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far ultraviolet
ultraviolet radiation with the shortest wavelengths ("farthest" from visible light in the electromagnetic spectrum)
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field of view
the area of sky visible in a telescope or binoculars
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filter
a device that transmits light of only certain wavelengths; used by astronomers to observe specific wavelengths or to reduce the light of exceptionally bright objects
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finder scope
a small, low-powered telescope attached to a larger telescope that helps the observer locate objects in the sky
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fireball
an extremely bright meteor; generally brighter than magnitude –4
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fireballs
extremely bright meteors; generally brighter than magnitude –4
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first quarter
the phase of the moon a quarter of the way around its orbit from new moon; the eastern half is illuminated during this phase
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flare
a sudden, violent outburst of energy from the surface of a star
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focal length
the distance from a lens or mirror to the point where it brings light to a focus
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focal ratio (f/ratio):
the ratio of the focal length of a lens or mirror to its diameter
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focus
the point at which rays of light passing through a lens (or reflecting off a mirror) converge
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focuser
the device on a telescope that holds an eyepiece and moves to allow an observer to bring light to a sharp focus
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fork mount
an equatorial mount in which the telescope swings in declination between the two prongs of a fork
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frequency
the number of wave crests or troughs that pass a particular point in a given interval of time (usually one second); usually expressed in hertz (cycles per second)
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full moon
the phase of the moon when it is halfway around its orbit from new moon and opposite the sun in the sky; the full disk is illuminated
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