How did scientists correct the Hubble Space Telescope’s focusing problem?

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Telescope Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.

Shortly after the Hubble Space Telescope (HST) was launched in 1990, scientists discovered that its primary mirror had been ground to the wrong shape due to faulty testing procedures. This error resulted in a spherical aberration, which caused the telescope to produce blurry images rather than the intended sharp, high-resolution views.1

To correct this focusing problem, NASA astronauts conducted a repair mission in December 1993 using the space shuttle Endeavour. During five space walks, the crew performed two primary tasks to restore the telescope's optical performance:

They replaced the original wide-field planetary camera with a new version.
They installed a corrective device containing 10 tiny mirrors designed to fix the light paths from the primary mirror to the telescope's other three scientific instruments.1

This mission was a complete success, allowing the HST to operate at its full potential and begin capturing the clear, detailed images of the universe for which it is now famous.1