Corrective Optics Space Telescope Axial - Second Generation Corrective Optics COSTAR is not an actual instrument, but a system of mirrors that refocus the light and correct for the aberration from HST's optical system for first generation instruments The FOC is the only instrument remaining today that uses COSTAR
Corrective Optics Space Telescope Axial Replacement; set of corrective optics installed in the 1993 HST servicing mission to correct the FOC, FOS, and GHRS for the spherically aberrated wavefront due to the misformed HST primary mirror COSTAR does not correct the wavefront as seen by the FGS