Time Dependent Trace Angles for the STIS First Order Modes

Single rows in STIS rectified spectral images and spectral extractions made with small extraction boxes are being analyzed in science programs where the highest obtainable spatial resolution is needed. Here we address a problem that has limited the accuracy of these data products: the spectral traces in the reference files often do not have the same tilt as those in the science exposures. By analyzing data for the most commonly used first order spectral modes, we have determined that the traces have been systematically rotating over time. We have delivered spectral trace reference files that apply the calibrated rotation to the most commonly used modes. We have also produced a PyRAF routine, MKTRACE, that measures the tilt of the trace in a science image and produces a corrected trace reference file for that image. Using corrected traces, we have analyzed the flux calibration of L grating spectra extracted with small boxes. We find substantial errors in the spectral shape of small-box extractions of G430L spectra due to uncalibrated systematic wavelength-dependent variability of the point spread function, which becomes broader at long wavelengths as it becomes narrower at short wavelengths.