Development and utilization of a point spread function for the Extrasolar Planet Observation and Characterization/Deep Impact Extended Investigation (EPOXI) Mission

The Extrasolar Planet Observation Characterization and the Deep Impact Extended Investigation missions (EPOXI) are currently observing the transits of exoplanets, a comet nucleus at short range, and Earth using the High Resolution Instrument (HRI) - a 0.3 m f/35 telescope - on the Deep Impact flyby spacecraft. The HRI is in a permanently defocused state with the instrument point of focus about 0.6 cm before the focal plane due to the use of a reference flat mirror that became a powered optic due to thermal warping during ground thermal-vacuum testing. Consequently, the point spread function (PSF) covers approximately nine pixels FWHM and is characterized by a patch with three-fold symmetry due to the three-point support structures of the primary and secondary mirrors. The PSF is also strongly color dependent varying in shape and size with change in filtration and target color. While defocus is highly desirable for exoplanet transit observations to limit sensitivity to intra-pixel variation, it is suboptimal for observations of spatially resolved targets. Consequently, all images used in our analysis of such objects were deconvolved with an instrument PSF. The instrument PSF is also being used to optimize transit analysis. We discuss development and usage of an instrument PSF for these observations.