Seeing - Celestron 127SLT User Manual

S e e i n g
S e e i n g
Seeing conditions refers to the stability of the atmosphere and directly affects the amount of fine detail seen in
extended objects. The air in our atmosphere acts as a lens which bends and distorts incoming light rays. The
amount of bending depends on air density. Varying temperature 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 determines the "seeing" quality. Under good seeing conditions, fine detail is visible
on the brighter planets like Jupiter and Mars, and stars are pinpoint images. Under poor seeing conditions,
images are blurred and stars appear as blobs.
The conditions described here apply to both visual and photographic observations.
Seeing conditions directly affect image quality. These drawing represent a point source (i.e., star) under
bad seeing conditions (left) to excellent conditions (right). Most often, seeing conditions produce images
that lie some where between these two extremes.
Figure 5-1
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