Corrective Optics Space Telescope Axial Replacement (COSTAR)

COSTAR was designed to optically correct the effects of the primary mirror's aberration on the Faint Object Camera (FOC), Goddard High Resolution Spectrograph (GHRS) and Faint Object Spectrograph (FOS). COSTAR was removed and returned to Earth during Servicing Mission 4. Its instrument bay was then reused by COS, the Cosmic Origins Spectrograph.

Credit: https://esahubble.org/about/general/instruments/costar/