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NASA has began a series of tests aimed at making the best of a bad job
out of the troubled Hubble Space Telescope. Last week, the agency attempted
to train two instruments aboard Hubble onto a number of stars to determine
how to compensate for Hubble’s faulty primary mirror, the source of the
problem.

Over the next two months, all the telescope’s other instruments will
be tested as well. Then NASA will advise astronomers how to adapt their
experiments to get the best data possible. ‘We expect now that about 50
to 60 per cent of the original science can be done,’ says Ed Weiler, NASA’s
programme scientist for Hubble. Most of the loss involves a wide field camera.
A faint object camera, a photometer and spectrometers will also be affected,
but not so seriously.

‘It’s the deep sky astronomy, looking back through time, that is most
affected,’ says Weiler. Objects of most interest are typically some 10 billion
light years away. The blurring of the images caused by the spherical aberration
of the primary mirror does not destroy resolution of objects; astronomers
can still distinguish objects within 0.10 arc seconds of each other. But
the ‘encircled energy’ captured by Hubble’s instruments, a measure of an
object’s brightness, is expected to remain only 15 to 20 per cent, down
from a projected 70 per cent. ‘The energy is being blurred out,’ Weiler
explains.

NASA will use a computer to ‘deemphasise the fuzz’ and enhance the bright
core of images, says Weiler. This will erase any ‘haloes’ created by the
mirror’s aberration. The tests will also help NASA to design a replacement
for the faulty mirror, to be launched by the space shuttle in 1993.

NASA concluded last month that a faulty optical reference device used
to measure the surface of the mirror during its manufacture caused the aberration
(This Week, 18 August).

The error in the device measured about one millimetre, which is consistent
with the amount of blurring in the images the telescope is sending back
to Earth. The board of inquiry investigating the ‘single but costly error’
by the mirror’s manufacturer has not determined how the slip-up occurred.
They are investigating the possibility that washers on the baseplate of
the lens assembly, which are 1.3 millimetres across, had been improperly
installed.