RECONSTRUCTING FLAT FIELDS FROM NONUNIFORM BACKGROUND-ILLUMINATION SOURCES

Precision CCD photometry over large fields of view and detection of ve ry faint or diffuse objects demand great care in the acquisition of fl at-field frames. Variations in the illuminated field, either in the sk y or with locally illuminated dome screens, will introduce spatial var iability, or gradient structure, in the flat-field frame. This is a co ncern for the Hubble Space Telescope which cannot make use of twilight or dome flats. The physics of generating flat-field frames must accou nt for the characteristics and structure of the CCD, the instrument an d telescope optics, and the illumination source. Recognizing that smal l angular displacements of the CCD, telescope, and instrument, with re spect to the external light source (dome, sky, etc.) yields a correspo nding linear translation of the image on the CCD with respect to the t rue 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 generali zed to two dimensions approximating an elliptic partial-differential e quation relating the offset CCD measurements to the flat-field functio n. This technique may benefit faint deep-field photometric wide field- of-view observations, the HST, and cameras on spacecraft and planetary landers.