for stellar photome try. Intrapixel sensitivity variations can be corrected for, however, by co nstructing a well-sampled PSF from a dithered data set. The reconstructed P SF is the convolution of the optical PSF with the pixel response. It can be evaluated at any desired fractional pixel location to generate a table of photometric corrections as a function of relative PSF centroid. A caveat is that the centroid of an undersampled PSF can also be affected by the pixel response function; thus sophisticated centroiding methods, such as cross-c orrelating the observed PSF with its fully sampled counterpart, are require d to derive the proper photometric correction.