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Astronomy & Telescopes Glossary - A
f-number - Another label for focal ratio.
Falling star - A meteor that is burning up in the Earth's atmosphere.
Field of View (FOV) - The maximum view angle of a lens. The number supplied by a manufacturer is the Apparent FOV. The True FOV (or Actual FOV)is found by dividing the Apparent FOV by the magnification.
Filter - A colored disk of glass that attaches in front of a telescope eyepiece (or objective lens), which passes only certain wavelengths of light while rejecting blocking others.
Finderscope - A small, low-power telescope attached parallel to the main telescope that aids in locating celestial objects and in pointing a telescope.
Flamsteed number - In each constellation, every star is assigned a number, whereby the the westernmost star in each constellation is given the number 1, the next star to the east is given the number 2, etc.
Focal length - The distance light travels from the primary lens or mirror to where it converges or is focused.
Focal ratio - This is found by dividing a telescope's focal length by its aperture. Any telescope with a focal ratio larger than f/12 would probably be said to be "slow", and any telescope with a focal ratio smaller than f/6, is said to be "fast".
Focuser - The telescope device that light rays to a focus. Common focuser designs include rack-and-pinion, Crayford, and helical.