The continuous undulating line in Figure A.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 A.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, BCKGRD 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.