As discussed in Chapter 8.1, global shifts in the location of the spectral format occur as a function of time and camera temperature. These global movements of the spectral format appear as displacements of the spectrum in both the spatial and dispersion directions of the low-dispersion SI. The resulting change in the starting wavelength (i.e., the wavelength value assigned to column 1 of the low-dispersion SI) can be compensated for through the application of zeropoint corrections derived from wavelength calibration (WAVECAL) images (Chapter 8.1). While this would result in the assignment of the correct wavelength zeropoint for each low-dispersion SI, it would also result in the assignment of a different starting wavelength for each image. Having spectra with offset wavelength scales would affect subsequent data analysis.
To alleviate this effect in low dispersion, zeropoint shifts in both the spatial and dispersion directions are applied to the image resampling vectors so that all spectra for a given camera have the same starting wavelength and the same spatial location within the low-dispersion SI. Furthermore, additional shifts are applied to the image resampling vectors so that both long wavelength cameras provide coverage of the same spectral range. The spatial centroid of large-aperture spectra is chosen to lie on or about line 51 of the low-dispersion SI for all cameras. The resulting centroid of small-aperture spectra is approximately at line 25. Due to the nature of the LWP geometry, this requires an additional adjustment to the LWP data in order to place the large-aperture spectrum at the top of the file. The common wavelength starting values are 1050Å and 1750Å for the short and long wavelength cameras, respectively. The amount of the zeropoint shift to be applied to each image is computed from the difference between the desired starting wavelengths and those predicted from the time- and temperature-corrected dispersion constants (Chapter 8.1). Image-to-image scatter in the precise location of the spectral format leads to 1-sigma errors in the predicted wavelength and spatial zeropoints of 0.3Å and 0.26 pixels, respectively, for the SWP and 0.6Å and 0.35 pixels, respectively, for the LWP.