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The diameter of the seeing disk, most often defined as the full width at half maximum (FWHM), is a measure of the astronomical seeing conditions. It follows from this definition that seeing is always a variable quantity, different from place to place, from night to night, and even variable on a scale of minutes.
This list contains a selection of objects 50 and 99 km in radius (100 km to 199 km in average diameter). The listed objects currently include most objects in the asteroid belt and moons of the giant planets in this size range, but many newly discovered objects in the outer Solar System are missing, such as those included in the following ...
The half flux diameter or HFD is a definition used by astronomers to define the star size in an astronomical image. Mainly due to the seeing, stars are not imaged as a dot but spread out like a Gaussian shape. [1] The half flux diameter defines the diameter of a circle around the bright center in which half of the star flux or energy is contained.
Chinese Giant Solar Telescope (CGST), an infrared and optical solar telescope, with light-gathering power equivalent to a 5 m diameter aperture. [31] [32] Advanced Liquid-mirror Probe of Astrophysics, Cosmology and Asteroids (ALPACA), a proposed Earth-based 8 metre telescope, by Arlin Crotts of Columbia University. [33] [34]
At locations selected for observatories, typical values for range from 5 cm for average seeing to 20 cm under excellent seeing conditions. The angular resolution is then limited to about λ / r 0 {\displaystyle \lambda /r_{0}} due to the effect of the atmosphere, whereas the resolution due to diffraction by a circular aperture of diameter D ...
an object of diameter 725.27 km at a distance of 1 astronomical unit (AU) an object of diameter 45 866 916 km at 1 light-year; an object of diameter 1 AU (149 597 871 km) at a distance of 1 parsec (pc) Thus, the angular diameter of Earth's orbit around the Sun as viewed from a distance of 1 pc is 2″, as 1 AU is the mean radius of Earth's orbit.
For typical seeing, the practical resolution limits are at mirror sizes much less than the mechanical limits for the size of mirrors, namely at a mirror diameter equal to the astronomical seeing parameter r 0 – about 20 cm in diameter for observations with visible light under good conditions.
Following advances in understanding the diffraction of light and astronomical seeing, astronomers fully understood both that the apparent sizes of stars were spurious and how those sizes depended on the intensity of light coming from a star (this is the star's apparent brightness, which can be measured in units such as watts per square metre ...