Exploring the Unknown - NASA's History Office

[3] WHAT CAN BE SEEN
Naturally, any estimate of the amount that can be seen from an extreme altitude must
be a function of both the resolving power of the camera system and the area that can be
scanned and recorded (or televised) and still retain usable data. Much of the discussion
and most of the figures in this section are the result of previous RAND studies conducted
by Dr. R. S. Wehner.
Using the relation (see Fig. 1)
EXPLORING THE UNKNOWN 187
AREA COVERAGE
Fig. 1—Viewing system
W = w = 2 tan a,
F d 2
where W = sensitive element width, in.
w = width of surface pictured per frame, mi.
F = focal length of camera, in.
d = optical range, mi.
a = angle of view, deg.,
and using Tables 1, 2, and 3, it is possible to compute the width of square surface viewed
and the angle of view for any given camera and aperture. This has been done and is summarized
in Table 4. As can be seen, if a limiting resolution 6 of 500 ft is [4] set, it is still possible
to obtain this resolving power under sunlight illumination with a contrast as low as
2.5 per cent (with a 5.0-in. aperture). Under moonlight, however, this resolution is possible
only with 100 per cent contrast, a very fast f/1.4 lens, and a minimum exposure time
of 0.25 sec; under light of the night sky illumination it is not possible at all. Assuming,
then, that the chosen limiting resolution is correct, the probability of obtaining identifiable
cloud photographs under any but sunlight illumination appears to be small.
6. The term “limiting resolution,” as used in the television field, refers to the greatest possible resolution
attainable by a given TV pick-up tube and is wholly dependent on the structural make-up of the tube itself.
As used in this report, limiting, minimum, or usable resolution is a quantity depending on scene contrast signalto-noise
ratio, aperture, f number of camera, etc., and is chosen to pick up the smallest object that it is desired
to view.