C

Carbon Fiber - A lightweight composite material that is used in certain telescope components (usually optical tubes and truss poles). 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 right ascension and declination, which are analogous to longitude and latitude, 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 aperture is 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 reflecting system.

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.

Comet - 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 - 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 Rods.

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 rack-and-pinion focuser, 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.