B

X dn dOn =------(Angular spacing of visibility lines) (7.5)

B sin On

Here dn = 1 yields the separation of adjacent bands. The bands normal to the baseline (On ~ 90°) are separated by |dOn | = X/B in agreement with (1), but the angular spacing increases toward the poles of the baseline as sin On decreases. In our equatorial observation (Fig. 2), this gives the increase in line spacing noted above when the source moves away from the zenith.

The effect of earth rotation is shown in Fig. 5c for the two telescopes placed at an arbitrary orientation, in this case with the baseline projecting to declination 8 = ±50°. The telescopes (at the center of the celestial sphere) are carried with the earth's rotation; hence the entire fringe pattern rotates about the earth's spin axis. As the pattern undergoes a 360° rotation, a source, being fixed on the celestial sphere, is scanned by a range of line spacings AO and line directions. This is best seen in the frame of reference of the earth in which the fringes are fixed and the stars move. The shaded track in Fig. 5c shows the circular track of a source (at 8 = 70°) in this rotating frame of reference.

During the earth rotation, the pointing directions of the telescopes must continuously change to keep the source in their individual fields of view. Sources not at high declinations will set and rise each day; they can not be observed when they are below the horizon. This earth occultation eliminates some of the possible line crossings and adversely affects the quality of the resultant images.

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