COSTAR

Hubble's primary mirror had not been curved correctly, causing a problem known as spherical aberration that resulted in blurred images. Although engineers designed Hubble with many replaceable components, the primary mirror was not one of them.

However, the ability for astronauts to upgrade the observatory in orbit ultimately led to a solution for this seemingly insurmountable problem. Hubble scientists and engineers devised a set of nickel- and quarter-sized mirrors to remedy the effects of the primary mirror on Hubble’s other instruments. Labeled the Corrective Optics Space Telescope Axial Replacement (COSTAR), this device could be deployed into the light paths of the telescope’s other instruments to focus their images properly.

COSTAR is seen here year's later after being returned to Earth, its prescription being built in to Hubble's newer instruments so COSTAR was no longer needed. The optics were deployed for contamination inspection and analysis.