2022AJ....163...78A

The Hubble Space Telescope Imaging Spectrograph (STIS) is a powerful tool for high spectral resolution ultraviolet (UV; 1150-3200 Å) studies of bright cosmic sources. However, achieving the superb wavelength precision inherent in its UV echelle channels is hampered by subtle camera distortions that are not fully compensated for by the CALSTIS pipeline. The systematics arise from the low-order (n = 2) bivariate polynomial dispersion model employed in the echellegram processing. The formulation does remarkably well given its simplicity (only seven terms in the current implementation), but cannot account for apparent higher frequency undulations in the STIS spectral images. Previous correction schemes have built elaborate distortion maps, with up to thirty-six terms, operating on the individual echelle orders in the pipeline "x1d" file, prior to merging the orders. There is, however, a more straightforward, although partial, solution: a polynomial formula in the wavelength domain applied to the order-merged spectrum; just n = 3 for most of the STIS settings, although up to n = 5 for a few of the more recalcitrant ones.