Observers using the STIS FUV MAMA detector are reminded that the instrumental background for this detector is highly variable. While the ETC adopts a rate of 1.5e-4 c/s/pixel, the actual rate can be much lower (~ 1e-5 c/s/pixel) immediately after the detector high voltage (HV) is turned on. The rate of increase after this period depends on the overall detector temperature, but can be as high as 4e-5 c/s/pixel/hour. After several hours rates up to 6e-4 c/s/pixel have been observed. While this extra detector "glow" is most intense in the upper left quadrant of the detector (see figure 7.20 in the STIS Instrument Handbook), very little of the detector is completely unaffected. Note that the ACS SBC detector suffers from a similar glow.
The count rates discussed above refer to the native MAMA detector pixels. When comparing to ETC results these values need to be scaled by the number of pixels included in an extracted spectral "pixel" or resolution element. For observers of bright targets, even the highest FUV MAMA dark rates will have a negligible impact on data quality. For observers of very faint targets, there are a number of possible mitigation strategies. For spectra of faint point sources, the observer should consider whether one of the COS FUV channels might be a better choice, as these have both much higher overall throughput and much lower detector background than comparable STIS modes. If STIS spectral observations are needed (e.g., to provide higher spatial resolution in the cross dispersion direction), the dark rate can also be minimized by putting the source near the bottom of the FUV MAMA detector below the worst of the extra detector glow. The "D1" aperture positions are intended for this purpose (see STIS IHB section 4.2.3). If neither of these strategies is adequate, the observers should discuss their needs with the Program Coordinator (PC) and contact scientist (CS) assigned to their program, to determine whether it might be practical to schedule their observations immediately after the detector HV has been ramped up, when the intrinsic detector background is lowest. Because of the need to schedule STIS MAMA operations around the South Atlantic Anomaly (SAA), in practice, the FUV MAMA detector is usually turned on only one time per day, and there is usually only about one orbit per day available where a detector background close to 1e-5 c/s/pixel can be expected, so it may not be possible to satisfy all such requests.