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.