Image Processing Reference
aged, the result is not an Airy function, but the light is spread out across the focal
plane. The characteristics identified in Eq. 1.1 are therefore no longer seen even in
short exposures. The effect of the atmosphere on the image spot for long exposures
has been characterized as (Hardy 1998)
where r 0 is the diameter of an average cell of “constant” refractive index in the at-
mosphere and the primes are to distinguish this from the diffraction-limited case.
At astronomical observatories, r 0 ranges from a few centimeters to more than a me-
ter; it can also be referred to as astronomical “seeing.” The seeing is said to be ex-
cellent when it is greater than 15 cm, average when around 10 cm, and poor when it
falls below 8 cm.
The effect of image jitter in the focal plane is shown in Fig. 1.5 for long-expo-
sure images, comparing the plane wavefront to either the motion of the receiver or
changing the angle of arrival of the wavefront due to atmospheric effects.
Figure 1.5 Effect of angle-of-arrival variations at the receiver showing beam spread-
ing in the focal plane.