Telescopes and the Stars
N
obody can be sure just when telescopes were invented,
but there is strong evidence that Leonard Digges, in
England, built a workable telescope in or around the year- Apparently it used both a lens and a mirror; we do
not know exactly what it looked like, and there is no firm
evidence that it was ever turned skywards.
The first telescopes of which we have definite
knowledge date back to 1608, and came from Holland.
During 1609 Thomas Harriot, one-time tutor to Sir Walter
Raleigh, drew a telescopic map of the Moon which shows
recognizable features, but the first systematic observations
were made from 1610 by Galileo Galilei, in Italy. Galileo
made his own telescopes, the most powerful of which
magnified 30 times, and used them to make spectacular
discoveries; he saw the mountains and craters of the
Moon, the phases of Venus, the satellites of Jupiter, spots
on the Sun and the countless stars of the Milky Way.
Everything he found confirmed his belief that Copernicus
had been absolutely right in positioning the Sun in the
centre of the planetary system – for which he was accused
of heresy, brought to trial in Rome, and forced into a
hollow and completely meaningless recantation of the
Copernican theory.
These early 17th-century telescopes were refractors.
The light is collected by a glass lens known as an objec-
tive or object-glass; the rays of light are brought together,
and an image is formed at the focus, where it can be mag-
nified by a second lens termed an eyepiece.
Light is a wave-motion, and a beam of white light is a
mixture of all the colours of the rainbow. A lens bends the
different wavelengths unequally, and this results in false
colour; an object such as a star is surrounded by gaudy
rings which may look pretty, but are certainly unwanted.
To reduce this false colour, early refractors were made
with very long focal length, so that it was sometimes nec-
essary to fix the object-glass to a mast. Instruments of this
kind were extremely awkward to use, and it is surprising
that so many discoveries were made with them. A modern
objective is made up of several lenses, fitted together and
made up of different types of glass, the faults of which
tend to cancel each other out.
Isaac Newton adopted a different system, and in 1671
he presented the first reflector to the Royal Society of
London. Here there is no object-glass; the light passes
down an open tube and falls upon a curved mirror, which
reflects the light back up the tube on to a smaller, flat
mirror inclined at 45 degrees. The inclined mirror reflects
ATLAS OF THE UNIVERSE
Principle of the Newtonian
reflector. The light passes
down an open tube and falls
upon a curved mirror. The
light is then sent back up
the tube on to a smaller, flat
mirror placed at an angle of
45°; the flat directs the rays
on to the side of the tube,
where they are brought to
focus and the image is
magnified by an eyepiece. Principle of the refractor.
The light from the object
under observation passes
through a glass lens (or
combination of lenses),
known as an object-glass
or objective. The rays are
brought to a focus, where
the image is enlarged by a
second lens, known as the
eyepiece or ocular. The equatorial mounting.
The telescope is mounted
upon an axis directed towards
the celestial pole, so that
when the telescope is moved
in azimuth the up-or-down
motion looks after itself. Until
recently all large telescopes
were equatorially mounted. The altazimuth mounting.
The telescope can move
freely in either altitude (up
and down) or azimuth (east to
west). This involves making
constant adjustments in both
senses, though today modern
computers make altazimuth
mountings practicable for
very large telescopes.EyepieceEyepieceEyepiece EyepieceObject-glassCurved
mirrorMirrorVertical
axisPolar
axisDeclination
axis
Horizontal
axisGerman
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