Subpixel sensitivity maps for a back-illuminated charge-coupled device and the effects of nonuniform response on measurement
A measurement program designed to investigate the variations in sensitivity of focal plane arrays on a subpixel scale has produced such information for a front-illuminated CCD device, results for which have been previously presented. New measurements have been made to provide information on sensitivity variations within a single pixel for a back-illuminated CCD. The measurements were made using a stable broadband light source and two high-precision translation stages. The pixel scans were obtained using four different spectral filters (three broadband and one narrowband). These results are compared to the pixel response functions of the front-illuminated CCD. These experimentally measured pixel response functions were used to determine their effect on photometric and astrometric measurements. The uncertainty introduced into such measurements, especially when using undersampled CCD data, is significant. It was also found that the form of the subpixel response is dependent on the wavelength of illuminating light. Therefore, the position of the optical center of weight of a pixel varies with wavelength.