After a continuous Chebyshev solution is computed for a particular
Pass 1 swath, a series of data pathology checks are performed on the
solution (see Section A.2.5). These tests are performed only on images of
point and extended sources containing continuum flux. If the solution
fails any of the pathology checks, an appropriate condition is relaxed
(e.g. the PSF is not used to calculate the true interorder background,
or the degree of the Chebyshev fit is decreased by one). The solution
is then recalculated iteratively. Since even adjacent Pass 1 swaths
sometimes can pass different pathology tests, their solutions can
occasionally be different. This is a subtle but often important source
of error in the final background solution. The results of these
differing solutions are manifested as ``spike structure" in
Figure A.4, as described below.
Figure A.3:
A depiction of the influence of interorder overlap
in progressively raising the intensity of (unit height) orders towards
shorter wavelengths (left). The dashed lines represent intensities the
orders would have if there were no overlap.