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Carbon Fiber - A lightweight composite material that is
used in certain telescope components (usually optical tubes and
Its primary advantage is its lower thermal expansion coefficient. This
means it expands and contracts less than standard aluminum when the temperature
changes. This is especially important for imaging purposes. Carbon
fiber is, however, more expensive than aluminum.
Catadioptric - A catadioptric telescope design
incorporates both reflecting and
refracting elements. A Schmidt-Cassegrain
is the most common example of a catadioptric telescope. Such a telescope
uses a set of mirrors to reflect the light and generate the optical power of the
system, but a full-aperture lens is also used to correct aberrations inherent in
the two-mirror design.
Celestial Equator - The celestial equator is the
line of Earth's equator projected out into space. It divides the celestial
sphere into northern and southern halves. Objects above the celestial equator
have positive declinations, while objects below the
celestial equator have negative declinations (analogous to latitudes on Earth).
Celestial Sphere - Imagining the stars projected
onto a sphere surrounding Earth allows an easy coordinate system for describing
their positions. The coordinate axes of the celestial sphere are
ascension and declination, which are analogous to longitude and
respectively, on Earth's surface. The celestial equator divides the
celestial sphere into northern and southern halves. 90°
above and below the equator are the north and south celestial poles.
Central Obstruction - Telescopes such as Newtonians and
Schmidt-Cassegrains have a central obstruction where part of the
blocked by the secondary mirror. This can
decrease contrast, although
the effect is very slight until the obstruction becomes very large (around 40%
of the aperture). Refractors do not have central obstructions which
contributes to their increased contrast versus other designs.
Chromatic Aberration - An optical aberration. Any
lens suffers from some level of chromatic aberration because all the different
wavelengths of light cannot be made to meet at exactly the same focus point.
Achromatic lenses suffer from noticeable amount of chromatic aberration which
can degrade the image quality and contrast. Apochromatic lenses are much
better corrected and the very best lenses have no noticeable chromatic
aberration. Only pure reflecting systems such as Newtonians are completely
free from chromatic aberration.
Clock Drive - For an equatorially mounted telescope to
track celestial objects (and compensate for Earth's rotation), it uses a clock
drive, which turns the mount once in 24 hours -- the same rate at which Earth turns.
Coatings - Glass does not transmit all of the light
that strikes it, and mirrors do not reflect 100% of the light that bounces off
them. Coatings are used to either increase the light transmission in a
refractive system or increase the reflectivity of a
Collimation - Collimation is the alignment of a telescope's
optics. An misaligned set of optics will cause image degradation.
Newtonian telescopes need to be collimated the most frequently, followed by
Schmidt-Cassegrain scopes. Barring any severe trauma (like dropping it
down a flight of stairs) a refractor should not normally need collimation.
Coma - An optical aberration. Coma produces
off-axis stars (near the edge of the field) that appear comet shaped, or flare
out away from the center of the field. Some telescopes have coma inherent
in their designs (Newtonians and commercial Schmidt-Cassegrains) and some use
exotic optical designs (such as Ritchey-Chrétiens)
to eliminate this aberration.
Comets are icy bodies that formed in the outer solar system, far beyond the
orbit of Pluto. Occasionally, comets are sent into the inner solar system
by the gravitational disturbances of passing stars. As it approaches the
sun, a comet's ice is sublimated (turned directly to gas) by the heat from the
sun. This causes a tail of gas and dust to form. The tail is blown
away from the sun by the solar wind, a constant flow of particles outward from
the sun. Comets can appear in the sky at any time, and they are generally
unpredictable, since they appear from the distant solar system. Some
comets become trapped in shorter orbits that bring them back around the sun more
often. Halley's comet, with a 76-year orbit, is the best known periodic
comet. Comets differ from asteroids in their
composition (asteroids are rocky, comets are icy) and where they formed in the
solar system (asteroids in close to the sun, comets farther away).
Cones are the cells in the retina of the eye which are sensitive to color.
However, they are insensitive at low light levels, which is why color is not
easily seen in deep-sky objects. See also
Constellation - Constellations are groups of stars that divide up the sky.
Originally constellations were distinct patterns in the sky used for
storytelling, astronomical prediction, and astrology. Every culture had
its own stories about the stars (although the correlation between cultures of
some pretty obscure constellations is uncanny). Eventually, astronomers
had to come to a consensus on what the constellations were for the purposes of
cataloging celestial objects. The sky was divided into 88 constellations,
which split up the sky like states divide up a country. And like states,
constellations come in all sizes and shapes. The largest constellation,
Hydra, covers 1303 square degrees of sky, while the smallest, Crux, covers just
68 square degrees. Some constellations (Orion, Scorpius, Leo) have
distinct, recognizable patterns while others are not obvious but still have boundaries that
help divide up the sky.
Corrector Lens - Some telescope designs use a full-aperture corrector lens to compensate for optical aberrations. The most well-known
designs are the Schmidt-Cassegrain and Maksutov-Cassegrain. A Schmidt
corrector lens is also sometimes called a corrector plate.
Corrector Plate - Common term for a Schmidt
corrector lens, such as that used on Schmidt-Cassegrain telescopes.
Counterweight - Used to balance a telescope. All
German equatorial mounts have counterweights as part of their design.
Fork-mounted telescopes may require counterweights if heavy accessories such as
2" eyepieces or cameras are attached.
Crayford Focuser - Unlike a
a Crayford focuser does not use gears to move the drawtube. Instead the
tube is moved by a roller and rides on bearings. This eliminates the
potential backlash that can result from the gears in a rack-and-pinion.