| Aberration |
In an optical system, any departure from a spherically converging final wave front. |
| Achomatic |
A refracting optical system without color correction. |
| Afocal Photography |
A type of eyepiece projection photography in which the camera lens is placed very close to a telescope's eyepiece |
| Airy Disk |
The central, brightest part of a normal Airy pattern. Its linear diameter is equal to 2.44ëf/D where ë is the wavelength of light in microns, f is the telescope's focal length and D is the telescope objective's diameter. |
| Airy Pattern |
The diffraction pattern of a telescope with central Airy Disk and surrounding diffraction rings. An ideal telescope objective produces a diffraction-limited Airy pattern from an infinitely small, luminous object point. |
| Alt-Azimuth Mount |
A type of telescope mount designed to move up and down (altitude) and left and right (azimuth). |
| Anti-Light Pollution (ALP) Filter |
A type of nebular filter that eliminates light frequencies from mercury and sodium vapor street lights. |
| Anti-Reflection Coatings |
Interference coatings deposited on a lens surface to increase light transmission. |
| Aperture |
The measurement of the diameter of the main lens of an optical system. |
| Aplanatic |
Simultaneously free of coma and spherical aberration. |
| Apochromatic |
An optical system without color aberration, Free of secondary spectrum. |
| Astigmatism |
An optical aberration that causes the image of a point light source to appear as an ellipse with the long axis of the ellipse shifting by 90° on opposite sides of the focal plane. |
| Averted Vision |
An observing technique in which a faint object image may be made to appear brighter to the eye by not looking directly at it. |
| Back Focal Length |
The distance from the final surface of an optical system to the focal plane. |
| Bandpass |
The selectivity of a filter or spectrohelioscope. |
| Barlow Lens |
A small negative lens that amplifies and relocates a telescope's beam when placed just inside focus. It is used to obviate the use of short-focus eyepieces and in negative lens projection photography. |
| Boundary Layer, Thermal |
The layer of relatively warm air on a telescope objective that is cooling down. The layer becomes harmful to definition if it becomes unstable, as by agitation in open-tube telescopes. |
| Camera Adapter |
A telescope accessory that receives a T-Ring Adapter for attaching a Single Lens Reflex (SLR) camera to a telescope. Also called T-Adapter |
| Camera Adapter Ring |
See T-Ring Adapter. |
| Cassegrain Telescope |
In its classical form, a compound reflecting telescope employing a parabolic concave primary mirror and a small hyperbolic convex secondary mirror to form images. Variations of the Cassegrain design have been developed since its invention, two of them being the Maksutov-Cassegrain and the Schmidt-Cassegrain. |
| Catadioptric Telescope |
A telescope employing a combination of mirrors and lenses to form an image, as in a Maksutov-Cassegrain or Schmidt-Cassegrain. |
| Catadioptrics |
An optical system that uses a combination of refractor and reflector lenses to produce a large aperture, compact optical system. |
| CCD |
Stands for "Charge Coupled Device". CCD chips are the detectors used in digital cameras. |
| Celestial Sphere |
An observationally practical model of the sky as a sphere with fixed stars that rotates around the North Star (Polaris). |
| Central Obstruction |
In Newtonian and Catadioptric telescopes, the obstruction caused by the secondary mirror. |
