A | B | C | D | E | F | G | H | I | J | K | L | M
N | O | P | Q | R | S | T | U | V | W | X | Y | Z

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Accretion An accumulation of dust and gas into larger bodies such as stars, planets and moons, or as discs around existing bodies.
Albedo A measure of the reflectivity of an object and is expressed as the ratio of the amount of light reflected by an object to that of the amount of light incident upon it. A value of 1 represents a perfectly reflecting (white) surface, whilst a value of zero represents a perfectly absorbing (black) surface.
Alt-Az Abbreviation for "Altitude-Azimuth". This refers to a telescope mount which has one axis moving parallel to the horizon (Azimuth) and one perpendicular to the horizon (Altitude).
Aperture Aperture is the diameter of the light-gathering portion of a telescope.
Aphelion The point in elliptical orbit of a planet, asteroid, or comet that is farthest from the Sun.
Apochromatic An apochromatic telescope uses three or more lens elements, one or more usually possessing special properties, to eliminate chromatic aberration. Apochromatic refractors are usually regarded as giving the best quality image but are by far the most expensive type of telescope for a given size.
Apogee The point in an orbit when a body orbiting the Earth, is farthest from the Earth
Apparent Field of View Apparent field of view refers to how wide a field of view an eyepiece will give at a certain magnification.
Arcminute An arcminute is a fraction of a degree. There are 60 arcminutes in a degree.
Arcsecond An arcsecond is a fraction of an arcminute. There ar 60 arcseconds in an arcminute, or 3600 arcseconds in a degree.
Astronomical Unit. (A.U.) This is the mean distance from the Earth to the Sun, i.e. 149,597,870 km. (approx 93 million miles)
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Barlow Lens A barlow acts to increase the effective focal length of a telescope providing higher magnification for any given eyepiece. For example, a 2X barlow will double the magnification of a given eyepiece.
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Central Obstruction Reflecting telescopes such as Newtonians and Schmidt-Cassegrains have a secondary mirror placed in the optical path of the scope to intercept light rays from the primary mirror and direct them to a convenient position for the eyepiece or camera. This secondary mirror obstructs a small portion of the primary mirror and is thus known as a Central Obstruction.
Chromatic Aberration When light passes through a lens, such as in a refracting telescope, the various wavelengths of light, from red to blue, do not all focus to the same point. It can be eliminated by using an apochromatic lens which causes all visible wavelengths to focus back to one point.
Chromosphere The layer between the photosphere and the corona in the atmosphere of the Sun, or any other star, mainly composed of excited hydrogen atoms.
Conjunction When two bodies appear to close together in the sky/
Corona Very hot, tenuous, outer layer of the solar atmosphere, fully ionized, affected by the solar magnetic field, region from which solar wind is emitted
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Dark Frame A dark frame is an image taken with the CCD’s shutter closed. This image records only the electronic noise. Then a picture is taken of the night sky which records both the object in the sky as well as the noise. Then the dark frame is subtracted to leave only an image of the subject.
Declination Analogous to lines of latitude on Earth, an astronomical object’s declination tells how far north or south the object lies from the celestial equator.
Deconvolution An algorithm-based method for eliminating noise and improving the resolution of digital data. For example, deconvolution algorithms are used to remove out-of-focus haze from confocal microscope images.
Degree A degree is a unit of measure for apparent size of celestial objects against the sky.
Diagonal A diagonal is a mirror (or sometimes a prism) placed in the optical path of a telescope which reflects incoming light at a 90-degree angle in order to bring the light to a convenient position.
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Eccentricity The measure of the degree to which an ellipse is not circular; ratio of the distance between the foci to the major axis. The greater the eccentricity, the more ‘flattened’ is the ellipse.
Elongation The angular distance between the Sun and any other solar system body, usually the Earth, expressed in degrees. The term Greatest Elongation is applied to the inner planets, Mercury and Venus. It is the maximum elongation from the Sun. At Greatest Elongation, the planet will appear 50% phase.
Equatorial Mount An equatorial mount has one axis parallel to Earth’s rotation axis.
Eye Relief The eye relief of an eyepiece is the distance your eye can be from the eyepiece and still see the entire field of view. 
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Focal Length The focal length is the length of the path through a telescope which incoming light follows. The magnification of a telescope is determined in part by the focal length of the scope.
Focal Ratio The focal ratio of a telescope relates the scope’s focal length to its aperture. The focal ratio of a telescope is the focal length divided by the aperture.
