There comes a point when the spacing of the interference pattern is equal to the shift between the two patterns
see
the image in Fig. 2~e! and the schematic in Fig. 3~e!. At this
point the two patterns are in complete registration and their
superposition produces a clear and quite distinct intensity
pattern. Indeed, the image of the intensity pattern is notice-
ably more distinct than the corresponding pattern obtained
from the single pinhole light source. That difference will be
discussed presently. The exact aperture separation at which
the two diffraction patterns are in complete registration is abit of a judgment call, if done visually. One would expect it
to happen at a separation of 8.8 cm but, as can be seen from
the image in Fig. 2~e!, the superposition of the two patterns
produces a clear and distinct pattern even at 9.0 cm. The
telescope size of 10 cm prevented exploration of the pattern
beyond 9.0 cm.