Due to residual small-scale geometric distortions introduced by the
IUE SEC Vidicon cameras, the dispersion solutions for low and high
dispersion are not precisely linear in nature. Residuals from a linear
fit to the emission-line positions in WAVECAL spectra show significant
second- and third-order terms. These distortions lead to wavelength
errors on the order of several Ångstroms (low dispersion) or several
kilometers per second (high dispersion) in some regions of the camera if
left uncorrected. A remapping (along the dispersion direction) of the
geometrically-corrected, rotated, linearized, and resampled image (SI)
data is necessary to eliminate these distortions and allow the use of a
linear dispersion relation. This remapping has been incorporated into
the resampling (*GEOM*)
step of the image processing system as
another vector field that is added to the existing vector fields that
describe the image rotation (
Chapter 7.1.2
) and geometric rectification
(
Chapter 7.1.3
). Higher-order terms, associated with fine scale shifts
in the dispersion direction analogous to the fine-scale shifts shown in
Figures
7.2
and
7.3
, are probably also present but cannot be corrected
because of the paucity of WAVECAL Pt-Ne features.

Analysis of many WAVECAL spectra has shown that the first-, second-, and
third-order dispersion terms for low-dispersion SI which have not been
linearized are very uniform over time and THDA. This allows the use
of a single third-order remapping vector for all low-dispersion images
from a given camera. In high dispersion, a similar condition exists
except that the remapping vectors are determined separately for each
order. The exact form of the correction for each camera is derived as
follows. First, a representative sample of WAVECAL images covering the
extremes in both observation date and THDA is chosen for analysis. The
number of images is typically on the order of 80-90. This sample of
images is initially processed without any attempt to apply the (as yet
unknown) linearization correction in the *GEOM* step. Third-order
Chebyshev dispersion solutions are derived for each of these uncorrected
images (using IRAF routines that are described in the next section) and
the mean dispersion coefficients for the entire sample are calculated on
a term-by-term basis. The mean dispersion coefficients are converted
into equivalent pixel-space coefficients, at which point they can be
used to compute the appropriate linearization correction vector to apply
to all subsequent images within the *GEOM* processing step. The
resulting low-dispersion linearization correction displacements for each
camera are shown graphically in Figure
7.1. These are included in the
*GEOM* processing of every low-dispersion image, so that the SI
reflects a linearized wavelength scale. Similar corrections are applied
to every order in high dispersion, yielding comparable results.

After the linearization correction is determined for a given camera, all WAVECAL images for that camera are processed with the correction applied (which is the normal processing mode) so that mean linear dispersion solutions and corresponding zeropoint dependencies with time and THDA can be derived as described in the following sections.