Telescope Glossary

Absolute magnitude: measurement of a star's intrinsic or true brightness.

Accessory tray: an attached shelf on the telescope's mount for diagonals, eyepieces, lenses, etc. The tray also can be a center brace for a tripod.

Achromat: also known as an achromatic objective lens, a refractor objective lens with elements of two types of glass, helping to reduce chromatic aberration.

Altazimuth mount: a two-axis mount for supporting and rotating a telescope freely in altitude and azimuth as in a standard photo tripod.

Altitude and azimuth: the method used to describe the location of an object in the sky as viewed from a particular location at a particular time.

Aperture: the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane.

Apochromatic: a photographic or refractor lens that uses exotic glasses or two or more lens elements to eliminate chromatic aberration.

Apparent magnitude: a measure of the brightness of a star to the naked eye. The brighter the star appears, the lower the magnitude value.

Arc minute: a unit of angular distance equal to a 1/60th of a degree. A one-degree angle has 60 arc minutes.

Arc second: a unit of angular measurement equal to a 1/60th of an arc minute. A one-degree angle has 3,600 arc seconds.

Astigmatism: most often encountered with inferior ground optics, an aberration in which light doesn't come to focus at the same plane.

Baffles: rings within a refractor's tube with apertures that prevent internal light scatter and add more contrast.

BAK-4 glass: a superior-quality glass for making optical prisms, yielding bright images and extreme contrast.

Balance system: the use of weights to counterbalance a telescope's optical tube assembly.

Barlow lens: named for English engineer Peter Barlow, a diverging lens which when used with other optics effectively increases the focal ratio of an optical system - up to double or triple.

Binocular: two small telescopes with a central focusing system used for viewing with both eyes.

Binocular tripod adapter: device used to couple a binocular to the pan head of a tripod.

BK-7 glass: an extremely common glass, also known as borosilicate glass, used in precision lenses.

Blower bulb: a hand-squeezed device that produces brief gusts of air to clean optical surfaces.

Camera adapter: a tele-extender or T-adapter that attaches to the telescope focuser's drawtube and is connected by means of an optional T-ring.

Cassegrain: a combination of a primary concave mirror or secondary convex mirror used in optical telescopes.

Catadioptric: the use of lenses and curved mirrors to make the image-forming optical system.

CCD camera: short for charge-coupled device, an analog shift register controlled by a clock signal that enables analog signals (electric charges) to be transported through successive stages (capacitors).

Center-mark: found on the primary mirror of a Newtonian reflector, a small centered mark that aids in collimating the optics.

Coated: the application of magnesium fluoride to at least one optical surface for enhanced contrast and light transmission.

Collimation: light whose rays are nearly parallel, thereby slowly spreading as it travels through space.

Collimation cap: a small cap with a hole in it to aid in quick optical alignment of a reflector's primary and secondary mirrors.

Coma: a common aberration in which star images turn increasingly comet-like or pear shaped away from the field's center.

CorrecTension (XT): a system for always-perfect telescope tube balance and motion.

Counterweight: the placement of a weight on a telescope mount to counterbalance the scope's tube assembly.

Crosshairs: a system of dots, rings, or cross wires - also known as reticle - in the finder scope or eyepiece focus to help in centering.

Crown glass: a type of glass produced from alkali-lime silicates containing approximately 10 percent potassium oxide.

Degree: the measurement equal to 1/360th of a circle. One degree approximately equals the diameter of two full moons side by side in the sky.

Dew shield: a metal or flexible plastic tube about the same diameter as the optical tube of a telescope that prevents dew from forming.

Dew zapper: placed around the telescope tube, a flexible heating strap that keeps the corrector's temperature above the ambient air's dew point.

Diffraction limited: an optical system with the ability to produce images with angular resolution that is as good as the instrument's theoretical limit.

Dispersion: the refraction at a slightly different angle of each wavelength of light when passing through a material at an angle. This causes chromatic aberration in lenses.

Dobsonian: an altazimuth mounted telescope developed and named for amateur astronomer John Dobson.

Double star/binary star: a star system consisting of two stars - optical doubles and physical doubles - orbiting around their common center of mass.

Dovetail bracket: a device used to hold finder scopes interlocking with a dovetail base on the optical tube and secured with a thumbscrew.

ED glass: extra-low dispersion glass with refractive properties that are superior over standard optical glass.

Electronic drive: a motorized system that allows electronic telescope tracking of celestial objects.

Emission nebula: a cloud of ionized gas emitting light of various colors. Orion and Lagoon nebulas are examples.

Equatorial mount: also known as German equatorial mount, a device used for easy tracking of celestial objects.

Erecting prism: a right-angled optical prism used to turn an inverted image upright.

Exit pupil: also known as the Ramsden disk, a virtual aperture in a telescope's eyepiece.

Eye guard: a rubber eyepiece cup that shields light from the side of the telescope.

