BBC Sky at Night Magazine
As the first images from Hubble came down to Earth, it was obvious all was not well and it wasn’t performing as it should. After a few weeks the problem was identified – the primary mirror was just 2 microns too shallow, around one 50th the width of a human hair. Hubble couldn’t reach the sharp focus it was meant to. Rather than collecting 70 per cent of starlight to a focal point, Hubble was only able to collect 10 to 15 per cent. Luckily the upgraded WFPC2 was being built and engineers could adjust the optics to compensate for the flaw.
This didn’t solve the problem for all the other instruments onboard, however. A set of smaller corrective mirrors, named the Corrective Optics Space Telescope Axial Replacement (COSTAR) were also developed to focus the images for the other scientific instruments. COSTAR consisted of five pairs of small mirrors on deployable arms, with 12 motors and over 5,000 individual parts, that could be positioned to intercept and refocus the light from the primary mirror into the two remaining spectrographs and FOC.
It launched on Space Shuttle Endeavour in December 1993, on the most complex Shuttle mission ever performed. It took the seven astronauts, 10 days and five spacewalks to install the new equipment. The mission went off without a hitch and when the first pin sharp images came through, they knew it had been a complete success. Hubble was back in business.