Glossary of Terms

Glossary

Accessory tray - an attached shelf on a telescope for diagonals, eyepieces, lenses, etc.
Achromat - also known as an achromatic objective lens, a refractor objective lens made with two types of glass to reduce chromatic aberration.
Altazimuth mount - two-axis support mount for a telescope freely in altitude and azimuth
Aperture – the diameter of a telescope’s primary opening.
Apochromatic – a photographic or refractor lens of exotic glass or two or more elements to eliminate chromatic aberration.
Apparent magnitude - a measure of star brightness to the naked eye
Arc minute – a unit of angle equal to 1/60 of a degree
Arc second - a unit of angle equal to 1/60 of an arc minute or 1/3600 of a degree
BAK-4 glass - superior-quality glass used in optical prisms
Barlow lens - filter lenses that extend the effective focal length of your telescope
Binocular - two small telescopes with central focusing for viewing with both eyes.
Binocular tripod adapter - device that couples binoculars to a tripod.
BK-7 glass - an extremely common glass used in precision lenses found commonly in less expensive binoculars.
Camera adapter - a tele-extender or T-adapter used for attaching cameras to telescopes
Cassegrain - a combination of a primary concave mirror or secondary convex mirror used in optical telescopes
Catadioptric - using lenses and curved mirrors to make the images
Center-mark - found on the primary mirror, this is a small centered mark that aids in collimating Newtonian reflectors.
Collimation – telescope fine tuning or adjustment.
Counterweight – a heavy object used to counter gravity in order to hold heavy telescopes in place
Crosshairs - a system of dots, rings, or cross wires, also known as reticle, used to center telescopes
Dew shield - a metal or flexible plastic tube that prevents dew from forming.
Dispersion - refraction of light at slightly different angles when passing through material causes chromatic aberration.
Dobsonian - an altazimuth mounted telescope developed and named for amateur astronomer John Dobson.
Double star/ binary star - A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system.
Dovetail bracket - device used to secure finder scopes to telescopes.
ED glass - extra-low dispersion glass with better refractive properties than standard glass.
Emission nebula - a cloud of ionized gas emitting light of various colors
Equatorial mount / German Equatorial Mount - a device used for easy tracking of celestial objects with one axis parallel to Earth’s rotational axis
Erecting prism - right-angled optical prism that corrects inverted images
Exit pupil - also known as the Ramsden disk, a virtual aperture in a telescope's eyepiece
Eyepiece – a small lens assembly used for viewing images through a telescope
Field of view (apparent) - the angular diameter, expressed in degrees, of the light seen by the human eye
Field of view (true) - the actual angle of sky seen through the eyepiece when attached to the telescope
Finder scope – a small attached telescope used to find and center objects to view
Focuser - eyepiece accessory used to focus objects
Galaxy - A giant aggregate of billions of stars, interstellar gas, and dust.
Globular cluster – (sometimes called global clusters) A tight, spherical grouping of hundreds of thousands of stars usually found near the centers of galaxies
Illuminated reticle eyepiece - a red-illuminated guiding eyepiece used in astrophotography.
Interpupillary distance – the distance between the centers of your pupils
Lens element – an optical lens or series of lenses used to create photographic or media-storable images
Light pollution – excessive or intrusive artificial light common around urban areas.
Magnitude - the brightness of stars. A magnitude 1 star is the brightest category. Magnitude 2 is half as bright.
Maksutov (MAK) - catadioptric telescope with full diameter meniscus lens, also called a aberration corrector plate
Messier object - a set of astronomical objects first cataloged in the late 18th Century by French astronomer Charles Messier.
Mirror cell - frame that holds the primary mirror of reflecting telescopes
Monocular - low-powered binocular used with one eye like a telescope
Motor drive - motorized drive assembly used to adjust a telescopes viewing direction
Multi-coatings - layers of magnesium fluoride with multiple anti-reflection coatings on some surfaces.
Nebula - A cloud of dust and gas in space, usually illuminated by one or more stars.
Newtonian reflector – Invented by Isaac Newton, these telescopes use a series of mirrors to collect and focus light in to the eyepiece.
NGC catalog - a well-known amateur astronomy catalog of 7,840 deep sky objects
Objective lens – the primary optics of a telescope that gathers and focuses light
Open cluster - A collection of young stars that formed together that may or may not be still bound by gravity.
Optical Tube Assembly (OTA) - a telescope's main tube
Parfocal - a lens that stays focused when magnification/focal length is changed.
Planetary nebula - A shell of gas surrounding a small, white dwarf star.
Planisphere – a hand-held star chart created by two spinning dials used to show the positions objects are in the night sky for a particular area.
Polar alignment - aligning a telescope’s equatorial mount rotational axis parallel to Earth's axis
Polar alignment scope - also called a polar axis finder, a small finder scope used in polar alignment
Power - magnification of a telescope
Rack-and-pinion focuser - device used to focus a telescope’s eyepiece
Reflector - an optical telescope using a single or combination of curved mirrors
Resolution - the ability to resolve celestial objects
Right ascension (R.A.) - amount of time between the rising of Aries and another celestial object used to locate objects in the sky.
Schmidt-Cassegrain - a catadioptric telescope that combines a folded optical path with a corrector plate
Secondary mirror – a smaller mirror designed to redirect light to the eyepiece of a telescope
Setting circles – star finding technique that uses a pair of graduated disks to find sky objects by their equatorial coordinates.
Slew - rapid turning of a telescope
Spherical mirror - curved mirror found in catadioptric telescopes
Star diagonal - angled flat mirror or prism adapter used in telescopes to change the eyepiece axis
Star hopping – star finding technique where you jump from one star to the next until you find the one you want.
T-adapter - camera adapter used for prime-focus astrophotography
T-ring - camera adapter for terrestrial, lunar, and planetary photography
Tele-extender – eyepiece camera adapter used for high-power lunar, solar, planetary, and extreme terrestrial photography
Tracking - small motor drive used to fix a telescope on a sky object as Earth rotates