Which Hubble instrument replaced the COSTAR system in 2009?

The Cosmic Origins Spectrograph replaced the COSTAR system aboard Hubble in 2009.

COSTAR was installed during Hubble's first servicing mission in 1993 to correct the spherical aberration in the telescope's primary mirror. The flaw had been discovered soon after launch and caused several original instruments to produce blurred images. COSTAR used corrective mirrors to redirect properly focused light to affected instruments.

By 2009, replacement instruments had been designed with their own corrective optics, so COSTAR was no longer needed. During Servicing Mission 4, astronauts removed it and installed the Cosmic Origins Spectrograph, commonly called COS, in the same axial instrument bay. COSTAR is now displayed at the Smithsonian's National Air and Space Museum.

COS is an ultraviolet spectrograph designed for highly sensitive studies of faint, compact sources such as stars and quasars. It complements the Space Telescope Imaging Spectrograph, or STIS, which was repaired during the same mission. Wide Field Camera 3 was also installed in 2009, but it did not replace COSTAR.