Now, what does one do about the 180° ambiguity in the typical autocorrelogram? Bagnuolo et al.10 describe a variation of the direct image-to-autocorrelogram numerical technique that resolves the quadrant ambiguity for most binary star systems. Instead of merely assigning the above-threshhold value to one, its multi-bit value is retained. The concept of aligning the frame multiple times on a larger canvas is replaced by an analysis of each of the unique pairs within the frame. In the case of the former concept, the net effect is to sample each unique pair twice, aligning the frame on each component of the pair. It is very easy to see that this will lead to a peak on either side of the central peak, exactly 180° apart. In the case of the latter concept, each unique pair is sampled only once, aligning the pair only on the brighter value. This will tend to emphasize the outlying peak that, along with the central peak, will define the true position angle of the binary star system. However, if both stars are about the same brightness, the method breaks down. This variation of the autocorrelogram is called the directed-vector autocorrelogram. Figure 17.4 shows a surface plot of a directed-vector autocorrelogram.
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