SCANNING PATTERN
To provide uniform scanning over conjugate image areas, a
Lissajous scanning pattern is used. This pattern is normally pro-
duced by two triangular waveforms for x and y deflection
which differ in frequency by the required frame frequency. The
frame frequency must be substantially higher than the band-
width of the correlation servoloop to filter out the frame fre-
quency components. The number of scanning lines need not be
large to detect zero- and first-order distortion. In the present
case, a scanning pattern with 31 lines across the diagonal is
used and the frame frequency is 560 hz.
Some advantages can be gained by orienting the scanning
pattern in the form of a diamond, as shown in Fig. 5. This pat-
tern can be generated by merely rotating the deflection system
through 45 degrees, or by maintaining alignment of the deflec-
tion coils with the x and y axes and using a more complex scan-
ning waveform. The diamond pattern has the advantage that the
scanning spot always moves in either the x or the y direction,
enabling the quality of correlation in these directions to be eval-
uated separately. Separate monitoring of x and y correlation
quality allows image areas of poor correlation to be covered
with greater reliability.
CORRELATION
The video signals from the photomultipliers are fed
through video amplifiers to the correlator, where they are split
into several frequency bands to allow the low spatial frequen-
cies to be correlated first, progressing to higher frequencies as
the accuracy of alignment is improved. The correlation process
consists of multiplying the two inputs in a balanced, linear,
four-quadrant multiplier. Two outputs are produced from the
correlator, as shown in Fig. 6. One output consists of the nor-
mal correlation function which peaks at the point of maximum
correlation, indicating correlation quality. The second output is
obtained by phase shifting the signals before multiplication, re-
sulting in the orthogonal correlation function which passes
through zero with a change of polarity at the point of maxi-
mum correlation. This output contains the desired information
on the magnitude and direction of image displacements between
the scanned images.
The multiband correlator provides both wide range pull-in
and high accuracy; an electronic channel selector is used to se-
lect the optimum channel, depending on the degree of correla-
tion between the two images. The correlator outputs are then
analyzed to determine the magnitudes of each of the various
distortion components. A schematic diagram of the correlator
and analyzer is shown in Fig. 7.
Fig. 5 — Diamond scanning pattern