Focal Reducer A focal reducer is used to decrease the focal ratio (and focal length) of a telescope. A smaller focal ratio yields a faster optical system and thus a shorter exposure time. The trade off is reduced image scale, but this is usually acceptable in return for a wider field of view and shorter exposure. Focal reducers typically attach to the back of the telescope, just ahead of the camera in the optical path. They are made in a variety of focal reduction factors, from 0.18x to 0.8x. Popular focal reducers for SCTs are 0.33x and 0.63x reducers, often called f/3.3 and f/6.3 reducers since most SCTs have an inherent focal ratio of f/10 and thus the reducers yield these new focal ratios. A focal reducer is a highly recommended accessory for imaging.
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Lagrangian Points Points in a two body gravity system of large objects (such as the Sun and Earth) where small objects can orbit the primary body and remain almost stationary relative to the secondary body.
Light Pollution The emission of stray light or glare from lighting fixtures in manners that counter the purpose of the light (which is to light what is below).
Light Year Not a unit of time! The distance travelled by light in one year, equal to 9.4607E12 km (5.88 * 10E12 miles or 63,240 AU).
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Magnitude The magnitude scale is used to compare the brightness of celestial objects. Objects with smaller magnitudes are brighter. The star Vega was selected to define magnitude 0.  Thus it is also possible to have negative magnitudes since the Sun, four stars, four planets, and Earth’s moon all appear brighter than Vega.  Sirius, for example, is the brightest star in the night sky at magnitude -1.46.
Meridian The meridian is the imaginary line in the sky dividing east from west. The meridian runs from north to south through a point directly overhead. An object is at its highest point in the sky when is crosses the meridian (with the exception of objects near the celestial pole which can also cross the meridian at their lowest point when passing under Polaris, the north star).
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Objective The term objective is used to refer to the lens of a refracting telescope. The objective lens of a refractor is simply the main lens at the front of the telescope.
Occultation This is when one celestial body, passes in front of and obscures, another.
Opposition An angle of 180 degrees between a planetary object, the Earth, and the Sun. More simply, these bodies lie on a straight line with Earth in the middle.
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Parallax The angular difference in apparent direction of an object seen from two different viewpoints.
Perigee The point in its orbit where the Moon, or planet is closest to the Earth.
Perihelion The place in the orbit of an object revolving around the Sun where it is closest to the Sun.
Photosphere Lowest part of Sun’s atmosphere where sunspots are seen.
Polar Alignment In order for a telescope on an equatorial mount to properly track the sky to compensate for Earth’s rotation, the mount must be correctly polar aligned.
Primary Mirror All reflecting telescopes have a primary mirror which gathers incoming light and sends it either to a secondary mirror or to a camera placed at the focus of the primary mirror.
Prime Focus When the eyepiece is removed from a telescope and replaced with a camera body it is said to be at prime-focus. It is the point at which a telescope’s optics direct incoming light.
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Quasar Compact, extra galactic, objects at extreme distances, which are highly luminous. They are thought to be active galactic nuclei. The name is an acronym for quasi-stellar radio source.
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Red Shift he lengthening of the wavelength of electromagnetic radiation caused by relative motion between source and observer. Spectral lines are red-shifted from distant galaxies, indicating that the galaxies are moving away from us due to the expansion of the Universe.
Refractor A telescope that uses as its primary optical element a lens. Binoculars are a type of refractor. In general, refractors are much more expensive to build and buy than are reflectors.
Right Ascension Analogous to lines of longitude, Right Ascension is divided into 24 hours increasing from east to west. Facing north, objects appear to rise on your right (east), hence the name Right Ascension.
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Secondary Mirror Most reflecting telescopes have a secondary mirror which reflects light from the primary mirror to a convenient position for the eyepiece or camera.
Seeing Seeing refers to the steadiness of Earth’s atmosphere.
Spherical Aberration A parabolic mirror will focus incoming light to a single point. A spherical mirror will focus incoming light to different points causing stars to no longer appear as nice round points of light. This distortion is called spherical aberration.
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T-Adapter A T-Adapter is used to attach a camera body to the telescope for photography.
Transparency Transparency is a term referring to the clarity of Earth’s atmosphere.
T-Ring A T-Ring is used to attach a camera body to a T-Adapter.
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Zenith The zenith is the point in the sky directly overhead from your location.  Directly opposite the zenith, below your feet, is the "nadir".