2.3 New Capabilities for Cycle 30

Spatial Scanning

Although spatial scanning with the STIS CCD is not a new capability in Cycle 30 (the first actual use of this available-but-unsupported mode was in Cycle 24), our understanding of its capabilities is evolving. For relatively bright targets, spatial scanning can allow many more photons to be collected in a single exposure (without saturating the CCD), better flat fielding, better removal of the fringing in near-IR spectra, and more robust removal of hot pixels and cosmic rays. Signal-to-noise ratios of 600–800 (in the 1D extracted spectra) have been achieved in several programs using this mode. To further explore the capabilities of spatial scanning with STIS, the STIS team conducted special calibration programs in Cycles 25 and 28 aimed at characterizing the reproducibility of the fluxes in time series observations (e.g., as commonly used for studies of transiting exoplanets). Some results from the initial analyses of those data can be found in the STIS September 2020 STAN; a paper containing a more complete discussion is in preparation. For more information about using spatial scanning with STIS, see Section 12.12.

Disabling Monthly MAMA Offsets

As discussed in Section 7.6.2, the position of first-order MAMA spectra on the detectors is offset slightly, on a monthly basis, in order to guard against possible uneven charge depletion in the detectors. Because the detector sensitivity varies slightly with offset from the nominal position, STIS flux calibration programs routinely disable the usual monthly offsets with use of the MSMOFF mode. Because the MAMA detectors have not shown any evidence of uneven charge depletion (see ISR 2021-02), beginning in Cycle 30 this previously restricted capability will be become available-but-unsupported for those GO programs requiring very high photometric accuracy or reproducibility. Scientific justification for the use of the MSMOFF mode is required for it to be approved. Please include this justification in your Phase I proposal (reach out to the Help Desk for help with the Phase I implementation) and speak with your STIS Contact/Instrument Scientist should you like to implement this in Phase II.