STS-61 was NASA's first Hubble Space Telescope servicing mission, and the fifth flight of the Space Shuttle Endeavour. The mission launched on December 2, 1993, from Kennedy Space Center (KSC) in Florida. The mission restored the spaceborne observatory's vision with the installation of a new main camera and a corrective optics package (COSTAR). This correction occurred more than three and a half years after the Hubble was launched aboard STS-31 in April 1990. The flight also brought instrument upgrades and new solar arrays to the telescope. With its very heavy workload, the STS-61 mission was one of the most complex in the Shuttle's history.

The Corrective Optics Space Telescope Axial Replacement (COSTAR) is an optical correction instrument designed and built by NASA. It was created to correct the spherical aberration of the Hubble Space Telescope's primary mirror, which incorrectly focused light upon the Faint Object Camera (FOC), Faint Object Spectrograph (FOS), and Goddard High Resolution Spectrograph (GHRS) instruments.