D

Declination - The celestial sphere has two coordinate axes, right ascension and declination. Declination is analogous to latitude on Earth. The celestial equator has a declination of 0°. Declination increases up to the north celestial pole, which has a declination of +90°. Declination decreases toward the south celestial pole, which has a declination of -90°. Declination is often abbreviated Dec.

Deep Sky Object - Deep sky objects (DSOs) are those outside our solar system. Often stars are considered separately from deep sky objects, so the usual list of deep sky objects includes galaxies, nebulae, open star clusters, and globular star clusters.

Degree - A unit of angular measure, equal to 1/360th of the full circumference of the sky. The full moon is 1/2 a degree in angular diameter. Degrees are further divided into arcminutes (1/60th of a degree) and arcseconds (1/60th of an arcminute, or 1/3600th of a degree).

Dew Shield - A tube that extends out from the front of a telescope to prevent moisture from forming on the front lens. Usually incorporated into the design of refractors and a common accessory for catadioptric telescopes. Also acts as a lens shade, blocking stray light. The terms lens shade and dew shield are often used interchangeably.

Diagonal - A diagonal is prism or mirror used to bend the optical path of a telescope 90° to provide a more comfortable viewing angle. 45° diagonals are also sometimes used for terrestrial viewing. Refractors and Cassegrain telescopes almost always use diagonals for visual observing. Newtonians technically have a diagonal mirror incorporated into the optical design, although this is as often just called a secondary mirror rather than a diagonal.

Diffraction - Assuming a perfect optical system, you would expect pinpoint stars to appear as perfect pinpoints in a telescope image. However, the wave nature of light prevents this from being the case. Waves of light are diffracted by the aperture of the telescope, and this diffraction causes stars to appear as disks instead of points. The size of this disk is proportional to the size of the aperture (the larger the scope, the smaller the disk size). Diffraction also occurs at any physical boundary in an optical system, such as the edge of the secondary mirror (central obstruction) or along the edges of spider vanes. Diffraction from spider vanes causes the familiar cross patterns seen around stars in some astrophotos.

Diffraction Limited - In a telescope with little or no optical aberrations, the limiting factor in the resolution of the optics will be determined by diffraction. A telescope in which the optical defects are such that the aberrated star image is smaller than the diffraction star size is said to be diffraction limited.

Digital Setting Circles - Digital setting circles (DSCs) use encoders to read the position of the telescope and provide feedback to the user so the telescope can be moved to a selected celestial target. They work basically like goto telescopes except that there are no high-speed motors for moving the telescope automatically; instead the user moves the telescope manually to the position indicated by the DSCs.

Diopter - A unit of measure of the optical power of a lens. This term is seen most often in reference to binoculars. Binoculars have a diopter adjustment to compensate for differences between an observer's left and right eyes. A scale on the binoculars indicates the difference between eyes in diopters (positive or negative).

Dispersion - An optical property of glass, describing how much a given type of glass spreads out the various colors of light as it passes through the glass. High-dispersion glass spreads light more, while low-dispersion glass spreads light less.

Distortion - An optical aberration. Distortion alters the scale of the image across the field. Barrel distortion causes the center of the image to be enlarged with respect to the corners, whereas pincushion distortion has the opposite effect. Distortion is normally only seen in wide-angle camera lenses and is not normally a major aberration in telescope designs.

Double Star - A double star is a pairing of two stars seen through a telescope. This can simply be two stars aligned along the same line of sight by chance, or binary stars bound by gravity and orbiting each other. Double stars can be quite beautiful objects, especially when there is a distinct color difference between the two stars. In addition to simple double stars, triple and quadruple stars exist as well. Examples of double stars include Alberio and Mizar.

Doublet - An objective lens that uses two elements is called a doublet. Doublet lenses are achromatic.

Dovetail - A dovetail is a bar used for mounting a telescope or accessories. A telescope on a German equatorial mount may have a dovetail on the bottom of the telescope to allow easy attachment of the scope to the mount and for balancing. Dovetails are sometimes mounted on top of a telescope tube for mounting accessories such as a guidescope or camera.