Eyepiece: an important part of the telescope, it magnifies the image. Most often, eyepieces consist of three or more elements.

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 telescope that mounts to the main telescope used for finding objects in the sky.

Finder scope, right-angle: a small telescope with built-in cross hairs for pinpoint accuracy while searching for celestial objects.

Flint glass: optical glass that has a low Abbe number and a relatively high refractive index.

Focuser: inserted into the eyepiece, it adjusts to bring objects into focus.

Galaxy: hundreds of billions of stars that are close together. Examples include the Milky Way (our galaxy) and the Andromeda Galaxy.

German equatorial mount: a mounting device that has one rotational axis parallel to the Earth's rotation axis. It's also called an equatorial mount.

Global cluster: highly concentrated mass of several thousands to millions of stars believed to be remnants of galaxy formation.

Guide scope: an inexpensive refractor that affixes to another telescope on the same mounting for astrophotography.

Illuminated reticle eyepiece: a red-illuminated guiding (or crosshair reticle) eyepiece used during astrophotography. It can be adjusted for brightness.

Interpupillary distance: the space between the centers of the two pupils of the observer's eyes. Binoculars can be adjusted for each individual's interpupillary distance.

LED: found in electronic displays, a light emitting diode sends out light at low voltage levels. Modern LEDs are available across the visible, ultraviolet, and infra red wavelengths with very high brightness.

Lens element: an optical lens or assembly of lenses used to make images of objects on either photographic film or on other media capable of storing images electronically.

Light grasp: the amount of light an optical system can gather. The aperture of a telescope's objective lens determines the light grasp.

Light pollution: excessive or intrusive artificial light that hinders visibility of stars.

Magnitude, absolute: measurement of a celestial object's intrinsic brightness. To determine the absolute magnitude of a sky object from observed apparent magnitude, its value is corrected for distance to the observer.

Magnesium fluoride: white crystalline salt composed of one magnesium ion and two fluoride ions used as coating applied to optical surfaces of refractors and eyepieces.

Magnification: the ratio between the apparent size of an object - or its size in an image - and its true size. For example, a telescope with a 1,200mm focal length and an eyepiece of 40mm has a 30x magnification.

Magnitude: how the brightness of a star or other sky object is measured. The lower the magnitude number the brighter the object.

Maksutov (MAK): a catadioptric telescope that uses a full diameter meniscus lens - also called a corrector plate - to correct off-axis aberrations such as coma found in reflecting telescopes while avoiding chromatic aberration.

Meniscus lens: a Maksutov telescope lens that has two spherically curved faces - one convex, the other concave - giving it the form of a shell.

Messier objects: a set of astronomical objects first cataloged in the late 18th Century by French astronomer Charles Messier.

Mirror cell: a frame designed to hold the primary mirror of a reflecting telescope.

Monocular: a low-powered, miniature telescope or spotting scope that is held in your hand like a binocular but used with one eye like a telescope.

Motor drive: a motorized drive system making it easier to find and track night sky objects. There are two basic types of motor drives - single-axis and dual-axis.

Multi-coatings: a layer of magnesium fluoride with some multiple anti-reflection coatings on some surfaces. It's a step up from fully coated but short of being fully multi-coated.

Near focus: the distance between the spotting scope or binocular and the nearest object you can focus on. You can generally focus closer with lower magnification and smaller aperture binoculars.

Nebula: an interstellar cloud of dust, hydrogen gas, and plasma. It originally was a general name for any extended celestial object, including galaxies beyond the Milky Way.

Newtonian reflector: a type of reflecting telescope invited by British scientist Sir Isaac Newton that uses a concave primary mirror and a flat diagonal secondary mirror.

NGC catalog: a well-known catalog of 7,840 deep sky objects - known as the NGC objects - in amateur astronomy containing 7,840 objects.

Objective lens: the lens or mirror in a telescope or binocular that gathers the light coming from the observed object and focuses the rays to produce the real image.

Ocular: an alternate type of lens that attaches to a telescope.

Off-axis guider: a device that allows employment of a lateral prism to capture a small outlying area of a telescope's field of view, allowing a view through a guiding eyepiece.

Open cluster: a group of as many as a few thousand stars formed from the same giant molecular cloud and still loosely gravitationally bound to each other.

Optical tube assembly (OTA): a telescope's main tube, complete with primary mirror or objective lens, focuser, and finder scope. A mount or tripod is not included.

Parabolic mirror: a parabola-shaped reflective device used to collect or distribute light, sound, or radio waves.

Parfocal: a lens that stays focused when magnification/focal length is changed. Some amount of focus error still occurs, but it's small enough to be insignificant.

Planetary nebula: formed when a star can no longer support itself by fusion reactions in its center. Examples include the Ring and the Dumbbell nebulas.

Planisphere: a star chart in the form of two adjustable disks rotating on a common pivot. It can be adjusted to display the visible stars for any time and date.

