The continuous undulating line in Figure 2 is the Chebyshev solution for an actual Pass 1 swath of an SWP image. The IOR area is denoted by a dashed triangle with vertices at spatial pixel numbers nc, nf, and nd. The normal execution of the background determination code requires that valid background pixels are sampled at these three points as well as the adjacent points nc-1, nf+1, and nd+1. In Figure 2 the rise from nd to nf is determined largely by the width of the PSF. This slope may vary because of circumstances for a particular exposure. For example, the slope of the leg of the IOR triangle is different for trailed images than for point-source images. Because this slope is a property of the swath itself, BASISMOD can use it to refine the overlap as estimated from the model PSF. The overlap between nc and nf depends primarily on source energy distribution and not exposure parameters. The effect of overlap near pixel nc appears to decrease in the figure, but this is an artifact of the rapidly decreasing camera sensitivity at short wavelengths.