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The scanning spot is moved, or deflected, along a search line, with 
a small geometrical rosette pattern superimposed on it. Selection of such 
a pattern is based upon the desirability of sampling the diapositive infor 
mation in a nondirectional fashion. Additionally, a high-frequency X-axis 
wobble is superimposed on the rosette pattern to enhance the detection of 
differential parallax. The pertinent characteristics of the scan pattern in 
clude a pattern generation time of 1/100 second and an X-axis wobble fre 
quency of 20,000 wobble cycles per second. The rosette pattern, composed 
of 12 leaves, is allowed to "pinwheel" about its axis of symmetry to assure 
complete sampling of the diapositive content. Thus, 100 independent 
samples of the diapositive information are obtained each second. 
The multiplier phototube, located within each Balplex projector, 
transforms the variations in the intensity of the scanning spot, brought 
about by the intensity modulation effects of each diapositive, into elec 
trical signals. The photomultipliers are mounted on specially constructed 
rotatable turrets which allow either projection of the diapositives, for 
stereomodel orientation, or receipt of the modulated scanner light spot, 
for automatic modes of operation. After obtaining the necessary electri 
cal signals, wide-band amplifiers raise the power of the signals to a 
level suitable for entry into the correlators. 
A beam-intensity-control feedback system continuously monitors 
and adjusts the intensity of the scanning spot to compensate for nonuni 
formities of the scanner tube phosphor. This action assures that the only 
intensity modulating effect the multiplier phototubes sense is due to the 
diapositive alone. It should be recalled that any variation in scanner 
light intensity will appear correlated, since it will appear in both signal 
channels at the same instant of time. 
The diameter of the rosette scan pattern is caused to vary accord 
ing to the terrain slope to further enhance the parallax detection capabilities 
of the correlators. Feedback signals are generated according to slope and 
contour angle measurements which are performed by the correlators. An 
idealized curve of the rosette pattern diameter as a function of terrain 
slope is shown in Figure 3.