This is the flown COSTAR (Corrective Optics Space Telescope Axial Replacement), a deployable optical bench built within a box frame unit designed to fit into the Hubble Space Telescope as an axial element in the instrument bay. It was installed on HST during the first servicing mission STS-61 in December 1993 to provide a means of correcting the spherical aberration of the primary mirror that was discovered soon after launch of the telescope in April 1990. COSTAR fits into one of the axial modules and contains 5 pairs of small corrective mirrors on deployable arms to send corrected light to other instruments on the Hubble: the Faint Object Camera, the Faint Object Spectrograph and the Goddard High Resolution Spectrograph. It was not designed to correct the beam for the WFPC since that required a far larger beam, and it was deemed advisable and feasible to replace WFPC with a slightly altered version to perform the needed optical corrections internally. COSTAR is a unique object in its function in HST history and in space history overall.

Elements of COSTAR were devised by a team of engineers at NASA and Ball Aerospace. The corrective mirrors were made by Tinsley Laboratories of Berkeley, CA, and the COSTAR was assembled was built by Ball Aerospace in Boulder Colorado.