STIS was installed in HST 14 years ago, and since then the STIS CCD detector has accumulated significant radiation damage. This has led to increased dark current, more hot pixels, and a significant decrease in the charge transfer efficiency (CTE) during detector readout. The impact of the CTE changes on STIS CCD data quality was recently discussed in STIS Instrument Science Report 2011-02. For faint objects, the impact on the signal-to-noise obtained can be severe.
In many cases, the best way to ameliorate this problem is to move the source closer to the detector readout. To this end, a number of years ago, new aperture positions were defined in the long STIS slits to place the target near row 900 of the CCD, much closer to the readout register. This reduces the number of transfers during the readout by about a factor of four. Use of the E1 aperture positions is strongly recommended for all but the very brightest or most extended spectroscopic point sources.