The new lens system was called @COSTAR (Corrective Optics Space Telescope Axial Replacement), and it consisted of several small mirrors that would intercept the beam from the flawed mirror, correct for the defect, and relay the corrected beam to the scientific instruments at the focus of the mirror.
When the HST was tested after the servicing mission, the images were not only vastly improved, but they were utterly astonishing, giving scientists an unprecedented new view of the universe. Now, all of the instruments placed in the HST have built-in corrective optics for the mirror's defect, and COSTAR is no longer needed.
Another major mission came to correct Hubble's orbit. The HST uses six gyroscopes to point itself at stars and galaxies very accurately, and it must stay in an exact position while a picture is taken. Three of the gyroscopes must work perfectly to stabilize the satellite. Unfortunately, four of the gyroscopes broke at the same time. Hubble was put in a special "safe mode" for two months. In this mode, astronomers can't make any observations.
Astronauts trained for the mission in Johnson Space Center's giant swimming pool.
