illumination

The astronomy/telescope term illumination refers to the light energy per unit time striking a unit area of the focal plane of a telescope where the image is to be observed or recorded. It is directly proportional to the area of the telescope's aperture, as well as depending upon the source, and the position of the portion of the image on the focal plane, i.e., for circularly-symmetric optics, illumination falls by distance from the center of the image. A telescope's zone of full illumination is a measure of how much the illumination falls off from the central value ("full" meaning above a some given percentage of that at the center, e.g., 75%).

Detailed equations describing the illumination allow derivation of the formula for the produced Airy disk.