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7.2.1 Aperture Alignment

The low-dispersion SI includes the spectral data for both the large and small apertures. Because the centers of the two apertures are not aligned along a line of constant wavelength in the rotated image space, it would be impossible to have a single dispersion solution apply to the entire image. Therefore, a shift along the dispersion direction is introduced for the small-aperture spectra so that both apertures are on a common wavelength scale. The small-aperture data for SWP are shifted by 0.8 pixels in the dispersion direction towards longer wavelengths, and for LWP and LWR the small-aperture data are shifted by 2.3 pixels towards shorter wavelengths. The shift is applied uniformly to all pixels within the small-aperture region of the low-dispersion SI, which is defined to extend from row 1 through row 33 of the image.

Karen Levay