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 spectroscopic observations of 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 (> 50 e-/pixel/s source count rate) or most extended spectroscopic point sources (> 5" diameter).
While efforts have been made to ensure that the STIS calibration near the E1 position is comparable to that at the central position, there are still a few circumstances under which the central position is preferable, e.g. IR observations with the G750L and long wavelength G750M observations that require an internal fringe flat observation. For point sources, the best fringe flat subtraction is obtained by putting the target at the default central position and taking the tungsten lamp fringe flat exposure using the small 0.1X0.09 aperture. At the E1 position, the 52X0.1 aperture needs to be used and this provides a less accurate emulation of the fringing pattern in an external point source. See section 11.2.3 of the STIS IHB for additional details. There is also some vignetting at the E1 position which slightly reduces the throughput. Observers unsure which strategy to use should discuss their needs with the Program Coordinator (PC) and contact scientist (CS) assigned to their program