The Corrective Optics Space Telescope Axial Replacement (COSTAR), removed from Hubble during Servicing Mission 4 in 2009, was an ingenious device created to solve a famous Hubble Space Telescope problem.

Soon after Hubble began sending images from space, scientists discovered that the telescope's primary mirror had a flaw called spherical aberration. The outer edge of the mirror was ground too flat by a depth of 4 microns (roughly equal to one-fiftieth the thickness of a human hair). The flaw resulted in images that were fuzzy because some of the light from the objects being studied was being scattered.

After this discovery, scientists and engineers developed COSTAR, corrective optics that functioned like eyeglasses to restore Hubble's vision.

By placing small and carefully designed mirrors in front of the original Hubble instruments, COSTAR --installed during the 1993 First Servicing Mission -- successfully improved their vision to their original design goals.

The optics of the Wide Field and Planetary Camera 2, which was also installed during the 1993 mission, were designed to correct for spherical aberration. This was the first instance of a Hubble instrument being constructed with built-in corrective optics.

All the instruments installed since were built with such internal corrections for spherical aberration, eventually making COSTAR unnecessary. During Servicing Mission 4, astronauts finally removed COSTAR to make way for a new instrument, the Cosmic Origins Spectrograph.

The COSTAR apparatus was built by Ball Aerospace.

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