Hubble’s troubles

In 1962 the US National Academy of Sciences proposed building a large telescope that would allow astronomers to study the universe. The new telescope would be placed in orbit which would enable it to make observations free from atmospheric interference. The Hubble Space Telescope (HST) was named after Edwin Hubble, in 1929 who had observed that distant galaxies were moving away from us therefore the universe was expanding.

In 1977 the US Congress approved funding for the HST and construction of the telescope began.

In 1981 the Baltimore, Maryland‑based Space Telescope Science Institute (STScI) became operational and the precision‑ground mirror of the telescope was completed.

In 1985, construction of the entire HST was completed and the ground control facility for the telescope was established at the Space Telescope Operations Control Center in Goddard.

The Hubble Space Telescope should have resolving power ten times better than any ground‑based telescope. It should be able to see objects which are fifty times fainter. In addition it would be able to observe wavelengths which are not detectable from the ground, particularly ultraviolet.

The launch of the HST was delayed due to the Challenger disaster in 1986, but in October 1989 the telescope was moved from Lockheed, California to its launch site at the Kennedy Space Center in Florida and on 24 April 1990 the HST was launched aboard the STS‑31 mission of the Discovery space shuttle.

As soon as the Hubble Space Telescope was deployed in space, it became apparent that the primary mirror was the wrong shape. The 2.4 m concave mirror was too shallow by 2 mm at the edge and this caused light from the outer part of the mirror to converge to an F/24 focal point some 38 mm behind the light from the central region. As a result star images were surrounded by haloes, several being seconds in diameter instead of being pin sharp and only a fraction of a second in diameter. The primary mirror was clearly suffering from a severe case of spherical aberration, and it was later found that this was the result of faulty testing in the optical works, because one test component was 1.3 mm out of position. Five 6‑hour space walks were required to repair the Hubble Space Telescope.

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.

The other instruments in the HST are a Faint Object Camera, a High Resolution Spectrograph, a Faint Object Spectrograph and a High Speed Photometer. To improve the images in these instruments NASA arranged to fit one corrective optical system common to them all, known as COSTAR. As COSTAR required some space the astronauts had to take out the least used of the instruments, the High Speed Photometer and replace it with COSTAR, which is in the form of a box about the size of a small refrigerator. In that box have been fitted ten mirrors, twelve DC motors, four movable arms and many sensors. As well as installing W17PC and COSTAR, astronauts in the Space Shuttle Endeavour replaced the solar cell arrays which have been troubled by jitter, and also three gyroscopes which are essential for measuring the telescope motions about its three axes of rotation. The telescope was equipped with three pairs of gyros, but one gyro in each pair had already failed – if one more gyro had failed the telescope would have become inoperable. According to Sky and Telescope, even if there had been no problems with HST’s optics or solar cells, NASA would have sent up a repair team just to replace the gyros.

In 1994 HST sent back images of the Orion Nebula. The images released by NASA depicted the births of planets near newborn stars.

In November 1995 NASA released images of the Eagle Nebula, which confirmed the birth of stars.

In 1996 “Deep Field” images were sent back by the telescope, providing an insight into the history of the universe, dating back more than 10 billion years.

During the second servicing mission in February 1997, scientists updated some of Hubble’s instruments and in October, NASA extended Hubble’s operations from 2005 to 2010.

In 1999 HST shut down when a fourth gyroscope on board the telescope failed. Servicing Mission 3A (STS‑103) was launched in December.

In 2002 Servicing Mission 3B was launched for the installation of the NICMOS Cooling System (NCS).

In 2003 HST viewed the core of one of the nearest globular star clusters, called NGC 6397.

The next servicing mission was cancelled after the Shuttle Columbia accident and the NASA Administrator decided to cancel all further HST on‑orbit servicing, including Servicing Mission 4, a decision based on the risks to the Shuttle astronauts associated with future HST servicing missions.