A typical sequence of real CCD images generated in this manner for a particular intersegment edge is shown in Figure 4 . As we approach the middle subimages of the sequence, the patterns become more and more coherent. Typical broadband sequence of CCD subimages. Conversely, the first and last subimages of Figure 4 bear little resemblance to any of the templates and therefore have a low degree of coherence. In Figure 5 we plot this coherence parameter vs. step height for a series of nume rically generated images corresponding to Eq.
Smooth curve is the best-fit Gaussian approximation.
For our purposes it makes little difference that the origin of the noise is theoretical rather than observational. The actual coherence parameters corresponding to the diffraction patterns in Figure 4 are shown in Figure 6 a. The top panel corresponds to the particular sequence of subimages shown in Figure 4 . These different coherence lengths define different modes of broadband phasing. The accuracy of a given mode is roughly proportional to the coherence length.