Instrument Science Report (STIS)
We confirm a long-term rotational drift of 0.0031 degrees/year of the STIS CCD based upon analysis of 50CCD flatfields spanning over 20 years of calibration data. Using the dust motes present in the flatfields, we extract the positions of the motes in each image, allowing us to develop a catalog of stable, high ‘signal-to-noise’ mote features and track their relative positions over time. We find that the motes appear to be moving at the aforementioned rate relative to an approximate center of rotation located at X=468.02, Y=411.18 in detector pixel coordinates. Given the relatively large errors in centroiding the unusually-shaped and often asymmetric motes, we perform an MCMC slope-fitting analysis to derive an uncertainty on the rotation of ±0.0001 degrees/year. Our derived rotation rate value is similar to two previous complementary CCD analyses: a measurement of spectral trace rotation in the grating L modes, and a time-dependent offset in detector true north position angle relative to the FITS header orientation keyword in science images. We therefore recommend that archival and future STIS CCD images should have their header information updated accordingly to account for this rotational drift. We also suggest similar corrections for rotational effects with respect to the reference files for spectral traces.
Ward-Huong, Kimberly; Lockwood, S.; Debes, J.; and De Rosa, R. J., "Long-Term Rotational Evolution of the STIS CCD Flatfields" (2022). Astronomy: Faculty Publications, Smith College, Northampton, MA.