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| Figure 1 | Figure 2 |
For comparison, Fig. 2 illustrates the fringe pattern (visibility) produced by a pair of equal sources centered in the same direction as Fig 1., but the separation of the sources is in the perpendicular direction, so the beating of the two patterns produces secondary "beat" fringes perpendicular to the primary fringes.
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| Figure 3 |
In the more general case, the sources can be in any direction, and the orientation of the source separation vector can be in any other direction. Figure 3 illustrates the visibility fringe pattern for a case where the source centroid has an azimuth of 6 degrees (yellow line perpendicular to lines of constant phase. The separation vector has an azimuth of 45 degrees (red line). The fringe pattern is seen to be slowly modulated with a period of about 6 modulation cycles.
The concentric circles in the image represent the sampling regions of 5 HESSI collimators. The innermost circle crosses the fringe maxima 3 times in one half revolution, but it is too small to "see" the "beating" modulation, having a diameter only half the separation of the "beat" fringes. The next larger circle crosses 5 main fringes, but is slightly smaller than the separation of the "beat" fringes, so the source separation is unresolved. The next larger(third) circle crosses 10 main fringes, and two of the "beat" fringes, so the separation is resolved. The 4th and 5th circles cross 2 and 4 "beat" fringes (respectively), and can also be used to determine the direction of the source separation vector.