Building Earth Observation Cameras

45Imaging Optics

aberration, and the major aberration is coma. Depending on the primary and

secondary surface proles, a number of variants of the Cassegrain telescope

are possible. Of all the three congurations, that is, Newtonian, Gregorian,

and Cassegrain, the Cassegrain telescope has the shortest tube length (dis-

tance between the primary and secondary) keeping the other parameters

constant. Therefore, most Earth observation cameras the use Cassegrain tele-

scope or one of its variants as the collecting optics.

(b)

(a)

FIGURE 3.7

Optical layout of two-mirror telescopes: (a) Gregorian and (b) Cassegrain.

46 Building Earth Observation Cameras