Arc-minutes: Measurement of arc length--equivalent to 1/60 of a degree
Arc-seconds: Measurement of arc length--equivalent to 1/3600 of a degree, or 1/206181.8 of radian
Astronomical Twilight: The time when the Sun is between 12° and 18° below the horizon; most objects can be observed however there is still some light scattered in the sky
Civil Twilight: The time when the sun is between 0° and 6° below the horizon; enough ambient light is around to still participate in most outdoors activities without artificial light
Infrared: Term for light with a lesser energy than that of the visible spectrum (but not quite micro- or radio-waves)
Light Spectrum: A means used to break down and describe light of varying frequencies, wavelengths, and energies
Nautical Twilight: The time when the Sun is between 6° and 12° below the horizon; this is the point at which artificial light is required for most outdoor activities, and enough bright stars for are visible for navigation
Night / Fully Dark: The time of the night after the Sun has set to more than 18° below the horizon
Supernova Classification: Supernova are classified with the letters SN (for SuperNova) followed by the year in which they occurred (eg. SN 1054)
Ultraviolet: Term for light with a greater energy than that of the visible spectrum (but not quite gamma or x-rays)
Aperture: The measurement of the diameter of the light collecting region of an optical tube
Bortle Scale: Scale describing the amount of light pollution; 10 is the brightest, 0 is an absolutely dark sky (most of the Atacama Desert, for example); Byfield MA is mostly Bortle 4
Focal Length: The distance from the lens or mirror to the focal point.
Light Pollution: Describes the amount of ambient light that is reflected or scattered in the atmosphere that limits night-sky darkness
Limiting Magnitude: The dimmest magnitude object that can be viewed by a certain optic
Magnification: The focal length of a telescope divided by the focal length of the eyepiece
Resolving Power: Describes the ability of a telescope to see distinctly small, separate objects
Alt-Az Mount: Telescope mount design that rotates on the Earth axes of Altitude and Azimuth to track an object, albeit allowing for field rotation
German Equatorial Mount: Telescope mount design that rotates on the celestial axes of Right Ascension and Declination to track an object and prevent field rotation
Newtonian: Telescope design using two mirrors to reflect light across the length of the optical tube an addition one time, creating a higher magnification than a standard refractor
Reflector: Telescope design that used mirrors (and sometimes glass) to focus light passing through an optical tube and extend focal lengths
Refractor: Telescope design that uses glass exclusively to focus light passing through the optical tube and remove abberations
Schmidt-Cassegrain : Telescope design using two mirrors to reflect light across the length of the optical tube an addition two times, creating a very high magnification