aberration — One of a number of optical distortions produced by an optical instrument, caused not by a defect in the instrument, but by the nature of its design, and the laws of optical physics.

achromat — A lens made of two pieces of glass (or, occasionally, other materials) with differing optical properties, specifically chosen to minimize chromatic aberration (which see).

altazimuth — A type of telescope mount, in which the telescope tube is rotated parallel to the horizon, or perpendicular to it, instead of around the axis of the Earth's rotation.

apochromat — A lens made of three pieces of glass (or other materials) with differing dispersions, chosen so as to reduce chromatic aberration to an even greater degree than in an achromat.

catadioptric — A telescope consisting of a lens and mirror combination, such as a Cassegraine, Schmidt-Cassegraine, or Maksutov design. In such telescopes, light enters the front of the telescope, is reflected off a "primary" mirror at the back of the telescope, and is then reflected off a "secondary" mirror at the front of the telescope. Such a design is common to reflecting telescopes of all types, but in a catadioptric telescope, the secondary mirror is curved in a way that changes the focal length of the telescope, and reflects the light through a hole in the center of the primary mirror, allowing the image to be viewed from the rear of the telescope, instead of (e.g.) at the front, as in a Newtonian reflector. In addition, a "correcting" lens is mounted at the front of the telescope, acting as a support for the secondary mirror, and correcting various "aberrations" common to ordinary reflecting telescopes, to provide a sharper, wider field of view.

chromatic aberration — A prismatic effect in an image produced by an optical instrument, caused by the tendency of lenses to disperse light, or to refract different colors of light by unequal amounts, thereby turning a point of light into a very small spectrum, and causing the image of an extended object to become less sharp, and fringed by the end colors of the spectrum.

declination circle — In the Equatorial coordinate system, a circle parallel to the Celestial Equator, at some fixed distance north or south of the Equator. Analogous to a parallel of latitude, in the Terrestrial coordinate system. Also, in a telescope, a circle used to indicate the declination that the telescope is pointing at.

doublet — A lens or eyepiece made of two pieces of glass (or other materials) selected so as to reduce the chromatic aberration of the lens.

equatorial — A telescope mount which features an axis parallel to the axis of rotation of the Earth, so that a simple rotation of the telescope around that axis allows it to follow the stars.

focus — One of two points used to construct an ellipse. A point on the major axis of the resulting ellipse, offset toward one end of the major axis in such a way that that end of the major axis is just barely the closest place to the focus. In a planetary orbit, the location of the Sun. Also, in a telescope, the place where light is brought to a focus, producing an image.

hour circle — An arc, in the Equatorial coordinate system, from the North Celestial Pole to the South Celestial Pole, of constant right ascension. Called an hour circle because right ascension is measured in time units. Analogous to a meridian of longitude, in the Terrestrial coordinate system. Also, in a telescope, a circle used to indicate the right ascension that the telescope is pointing at.

triplet — A lens or eyepiece made of three pieces of glass (or other materials) selected so as to reduce the chromatic aberration of the lens.