32
Seeing
Seeingconditions refers to the stability of the atmosphere and directlyaffects the amount of fine detail seen in extended
objects. The air in our atmosphere acts as a lens which bends and distorts incominglight rays. Theamount of bending
dependson air density. Varyingtemperature layers have different densities and, therefore, bend light differently. Light
rays from the same object arrive slightly displaced creating an imperfect or smeared image. These atmospheric
disturbances vary from time-to-time and place-to-place. The size of the air parcels compared to your aperture
determinesthe "seeing" quality. Undergood seeing conditions, fine detail is visible on the brighter planets like Jupiter
andMars, and stars are pinpoint images. Underpoor seeing conditions,images are blurred and stars appear as blobs.
Theconditions described here apply to both visual and photographicobservations.
Figure7-1
Seeingcon ditions directly affect image quality. Thesedrawings represent a point
source(i.e., star) under bad seeing conditions (left) to excellent conditions(right).
Most often, seeing conditions produce image s that lie some where between these two
extremes.