Airy Disk- Because of the physics of light, it cannot be focused to an exact pin-point. Instead, due to diffraction, it is spread about. Named after Sir George Airy who developed the mathematical description of the point spread function. The Airy Disk, in arc-seconds and radians (Lambda= wavelength of light, D= Diameter of objective):
Airy Disk (radians)= 251600 x Lambda / D
Airy Disk (arc-seconds)= 1.22 x Lambda / D
Alt-Az Mount- A telescope mount designed to be moved independently in altitude (up-down) and azimuth (rotational) coordinates.
Dawes Limit- Named after double-star observer William R. Dawes. He established an experiment to determine the resolving power (ability to distinguish two close-together objects from an image as separate) of telescopes. The Dawes Limit, in arc-seconds (D=Diameter of objective):
4.6/D (Inches)
116/D (Millimeters)
Dobson, John- Telescope maker and inventor of the Dobsonian Telescope.
Dobsonian Telescope- Invented by John Dobson, a popular type of alt-az Newtonian telescope design, characterized by large-area bearing sliding on Teflon pads, using friction to hold position.
Equatorial Mount- A telescope mount designed to move in a coordinate system tied to the rotation of the Earth. Used in conjunction with a motorized drive, stellar objects can be "frozen" in the field of view.
f/number- Or Relative Aperture. The ratio of the focal length to the mirror diameter. A mirror below f/5 (deeply-curved and short focal length) is considered "fast", while above (shallow-curved and long focal length) is considered "slow".
Focal Length- The distance from a mirror or lens where light reflected off or refracted through will converge to.
Mirror box- In a Dobsonian telescope, holds the optical tube assembly and rests in the rocker. Aims the telescope in altitude coordinates.
Mirror Cell- Designed to securely hold a mirror in a reflector-type telescope.
Newtonian Telescope- Invented by and named after Sir Isaac Newton, a reflector telescope consisting of main curved mirror which reflects an image to a smaller secondary flat mirror.
Optical Tube Assembly- Consists of the optics, mounting hardware, and eyepiece.
Primary Mirror- In a Newtonian telescope, the objective or main light-gathering mirror.
Rocker- In a Dobsonian telescope, the rotational (azimuth) base of the telescope.
Secondary Mirror- In a Newtonian telescope, reflects light from the primary mirror to the eyepiece.
Rich-Field Telescope- A telescope designed to provide the richest field of stars possible in any single view. Such a telescope usually needs a f/6 or faster mirror.
