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Glossary of Astronomy Terms
| Term | Definition |
|---|---|
| Aberration | Any optical defect and/or design error which causes any of the processed light to deviate from reaching the focal point, therefore reducing the quality of the image. |
| Absolute Magnitude | The apparent brightness a star would have if placed at a distance of 10 parsecs from the earth. |
| Achromatic Lens | A refractor lens, made of two or sometimes three separate lenses, which has the effect of bringing most of the viewed colours to a sharp focus, thus reducing chromatic aberration. |
| Alt-Azimuth | A simple mount that allows movement in altitude (up and down) and in azimuth (side to side). |
| Aperture | The diameter of the primary mirror or lens. |
| Apochromatic Refractor | A refractor telescope that uses special glass to reduce chromatic aberration and produce a high-quality image. |
| Astrophotography | The process of taking photographs of astronomical objects or events. |
| Barlow Lens | A lens that increases the magnification of a telescope by extending the focal length of the telescope. |
| Casual Observer | An amateur astronomer who observes the night sky as a hobby or for enjoyment. |
| Catadioptric Telescope | A telescope that uses a combination of mirrors and lenses to form an image. |
| Celestial Sphere | An imaginary sphere surrounding the Earth to which all celestial objects are considered to be attached. |
| Chromatic Aberration | An optical defect in which different colors of light are focused at different distances from the lens, causing color fringing and reducing image sharpness. |
| Coma | An optical aberration that causes off-axis objects to appear distorted, resembling a comet with a tail. |
| Declination | The angular distance of a celestial object north or south of the celestial equator. |
| Dew Shield | A device attached to the front of a telescope to prevent dew from forming on the lens or corrector plate. |
| Diffraction | The bending of light waves as they pass near the edge of an object, causing interference patterns and reducing image sharpness. |
| Dobsonian Telescope | A type of reflector telescope with a simple, low-cost mount consisting of a box or rocker that allows the telescope to pivot in altitude and azimuth. |
| Double Star | Two stars that appear close together in the sky and are usually gravitationally bound to each other. |
| Eyepiece | A lens or group of lenses used to magnify the image produced by a telescope or other optical instrument. |
| Focal Length | The distance between the lens or mirror and the focal point, usually expressed in millimeters. |
| Focal Ratio | The ratio of the focal length to the aperture of a telescope, which determines the telescope's light-gathering power and ability to produce a sharp image. |
| Galaxy | A large system of stars, gas, and dust held together by gravity, usually with a roughly spherical or disk-like shape. |
| Guidescope | A small telescope or other device used to guide a larger telescope during astrophotography or other precision observations. |
| Light Pollution | The presence of artificial light in the night sky, which can interfere with astronomical observations and make it difficult to see faint objects. |
| Magnification | The degree to which an optical instrument magnifies an image, usually expressed as a number or ratio. |
| Messier Catalog | A catalog of 110 astronomical objects compiled by Charles Messier in the late 18th century, including many galaxies, star clusters, and nebulae. |
| Mount | The mechanism that holds and moves a telescope or other optical instrument, usually consisting of a tripod or other support and one or more axes of rotation. |
| Nebula | A cloud of gas and dust in space, often illuminated by nearby stars or other sources of radiation. |
| Newtonian Reflector | A type of reflector telescope with a parabolic mirror and a flat secondary mirror mounted on a central strut, usually with an open tube assembly. |
| Objective Lens | The main lens or curved mirror in an optical instrument, which gathers and focuses light to form an image. |
| Parallax | The apparent shift in position of a nearby object against a more distant background when viewed from different positions, caused by the observer's motion. |
| Planetary Nebula | A type of emission nebula consisting of a glowing shell of gas ejected by a dying star, often with a bright central star or other features. |
| Prime Focus | The focus point of a telescope or other optical instrument when no eyepiece or other device is used, usually at the end of the optical tube or on a camera sensor. |
| Reflector Telescope | A type of telescope that uses a curved mirror to gather and focus light, usually with an open tube assembly and a secondary mirror to deflect the light to an eyepiece or other device. |
| Refractor Telescope | A type of telescope that uses lenses to gather and focus light, usually with a long, closed tube assembly and an eyepiece at the end. |
| Right Ascension | The angular distance of a celestial object eastward along the celestial equator from the vernal equinox, usually measured in hours, minutes, and seconds. |
| Setting Circles | Circular scales attached to the telescope. They are marked off in degrees of Declination and hours of Right Ascension. Together, the circles allow the position of a known object to be found by setting the dials to the equatorial coordinates. |
| Spherical Aberration | A blurring of the image caused by the inability of a spherical mirror to focus all light from infinity to one focal point. Light rays from the edge of the spherical mirror focus to different points than those from the centre. |
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