A measure of the blurring and degradation of the image of astronomical objects caused by → turbulence in the Earth's atmosphere, including the telescope environment. Seeing causes the images of stars to break up into → speckle patterns, which change very rapidly with time. See also → Fried parameter; → differential image motion monitor.
M.E. seen, from O.E. seon; cf. O.S., O.H.G. sehan, M.H.G., Ger. sehen, M.Du. sien, Goth. saihwan, from PIE base *sekw- "to see."
Šekân "wrinkle, plait; curl; rupture, breach," variant of šekan "fold, curl; ripples on water," from šekastan "to break, split;" Mid.Pers. škastan "to break;" Av. scind-, scand "to break, cleave;" Proto-Iranian *skand- "to break, cleave;" PIE sken- "to cut off."
gerde-ye šekân, disk-e ~
Fr.: tache de seeing
The angular size of a stellar image for long exposures, as determined by the ratio λ/r0, where λ is the wavelength and r0 the typical size of → turbulence patches. → Fried parameter. The most common seeing measurement is the → full-width at half-maximumof the seeing disk. → Airy disk.
Fr.: moniteur de seeing