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2. Automated High Accuracy Comparator 
The functioning of this system is somewhat more complex than 
that of the previewer. This complexity, however, is built into the system 
itself, and does not constitute a burden to the operator. 
As in the case of the previewer, the photographic plate to be 
scanned in the comparator must first be aligned with the axes of the ma- 
chine. This is done by the operator in the same manner as for the pre- 
viewer, 
If the zero reference for the High Accuracy Comparator read- 
out is to coincide with that for the previewer, the previewer zero-position 
image is centered in the comparator field of view, and the comparator co- 
ordinate counters are set to zero, However, if a different zero reference 
is desired for the comparator readout, then, after the above operation, 
the table is translated until the desired zero point is reached. At this 
point, the operator instructs the programmer to store the zero offset. The 
coordinates of the new point with respect to the previewer zero are auto- 
matically read from the coordinate counters into a storage unit provided 
in the digital subsystems. Thereafter the previewer coordinates are used 
to drive the comparator table to the rough image positions, but the final 
accurate coordinate readouts are modified by the stored offset data. This 
system permits any number of translations of the original previewer co- 
ordinates to be made without repeating the previewing operation. 
After the zero reference has been established in one of the above 
ways the rough coordinates for the following image are read into the com- 
parator by an IBM card reader, under control of the programmer. The 
distributor routes the rough metric coordinates to the digital subsystem, 
where they are converted to equivalent wavelengths of light, to make them 
compatible with the comparator coordinate counters. The digital compar- 
ator detects the difference between the desired and actual coordinates and 
causes the drive system to translate the table to reduce the error. 
Reduction of the error in both axes to the point where the de- 
sired image is in the field of view causes the programmer to transfer con- 
trol of the drive system to the automatic image sensing equipment, This 
equipment translates the table until the image is centered in the optical 
field of view. It then causes the programmer to read out the accurate co- 
ordinates from the counters (which have been updated to the final position) 
and to read out the image constants from the distributor. 
Readout of the coordinates requires a reverse conversion, in 
which light wavelengths are converted to microns. Final readout is there- 
fore directly in microns. The output record is punched into an IBM card 
and printed on paper tape. In each case, the format is controlled by the 
patching of the programmer plugboard. The micron readout is also avail- 
able as a visual display.