|
II.
Optical Telescopes
There
are two main kinds of optical telescopes—refracting and reflecting.
Refracting telescopes use a lens to magnify objects; reflecting telescopes
use a curved mirror.
A.
Refracting Telescopes
Refracting
telescopes, or refractors, use a glass lens to bend, or refract, starlight
and bring it to a focus. The lens is convex, meaning that the center of
the lens is its thickest part, and the lens becomes thinner toward its
edges. A convex lens bends light at the edge of the lens to a greater
angle than light coming through the center, so all of the rays converge to
a focus. The distance between the lens and the place where the rays
converge is called the focal length of the lens. A refracting telescope's
light-gathering power is proportional to the size of the objective, or
main, lens and to the ratio of the focal lengths of the objective lens and
the eyepiece.
Refracting
telescopes are typically hampered by chromatic aberration, which
causes different colors of light to come to a different focus because
every color has its own degree of refraction. Chromatic aberration causes
the image of a star or planet to be surrounded by circles of different
colors.
Another
fundamental limitation of refractors is that lenses with diameters beyond
40 in (100 cm) are impractical because they weigh more than half a ton and
sag under their own weight, distorting the starlight. They cannot be
supported from behind, as optical mirrors are.
B.
Reflecting Telescopes
A
reflecting telescope uses a precisely curved mirror instead of a lens to
collect starlight. The mirror is concave—that is, shaped like the inside
of a dish—a shape that brings reflected light waves to a focus at a
point above the mirror. Reflecting telescopes are especially useful for
gathering light from dim objects. A reflecting telescope's light
sensitivity increases with the square of the diameter of the telescope's
mirror, so doubling the mirror's diameter increases light-gathering power
by a factor of four. Not only can a larger telescope see fainter objects,
but it can also obtain the data in a fraction of the time required for a
smaller telescope. Larger reflecting telescopes can typically detect
objects that are a millionth or a billionth the brightness of the faintest
star seen by the human eye.
The
ideal mirror for a reflecting telescope has a parabolic
or hyperbolic shape that brings distant light rays to a precise focus.
Such mirrors are difficult to make because the curvature of the mirror's
surface changes with its distance from the center, unlike a simpler,
though not as precise, spherical reflector. A telescope mirror is cast
from special molten glass that will not significantly expand or contract
with temperature once it cools and hardens. Pyrex glass has been commonly
used, and newer materials include borosilicate glass and a glass-ceramic
composite.
The
molten glass is cast as a mirror blank, a flat, thick disk that
approximates the size of the finished mirror. It then must cool slowly to
avoid cracking. Once cooled, the flat mirror blank's surface is ground and
polished to the right shape using a computer-controlled polishing tool
that rubs a liquid slurry of fine abrasive across the glass. This process
must be extraordinarily accurate—differences in the surface must be
smaller than a fraction of the width of a human hair. A fine layer of
aluminum is deposited on the glass to create a reflective surface.
A
technique that reduces some of the time needed to grind a mirror to shape
was developed in the 1990s. The glass is spun into the desired shape while
it is still molten. Rotational forces move some of the glass toward the
edge of the spinning container and into a shape called a paraboloid. After
it cools, a spin-cast mirror does not require laborious grinding to remove
excess glass.
Astronomers
seek ever-larger mirrors to increase the power and efficiency of
telescopes. However, huge mirrors are expensive and difficult to make, and
they are challenging to move while tracking celestial targets. One
particularly daunting problem is that a solid glass mirror is heavy. The
200-in (508-cm) Hale telescope on California's Mount Palomar weighs 14
tons.
In
the 1990s a daring and innovative design broke the mirror size barrier.
Each of the twin Keck telescopes located in Mauna Kea, Hawaii, combined 36 hexagonal
72-in (183-cm) mirrors together, like bathroom tiles, to behave like one
immense 400-in (1,016-cm) mirror having four times the collecting power of
Palomar.
In
some telescopes designed in the 1990s, the mirror's weight has been
dramatically reduced by sandwiching a honeycomb pattern of glass ribs
between a thin, but rigid, concave mirror and a flat back plate. Engineers
have even developed meniscus mirrors—mirrors that are too thin to
support their own weight. An adjustable framework supports the meniscus
mirror, and servomechanical actuators, controlled by computer,
continually adjust the shape of the mirror as it tracks celestial targets.
Actuators are also critical to the operation of segmented mirror
telescopes, like Keck, that require that a number of smaller mirrors
operate as if they were one large mirror.
C.
Resolution
An
optical telescope's resolution—the ability to see fine
detail—increases with mirror or lens size. However, Earth's turbulent
atmosphere provides a practical limit on resolution because it blurs
incoming starlight. This effect makes stars appear to twinkle at night.
Ground-based telescopes can typically see objects as small as 1 second of
arc, about the apparent width of a U.S. quarter at 50 km (30 mi).
With
the use of computers, astronomers are developing adaptive optics that
essentially take the blur out of starlight. Astronomers hope that
computers will be able to analyze the blurring created by the atmosphere
and compensate for it by rapidly distorting the mirrors in a reflecting
telescope.
D.
Optical Interferometry
A
new technique in optical astronomy is to combine signals from telescopes
in separate locations so that the resulting image is equal to that
received from one giant telescope, a method called optical interferometry.
The European Southern Observatory started construction of the largest
optical interferometer in 1996. The Very Large Telescope (VLT) is located
in the Atacama Desert in northern Chile and will combine the light from
four 315-in (800-cm) telescopes, producing an image equivalent to that of
a 630-in (1,600-cm) telescope. The first telescope was installed in 1998,
and the observatory expects the entire telescope to be finished in 2002.
Optical
interferometers are useful for resolving the separation between relatively
bright, closely paired objects, such as double stars. Astronomers hope
this technique will eventually make it possible to directly image small,
Earth-sized planets orbiting distant stars.
E.
Recording Images
Throughout
most of the history of astronomy scientists have viewed celestial objects
through a telescope's eyepiece. When photography was invented in the
1800s, one of its first applications was to attach a camera to a telescope
to make a photograph of the Moon. Photography permitted astronomers to
record and archive what they saw. Photographic time exposures exceeded the
eye's sensitivity and recorded very faint objects, often in rich colors.
Today,
photographic film in telescopes has been largely replaced by solid-state
detectors called charge-coupled
devices (CCDs). These thumbnail-sized silicon chips are divided into
millions of picture elements, called pixels,
that convert incoming starlight into an electric charge that is read by
computer. The resulting mosaic of bright and dark pixels creates a
picture. CCDs provide much greater sensitivity and contrast than
photographs do, and the image is automatically recorded in digital form
for subsequent storage and enhancement by computer image processing. CCDs
can also record more wavelengths of light than cameras can, from the
visual edge of the ultraviolet region to the near-infrared.
|