Polar alignment: the act of aligning the rotational axis of a telescope's equatorial mount so that it's parallel the Earth's axis.

Polar alignment scope: also called a polar axis finder, a small finder scope built into the right ascension axis of an equatorial mount for greater polar alignment.

Power: the magnification of a telescope. For example, 40x is known as 40 power.

Primary mirror: the principal light-gathering surface, or objective, of a reflecting telescope.

Prism: a solid glass cut with flat surfaces used in star diagonals and binoculars made from either BK-7 or BAK-4 glass.

Prisms, porro: named for its inventor, Ignazio Porro, a reflective prism used in an optical to alter the orientation of an image.

Prisms, roof: in general, any kind of reflective optical prism containing a section where two faces meet at a 90 degree angle. Also called a Dach prism.

Pyrex: high-quality glassware originally made from thermal shock-resistant borosilicate glass, an ideal material for making primary mirrors or reflecting telescopes.

Rack-and-pinion focuser: a device used to move the drawtube of a telescope to allow the eyepiece to be correctly focused.

Reflection nebula: interstellar clouds of dust which are simply reflecting the light of a nearby star or stars. Reflecting nebulas appear blue in long-exposure images.

Reflector: an optical telescope using a single or combination of curved mirrors that reflect light and form an image. The best-known reflectors are Newtonians and Maksutov-Cassegrains.

Reflex sight: lens-less optical or computing sights that reflect a reticle image or images onto a combining glass for superimposition on a target.

Refractive index: a measure of how much the speed of light - or sound waves - is reduced inside a medium. Different glass types have different refraction indices.

Resolution: the ability to resolve celestial objects - one of the key functions of a telescope. The higher the telescope resolution, the more details that can be seen from the images obtained on it.

Reticle: cross wires, dots, or rings used for precise alignment of a finder scope or eyepiece.

Right ascension (R.A.): astronomical term for one of two coordinates of a point on the celestial sphere when using the equatorial coordinate system, somewhat analogous to longitude on Earth. The other coordinate is the declination.

Schmidt-Cassegrain: a catadioptric telescope that combines a folded optical path with a corrector plate for a compact astronomical instrument.

Secondary mirror: a small, flat, or curved mirror that diffracts incoming light and transfers some of the energy that would be in the central spot of the diffraction pattern to the outer rings.

Setting circles: a pair of graduated disks on telescopes equipped with an equatorial mount to find sky objects by their equatorial coordinates often used in star charts or ephemeris.

Sidereal rate: the rate of movement of the stars across the sky as the Earth spins. It's the equivalent to one rotation per 23 hours, 56 minutes, 4 seconds.

Silicon dioxide: also known as silica, an oxide of silicon that has a more durable layer to better protect aluminum coatings.

Silicon monoxide: an amorphous brown, hard, abrasive material used as thin surface films to protect optical mirrors' aluminum coatings.

Slew: rapid turning of a telescope, manually or electronically, about its mount's axes of rotation.

Spherical mirror: a curved mirror found in catadioptric telescopes.

Spider: found at the front of a reflecting telescope tube, a 3- or 4-vaned frame that supports the secondary mirror.

Spiral galaxy: a disk galaxy with a whirlpool or pinwheel shape belonging to one of the three main galaxy classes which forms part of the Hubble sequence.

Spotting scope: a portable telescope optimized for observing terrestrial objects. The magnification of a spotting scope is typically on the order of 20X to 60X.

Star diagonal: an angled flat mirror or prism used in telescopes that permits viewing from a direction perpendicular to the usual eyepiece axis.

Star hopping: a method of moving a telescope in successive steps to locate a desired target without computer-mediated assistance.

T-adapter: a camera adapter that turns a telescope into a giant camera lens for prime-focus astrophotography.

T-ring: a device that attaches a camera to the main focus of a telescope for terrestrial, lunar, and planetary photography.

Teflon: a trademark material formally called polytetrafluoroethylene used as a bearing surface.

Tele-extender: a camera adapter used for high-power lunar, solar, and planetary photography as well as for extreme terrestrial photography. It fits over the telescope's eyepiece.

Terrestrial: daytime observing of birds, landscape, or seascape using a telescope, spotting scope, or binoculars.

Terrestrial scope: a scope used for daytime or low-light observation of birds and nature.

Tracking: the use of a small motor drive to continuously fix on a sky object as the Earth rotates.

Tripod: a three-legged device on which a telescope, camera, spotting scope, or binocular attaches. It can have a swivel or pan head.

Variable star: a classification of a star that changes apparent brightness over time as seen from Earth. Examples include Delta Cephei, Betelgeuse, and Algol.

Waterproof: different from water resistant, the capability of being splashed with water but causing no internal leakage.

Worm gear: a pair of matching elements of which the worm is a spirally-cut gear that has the shape of a screw that when, rotated, uniformly moves a telescope mount's axis.

Zenith: the direction pointing directly above the observer's head.