The Mammoth Book of Space Exploration and Disasters

On 2 December 1993 the first servicing mission (STS-61) was launched aboard space shuttle Endeavour and on 4 December the mission commander, Colonel Richard Covey, piloted the shuttle to within 30 ft of the telescope. The astronauts, in pairs, did the necessary work during five spacewalks, a record for a single mission and four of the telescope’s six gyroscopes were replaced. On 5 December the two solar panels, which had been vibrating as a result of extreme changes in temperature, were replaced. On 6–7 December two astronauts replaced the Hubble’s primary camera, which had the flawed mirror, and also replaced two magnetic sensors, which measured the telescope’s position in the magnetic field. On 13 December the shuttle landed at Cape Canaveral, but it was another month before astronomers saw the first photographs from the repaired telescope. On 13 January 1994, NASA officials released photographs taken after the repairs, images that were much clearer than those taken earlier. One subject of the new photographs was the core of a galaxy 50 million light years distant. The astronauts had installed COSTAR (Corrective Optics Space Telescope Axial Replacement) to rectify the problem with the telescope’s primary mirror.

The instrument most affected by the poor images was the Wide Field and Planetary Camera, (WFPC), which the astronauts had completely replaced with a new one (WFPC2). WFPC2 incorporates secondary mirrors that have been deliberately figured high at the edges so that they introduce spherical aberration of the same magnitude as that of the HST primary mirror but of the opposite nature so that they should send corrected images to the CCD sensors.

The F/24 light beam from the main telescope is first fed to one of the instruments by a movable mirror. In the case of the WFPC the light then goes to a pyramid mirror which directs the beam into one of four CCD cameras. Each camera has a different focal length and one is chosen to give the most suitable size of image on the CCD sensor. The cameras are of the Cassegrain mirror type and it is the Cassegrain secondary mirror of each camera which has been specially figured with the right amount of spherical aberration.

Incredibly this corrective aberration has been incorporated in mirrors which are only one centimetre in diameter. Alignment of these mirrors is extremely critical so they are provided with positional adjustments which can be operated from the ground control centre.