2.1 CTE Effects and the E1 Aperture Positions

As the STIS CCD detector has accumulated radiation damage over time, the Charge Transfer Efficiency (CTE) has decreased (see Section 7.2.6). For faint sources observed near the center of the CCD detector, this can result in loss of 18% or more of the detected signal during the readout. Since the amount of these CTE losses depends on both the observed signal and background counts, there is no simple way to correct for these losses, and they can significantly affect the shape of a measured spectrum. Noticeable effects can be seen even in well exposed spectra. In addition to its effects on the counts from the observed astronomical source, CTE effects redistribute some of the electrons in hot pixels and cosmic rays into "tails" that lag behind during the readout. These tails add significant background noise to long exposures that is not taken into account by the STIS Exposure Time Calculator (ETC), and which can be difficult to remove.

All of these effects can be significantly ameliorated by moving the location of the source image on the detector closer to the amplifier, thereby reducing the number of parallel transfers that occur during the readout. To this end, new aperture positions (52X0.05E1, 52X0.1E1, 52X0.2E1, 52X0.5E1, and 52X2E1) have been defined near row 900 on the STIS CCD detector for use with the STIS first order gratings. The use of these new aperture positions is strongly recommended for the observation of faint sources. For high signal-to-noise observations of bright targets we recommend continuing to use the regular aperture positions near the center of the detector. Extensive calibration observations are planned during Cycles 11 and 12 to ensure that the calibration at the E1 aperture positions is of the same quality as it is for sources observed at the usual location on the STIS CCD. Further information regarding the use of these new aperture positions can be found in Section 7.2.7.