O
f all the many and varied telescopes
available
for
use
by
amateur
astronomers and nature watchers, all
can be categorised into three types: the
re f r a c t o r, the reflector & the catadioptric.
Each have their relative strengths and weak-
nesses, but they all have a common function:
to gather and focus light from distant objects
to produce a bright image that may be magni-
fied. In this respect it is the aperture (i.e. the
diameter of the main mirror or lens) of the
telescope that performs a critical function.
With larger apertures, more light is gathered
so fainter objects may be perceived and the
resolving power (i.e. the ability to see fine
detail) is increased.
When comparing telescopes of similar type
but of differing apertures, an instrument that
has a main lens or mirror twice the size of
another gathers four times as much light, not
twice. We are comparing the collecting areas
of the larger and smaller telescopes, which is
proportional to the squares of the apertures.
So, for example, a 120mm telescope gathers
2.25 times (225%) as much light as an instru-
ment of 80mm aperture (120/80 = 1.5, 1.5
squared = 2.25).
When it comes to seeing fine detail in an
image, we are simply comparing apertures:
a 120mm telescope will enable you to
perceive lunar craters, for example, half the
size of those visible in a 60mm instrument.
No amount of magnification applied to the
smaller telescope will show you that which
will be visible in the larger instrument, though
both images may appear equally sharp. The
larger telescope merely forms its images out
of smaller 'dots'.
However, the resolving power (as it is cor-
rectly termed) of even the smallest telescope
is awesome: a 60mm telescope is capable of
resolving detail as small as a £1 coin at a dis-
tance of 2.35 km - nearly 1.5 miles!
< The refractor:
This is the type of
instrument that the lay-
man thinks of when
conjuring up a mental
picture of a telescope.
At the end of the tube
furthest away from the
o b s e rver there is an
objective lens (or rather
two glass elements
sandwiched together
for reasons to be ex-
plained in a moment)
that gathers light from
t h e object under scru t i n y,
to form an image at the
other end which is
viewed by means of an
eyepiece.
The objective lens can-
not be made of a single
piece of glass since
such an element is
incapable of bringing
light of differing wave-
lengths to a common
focus, introducing a
prismatic effect that
causes bright objects
to be surrounded by
false rainbow colours.
This undesirable quality
of refractors is virtually
eliminated by making
the objective out of two
glass elements with
optical characteristics
that effectively 'cancel
out' the false colour.
2
Types of Telescope...
