EXPLORING THE UNIVERSE
the light to the side of the tube, where an image is formed
and enlarged by an eyepiece as before. A mirror reflects
all wavelengths equally, so that there is no false colour
problem. Newtonian reflectors are still very popular,
particularly with amateur astronomers, but there are other
optical systems such as the Cassegrain and the Gregorian,
where the light is reflected back to the eyepiece through a
hole in the centre of the main mirror.
Newton’s first reflector used a mirror only 2.5 centi-
metres (1 inch) in diameter, but before long larger
telescopes were made. In 1789 William Herschel, a
Hanoverian-born musician who lived in England, built a
reflector with a 124.5-centimetre (49-inch) mirror, though
most of his work was carried out with much smaller
instruments. Then, in 1845, came the giant 183-centimetre
(72-inch) reflector made in Ireland by the third Earl of
Rosse, who discovered the spiral forms of the star systems
we now call galaxies. The Rosse reflector remained the
world’s largest until the completion of the Mount Wilson
2.5-metre (100-inch) reflector in 1917.
Admittedly the Rosse telescope was clumsy to use,
because it was slung between two massive stone walls
and could reach only a limited portion of the sky.
Moreover, a celestial object moves across the sky, by
virtue of the Earth’s rotation, and the telescope has to
follow it, which is not easy when high magnification is
being used. In 1824 the German optician Josef Fraunhofer
built a 23-centimetre (9-inch) refractor which was mech-
anically driven and was set up on an equatorial mount, so
that the telescope rides the axis pointing to the pole of the
sky; only the east-to-west motion has to be considered,
because the up-or-down movement will look after itself.
Until the development of modern-type computers, all
large telescopes were equatorially mounted.
The late 19th century was the age of the great refrac-
tors, of which the largest, at the Yerkes Observatory in
Wisconsin, USA was completed in 1897. The telescope
has a 1-metre (40-inch) object-glass, and is still in regular
use. It is not likely to be surpassed, because a lens has
to be supported round its edge, and if it is too heavy it
will start to distort under its own weight, making it use-
less. Today almost all large optical telescopes are of the
reflecting type, and are used with photographic or elec-
tronic equipment. It is not often that a professional
astronomer actually looks through an eyepiece these
days. The modern astronomer observes the skies on a
computer or TV screen.The Rosse reflector.
This telescope was built by
the third Earl of Rosse, and
completed in 1845. It had a
183-cm (72-inch) metal
mirror; the tube was mounted
between two massive stone
walls, so that it could be
swung for only a limited
distance to either side of
the meridian. This imposed
obvious limitations;
nevertheless, Lord Rosse used
it to make some spectacular
discoveries, such as the spiral
forms of the galaxies. The
telescope has now been fully
restored, and by 2001 was
again fully operational. This
photograph was taken in 1997.▲ Herschel’s ‘forty-foot’
reflector was completed in- The mirror was 124 cm
(49 inches) in diameter, and
was made of metal; there
was of course no drive, and
the mounting was decidedly
cumbersome. The optical
system used was the
Herschelian; there is no flat,
and the main mirror is tilted
so as to bring the rays of
light directly to focus at the
upper edge of the tube – a
system which is basically
unsatisfactory.
▲ The Yerkes refractor.
This has a 101-cm (40-inch)
object-glass. It was
completed in 1897, due to
the work of George Ellery
Hale, and remains the largestrefractor in the world; it is
not likely that it will ever be
surpassed, because a lens
has to be supported round
its edge, and if too heavy will
distort, making it useless.B Atl of Univ Phil'03stp 31/3/03 3:55 pm Page 15