When buying a telescope, it's important to be familiar with common terms used in astronomy and optics. This will given you a better understanding of how telescopes work and also enable you to ask the pertinent questions once you're ready to purchase (and use) your brand new instrument. In no time, you'll be observing the wonderful expanse of the evening sky too!
Note: Astronomical terms denoted by (A) and telescope terms by (T).
Aberration: Degradations in the appearance of an image, resulting from an optical system's design of improper alignment. The most common are: chromatic aberration, spherical aberration, coma, field curvature, and astigmatism. (T)
Altitude: The angular distance of an object from the horizon. For reference, the zenith directly above is 90° from the horizon and the horizon is 0°. (A)
Angular Distance: The angle through which a telescope must turn from one object to observe another. For instance, from the horizon to the zenith, it would be 90°. (A)
Aperture: Defines the size of a device's prime objective. This determines how much light is allowed into the scope.
Astigmatism: An aberration that spreads the image into what appears to be an ellipse or line. (T)
Azimuth: The angular distance around the horizon measured eastward in degrees, relative to true north from the horizon point. (A)
Celestial Equator: The line of declination which lies right above the earth's equator, as if the equator were transferred to the sky. (A)
Chromatic Aberration: An aberration in which all visible colors are not in focus at the same plane. (T)
Collimation: The alignment of optical components within a device. Misalignment could lead to aberration. (T)
Coma: An aberration which gives bright objects a flared appearance. (T)
Declination: The astronomical equivalent to latitude, it refers to the angle of a celestial object in relation to the celestial equator. If above the northern hemisphere, the object has "positive declination"; if in the southern hemisphere, the object has "negative declination." (A)
Ecliptic: The sun's apparent path across the sky during the course of a year. (A)
Eyepiece: Also known as the ocular, the eyepiece magnifies the image. Best results are when the optics match up. (T)
Field of View: The amount of sky that can be seen through a telescope's eyepiece. (T)
Focal Length: The distance from where light enters a telescope to the point where the image is focused. (T)
Focal Ratio: The ratio between a telescope's focal length and objective size. The field of view widens in proportion to the focal ratio; the smaller, the wider the field. (T)
Limiting Magnitude: The faintest object that can be seen with a telescope. (T)
Magnification: The relationship between the telescope's and eyepiece's focal lengths. The greater the difference in the two focal lengths, the greater the magnification. The smaller the magnification, the shaper the image as light will be more concentrated versus being spread across a wider field. (T)
Objective: The front lens (or mirror) of a telescope. The larger the diameter, the more light that will be pass through and be collected. (T)
Parabolic Mirror: Formed in a precise curve which allows incoming light rays to be focused onto the same point. (T)
Right Ascension: The astronomical equivalent to longitude, it's measured in hours, minutes and seconds. When discussed, it's referred to as "hour circles." (A)
Resolving Power: A telescope's ability to view objects spaced closely together. Also called Dawes' Limit. (T)
Spherical Aberration: An aberration in which light is prevented from the perimeter is prevented from focusing at the same point as light from the center. (T)
Zenith: The point in the sky located directly above the observer's head. This obviously changes in direct relation to the observer's location. (A)
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