The HST mirror.
The mirror of the Hubble
Space Telescope was
perfectly made – but to the
wrong curve! Human error
resulted in an unacceptable
amount of spherical
aberration. Fortunately,
the 1993 repair mission
restored the situation.The Hubble Space Telescope
O
ne of the most ambitious experiments in the history of
science began on 24 April 1990, with the launch of the
Hubble Space Telescope (HST) – named in honour of the
American astronomer Edwin Hubble, who was the first to
prove that the objects once called spiral nebulae are inde-
pendent star-systems. The HST is a reflector with a 2.4-
metre (94-inch) mirror; it is 13 metres (43 feet) long, and
weighs 11,000 kilograms (24,200 pounds). It was launched
in the Space Shuttle Discovery, and put into a near-circular
orbit which takes it round the Earth in a period of 94 min-
utes at a distance of almost 600 kilometres (370 miles).
It is an American project, controlled by NASA, but
with strong support from the European Space Agency; the
solar panels, which provide the power for the instruments,
were made by British Aerospace in Bristol. Five main
instruments are carried, of which the most important areATLAS OF THE UNIVERSE
probably the Wide Field and Planetary Camera (WFPC)
and the Faint Object Camera (FOC). Operating under
conditions of perfect seeing, high above the atmosphere,
the HST was expected to far outmatch any Earth-based
telescope, even though its mirror is so much smaller than
that of instruments such as the Keck.
The first images were received on 20 May 1990, and it
was at once plain that the results would indeed be superb;
the HST can ‘see’ more than any ground-based instrument
could hope to do. Moreover, its range extends from visible
light well into the ultra-violet. Yet there was also an unwel-
come discovery. The mirror had been wrongly made, and
was of an incorrect shape; it was too ‘shallow’ a curve. The
error was tiny – no more than 0.002 of a millimetre – but it
was enough to produce what is termed spherical aberration.
Images were blurred, and it was said, rather unkindly, that
the telescope was short-sighted. Some of the original pro-
grammes had to be modified or even abandoned.
Regular servicing missions had been planned during
the estimated operating time of fifteen years. The first of
these was undertaken in December 1993 by a team of
astronauts sent up in the Space Shuttle Endeavour. They
‘captured’ the telescope, brought it into the Shuttle bay,
and carried out extensive repairs and maintenance before
putting it back into orbit. The WFPC was replaced, and
extra optical equipment was introduced to compensate for
the error in the main mirror.
Several servicing missions have since been carried out,
and the HST has surpassed all expectations. In 10 years or
so it will be succeeded by the James Webb Space
Telescope, which will have a much larger mirror, but will
be so far from Earth that servicing missions will not be
possible.▼ Inside the Sagittarius
star clouds. This picture,
taken in 1998, shows a
narrow dust-free region in
the star clouds which lie
in front of the centre of the
Galaxy. Many of the brighter
stars show vivid colours,
showing that they are at
different stages in their
evolution.B Atl of Univ Phil'03stp 31/3/03 4:01 pm Page 30