Word: prism
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Dates: during 1950-1959
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...camera itself does not move; mounted rigidly in the airplane, it is eight feet high, has a focal length of 48 in., weighs about 1,500 Ibs. Protruding below the plane's belly is a 90-lb. prism that rotates across the airplane's line of flight (see diagram). The prism, acting like a swinging mirror, throws into the camera lens a constantly changing view of the ground below. First the prism looks at the horizon on one side; then its glance sweeps under the airplane, then up to the horizon on the other side...
Since the image formed by the lens is a, moving one, the film must move in stept with it. In the Perkin-Elmer camera, the film is 18 inches wide and is carried in reels weighing 400 Ibs. Every time the prism makes its sweep, about ten feet of film race past the slit where the image forms. A complicated mechanism makes the film move slightly slantwise during part of its rush. This is designed to compensate for the forward motion of the airplane and keep the image from "drifting" on the film...
...expedition will use a new type of telescope for the job, a large Baker-Schmidt model that sees three times further than any present wide-range instrument. Part of the telescope is the largest prism in the world, almost a yard wide...