A Porro prism binocular will inherently produce an intrinsically brighter image than a roof prism binocular of the same magnification, objective size, and optical quality, as less light is absorbed along the optical path.
Seeing is the astronomer's term for the relative optical quality of the Earth's atmosphere. Optical quality is defined as the steadiness and absence of distortion in a telescopic image across an interval of observation.
Resolving power is primarily a function of aperture; however, optical quality, collimation, tube currents, etc., can have great effects.
Diffraction limited A measure of optical quality in which the performance is limited only by the size of the theoretical diffracted image of a star for a telescope of that aperture.
Direct motion Another term for prograde motion.
Diffraction Limited
The point at which optical quality is good enough that the limits of viewing detail are determined by the physical properties of light, and not any optical defects in the telescope. See also RESOLUTION.
These images served to evaluate provisionally the performance of the new telescope, mainly in terms of mechanical and optical quality. The ESO staff were very pleased with the results and pronounced YEPUN fit for the subsequent commissioning phase.
Point Spread Function (PSF) - A mathematical description of how the light from a theoretical point source like a star is spread out by seeing, diffraction, optical quality, tracking accuracy, and the resolution of the sensor.
We learn that the light dispersed in the direction of primary vibration is not only of higher order in the difference of optical quality, but is also of order k 2 c 2 in comparison with that dispersed in other directions, ...
of the mirror will not change significantly as the temperature of the telescope changes during the night. Since the back of the mirror serves only to provide the desired form and physical support, it does not have to meet the high optical quality ...
 See also: Time, Universe, Sky, Night, Telescope
  
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