Research telescopes that use ordinary (optical) light
are basically light funnels. Their purpose is not so much to make
things look bigger or closer, but to make them look brighter. Many
astronomical objects are big enough to see with the naked eye; they
just aren't bright enough. Therefore the most important feature of
an optical telescope is the size of the area over which light is
collected - the bigger the size, the more photons are collected,
the brighter the
The first astronomical telescope was designed and used
by Galileo Galilei. He used two lenses, one an objective lens to
focus the light, and one an eyepiece. Such a telescope is called
a refracting telescope.
An alternative was designed by Isaac Newton, which
uses a mirror to bring the light to a focus. Such telescopes are
called reflecting telescopes.
There are problems with both kinds of telescope. Refractors
have the difficulty that lenses refract different wavelengths of
light by different amounts, so the focus is at a different point
for blue light than for red lightthis is called chromatic aberration.
Also, the objective lens must be held by the edges. This means that
bigger more powerful lenses become difficult to deal with, since
a huge piece of glass has to be held by a very thin edge.
Almost all astronomical research telescopes are now
reflecting telescopes. These can be supported all across the back
of the primary mirror, and this has many advantages as the mirrors
get large. The problem with a reflector is where to put your head
(or photographic plate or detector). If you stare down at the mirror,
you get in the way of the light coming from the star. The solution
is generally to put another smaller "secondary" mirror in front of
the primary mirror, to deflect the light to some other position where
detectors can be placed. Various configurations of secondary and
additional mirrors are in general use, such as prime, cassegrain
and nasmyth focii. Very big telescopes sometimes use a prime focus
cage where the secondary mirror ordinarily sitsthe observer (or
instrument) then rides around in this cage at the front end of the
telescope while observations are being carried out.
For many years the biggest reflecting telescope was
the Palomar 200" (5m) telescope, a marvel of mid-20th century engineering.
Within the past decade, technical advances have made it possible to make bigger
While the 200" is still
in active use, there are now many larger telescopes
in use or under construction. Modern telescopes also have improvements in resolution
and field of view that make them
more powerful than the 200" and its smaller contemporaries. One of the first
examples of a new technology telescope was the WIYN 3.5m telescope at Kitt Peak, Arizona, completed in 1994.
Interestingly, the famous Hubble
Space Telescope (HST) is not a particularly large telescope—it
has only a 2.4m mirror, about half the diameter of the Palomar 200",
and a quarter that of the Keck telescopes. The advantage of the
HST is that it is above the atmosphere, and therefore does not have
degraded resolution due to the Earth's atmosphere. Shortly after
launch, HST was found to have a problem
with the primary mirror which reduced its resolution considerable.
After this problem was solved, the resolution (measured by a quantity
known as seeing)
was about 10-15 times better than a typical ground-based telescope.
It is the high resolution that allows HST to create such spectacular
images. But there are many astronomical projects for which Hubble
is simply too small—for these projects, astronomers continue
to rely on ground-based optical telescopes.