IN-ORBIT PERFORMANCE OF THE COSTAR-CORRECTED FAINT OBJECT CAMERA

The improvements due to the Corrective Optics Space Telescope Axial Replacement (COSTAR) on imaging with the Faint Object Camera on board the Hubble Space Telescope are presented. The encircled energy performance is dramatically improved, such that 85% of the total light in the PSF is now enclosed within a circle of radius 0''.1 at 486 nm wavelength, compared to 18% in the spherically aberrated PSF. This is equivalent to a sensitivity increase of 1.6 mag. The effective angular resolution is also improved from 66 to 43 mas at 486 nm. These improvements are slightly offset by a 20% lower total throughput at visible wavelengths. The plate scale is changed from 22.3 mas pixel-1 to 14.35 mas pixel-1, resulting in a decrease in the field of view from 11 x 11 arcsec2 to 7.3 x 7.3 arcsec2 for the workhorse 512 x 512 format.