Reconstructing Flat Fields from Non-Uniform Background Illumination Sources

Abstract

Precision CCD photometry over large fields-of-view and detection of very faint or diffuse objects demand great care in the acquisition of flat-field frames. Variations in the illuminated field, either in the sky or with locally illuminated dome screens, will introduce spatial variability, or gradient structure, in the flat field frame. This is a concern for the Hubble Space Telescope which cannot make use of twilight or dome flats. The physics of generating flat-field frames must account for the characterisitics and structure of the CCD, the instrument and telescope optics, and the illumination source. Recognizing that small angular displacements of the CCD, telescope, and instrument, with respect to the external light source (dome, sky, etc.) yields a corresponding linear translation of the image on the CCD with respect to the true flat-field function, we can derive a functional equation that can be solved and hence enables a flat-field frame to be generated that is independent of the illumination source. This equation can be generalized to two-dimensions approximating an elliptic partial differential equation relating the offset CCD measurements to the flat-field function. This technique may benefit faint deep-field photometric wide field-of-view observations, the HST, and cameras on spacecraft and planetary landers. (SECTION: Astronomical Instrumentation)