What corrective package was implemented to fix the focus issue for instruments like the FOS and GHRS?

Answer

COSTAR

To correct the spherical aberration for most of the scientific instruments that relied directly on the primary mirror's focus, engineers developed the Corrective Optics Space Telescope Axial Replacement, known universally as COSTAR. This instrument was ingeniously designed as an external package containing four small, precisely shaped relay mirrors. When installed into an axial instrument bay, COSTAR intercepted the flawed light beam, passed it through this series of relay mirrors which introduced an opposite, complementary distortion, thereby canceling out the spherical aberration before the light reached the detectors of instruments like the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS).

What corrective package was implemented to fix the focus issue for instruments like the FOS and GHRS?

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