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.