| Dark Matter |
The undetected matter in the universe which is not luminous. We know of its existence because of how clusters of stars and galaxies rotate. |
| Dawes Limit |
The limit of a telescope's angular resolution, expressed in arc seconds and equal to 4.56 divided by the telescope's aperture in inches. |
| Declination |
The coordinate on the sky exactly analogous to latitude on Earth, measured north and south from the celestial equator. |
| Deep-Sky |
A term used for dim, non-planetary, non-lunar astronomical targets, like nebulae and galaxies. The region of space beyond the solar system. |
| Definition |
The ability of a telescope to reveal, in an extended object image, the contrast between two areas having nearly the same brightness. |
| Deformed Optics |
Optics that have become stressed from being pinched in their cells. |
| Degree |
A measurement of an angle where 360 degrees would be a circle. |
| Deimos |
The smaller and outermost of the planet Mars' two moons. Deimos was discovered by Asaph Hall on August 11, 1877. In Greek mythology, Deimos was the son of Ares (Mars) and Aphrodite (Venus) and was the personification of "Panic". |
| Delphinus |
The constellation "The Dolphin" |
| Desdemona |
A moon of Uranus was discovered by Stephen P. Synnott (Voyager 2) on January 13, 1986. Desdemona is also designated as "Uranus X" and "S/1986 U6". Desdemona is the wife of Othello in William Shakespeare's play Othello. |
| Despina |
A moon of Neptune was discovered by the Voyager Imaging Team on August 2, 1989. Despina is also designated as "Neptune V" and "S/1989 N3". In Greek mythology, Despina was a nymph who was a daughter of Poseidon. |
| Dewing |
Nighttime condensation of water on exposed optical surfaces after the dew point has been reached. It may be delayed through the use of a dew cap. |
| Diagonal |
An optic accessory with a mirror or prism used to place an eyepiece in a comfortable position for the observer using a telescope and to re-arrange the orientation of the image. |
| Diffraction |
A modulation of light waves caused by interference in response to an obstacle in the path of propagation. |
| Diffraction Pattern |
The image of a point light source formed by a telescope. Also called Airy Pattern. |
| Digital Photography |
Photographic imaging in which images are digitized and stored in a computer instead of recorded on film. |
| Digital Setting Circles |
A computerized setting circle system that simplifies locating celestial objects. |
| Dione |
A moon of Saturn was discovered by Giovanni Cassini in 1684. Dione also is designated as "Saturn IV". In Greek mythology, Dione was a goddess and mother to Aphrodite. |
| Direct Objective Photography |
A photographic technique that substitutes the telescope objective for the lens of the camera, focusing the image formed by the objective directly onto the film. Also called Prime Focus Photography. |
| Distortion |
An optical aberration in which magnification varies across the field of view. In this aberration, the shape of the image is not a true copy of the object, even though it may be in sharp focus. |
| Dobsonian Mount |
A type of alt-azimuth telescope mount designed to hold large Newtonian reflector telescopes. The mount consist of a rocker box which allows the telescope to move up and down which rests on a rotating platform which allows the telescope to be move left and right. |
| Doppler Equation |
The relationship which describes how the change in frequency of light received from an object depends on the objects velocity from us. |
| Dorado |
The constellation "The Goldfish" |
| Draco |
The constellation "The Dragon" |
