|ACS Instrument Handbook for Cycle 26|
4.5.1 SBC Scheduling PoliciesThe STIS MAMA control electronics are subject to resets due to cosmic-ray upsets. Therefore, STIS MAMAs are operated only during the contiguous orbits of each day that are free of the South Atlantic Anomaly (SAA). Even though the design of the ACS MAMA control electronics in the SBC was modified so that they would not be susceptible to cosmic-ray hits, the background count rate still exceeds the bright object limits for the SBC during SAA passage. Consequently, the SBC will in general only be scheduled for use during SAA-free orbits.The MAMA is a photon-counting detector: as each event is recorded, the buffer memory for the corresponding pixel is incremented by one integer. The buffer memory stores values as 16 bit integers; hence the maximum number it can accommodate is 65,535 counts per pixel in a given ACCUM mode observation. When accumulated counts per pixel exceed this number, the values will wrap, i.e., the memory resets to 0. As an example, if you are counting at 25 counts/second/pixel, you will reach the MAMA "accumulation" limit in ~44 minutes.One can keep accumulated counts per pixel below this value by breaking individual exposures into multiple identical exposures, each of which is short enough that fewer than 65,535 counts are accumulated per pixel. There is no read noise for MAMA observations, so no penalty is paid in lost signal-to-noise ratio when exposures are split. There is only a small overhead for each MAMA exposure (see Section 8.2).
4.5.3 MAMA DarksMAMA detectors have intrinsically low dark currents. Ground test measurements of the ACS MAMA showed count rates in the range 10–5 to 10–4 counts/s/pixel as the temperature varied from 28°C to 35°C. The count rate increases by about 30% for one degree increase in temperature. A recent study of in-flight data investigated how the temperature impacts the dark rate when the detector is left on for a long period of time (ACS ISR 2017-04). The dark rate remains as stable as ever. It is 8.11 x 10–6 counts/s/pixel while the instrument is below ~25°C (Figure 4.13). Above that temperature, the central region of the detector begins to experience an increasingly higher dark rate (Figure 4.17). That being the case, dark frames cannot be subtracted from science observations like they are with CCDs. Unlike CCDs, MAMA dark images are not meant to capture hot and warm pixels. Instead, dark rate in MAMA detectors shows up as random background noise. For the purposes of scientific analysis, the dark rate can be subtracted along with the background noise, at least when the detector is still below ~25°C. Above that temperature, users should still be able to do a local subtraction, but should be aware of the pattern present in the dark rate. Alternatively, users can place their target in the bottom left corner of the image if they plan to use the SBC for visits longer than 2 orbits.Users should work with contact scientists during the Phase II reviews to address any issues related to the dark rates in SBC.Figure 4.16: SBC Dark Rate and Operating Temperature.Top panel shows the dark rates vs. temperature. The bottom panel shows how the temperature changed from the time the instrument was turned on until the end of the observations. Dashed lines correspond to bad data not used in the analysis.Figure 4.17: Dark Rate Image Comparison.Left: Dark rate image taken immediately after the detector was turned on (from proposal ID 14513). Right: Dark rate image from the same program taken after the detector had been on for 5 hours. The increased dark rate visible to the top-right of the image drives the increased average of the entire detector, while the dark rate in the bottom-left corner remains constant throughout.MAMA detectors are capable of delivering signal-to-noise ratios of order 100:1 per resolution element (2 × 2 pixels) or even higher. Tests in orbit have demonstrated that such high S/N is possible with STIS (Kaiser et al., 1998, PASP, 110, 978; STIS ISR 1998-16). For targets observed at a fixed position on the detector, the signal-to-noise ratio is limited by systematic uncertainties in the small-scale spatial and spectral response of the detector. The MAMA flats show a fixed pattern that is a combination of several effects including beating between the MCP array and the anode pixel array, variations in the charge-cloud structure at the anode, and low-level capacitive cross-coupling between the fine anode elements. Intrinsic pixel-to-pixel variations are of order 4% but are stable to < 1%. Photometric accuracy can be improved by averaging over flat field errors by dithering the observation (see Section 7.4).4.5.5 SBC FlatfieldFigure 4.18: MAMA Flat FieldThe SBC requires two types of flat fields: the “pixel-to-pixel flats” (or P-flats), which take care of the high-frequency structures in the detector sensitivity, and the “low-order flats” (or L-flats), which handle the low-frequency components. Current P-flats (ACS ISR 2005-04) using the on-board deuterium lamp were found to be independent of wavelength. Recent analysis shows that the structure in the P-flat changed sometime between 2005 and 2007 (ACS ISR 2016-02). Random pixel to pixel changes were found to be small, but coherent changes were as large as 4% peak to peak. A new P-flat from this program will be incorporated into the calibration pipeline.The P-flat in Figure 4.18 shows the effect of the disabled broken anode for rows 600 to 605 and of the shadow of the repeller wire around column 577.Low-frequency flatfield corrections for the SBC imaging modes have been derived using multiple exposures of the globular cluster NGC6681 (ACS ISR 2005-13). Variations of ±6% (full range) were found for the F115LP and F125LP filters, ±8% for the F140LP and F150LP filters, and ±14% for the F165LP filter. The F122M filter was not included in this analysis due to lack of sufficient data. The L-flat shows a similar general pattern in all filters, with the required correction increasing with wavelength. Yearly observations of this cluster taken from launch up to the current cycle are being used to produce new L-flat corrections that will be incorporated into the calibration pipeline within the current cycle (Avila & Kossakowski, 2016).Analysis of this stellar data showed a decline in the average UV sensitivity with time at a level of 2 to 4% per year over the first 1.6 years of operation. After that, the sensitivity remained within a few percent until 2014. At that time another drop in sensitivity was observed which brings the total amount of sensitivity loss since launch to ~8% (Figure 4.19). These effects were also observed for the STIS FUV-MAMA detector ( STIS ISRs 2003-01 and 2004-04). This loss was observed in the F125LP filter, but preliminary analysis shows that the other imaging filters show the same qualitative behavior. Work is ongoing to fully characterize the sensitivity changes of the rest of the filters.4.5.6 SBC NonlinearityThe MAMA detector becomes nonlinear (i.e., photon impact rate not proportional to detected count rate) at globally integrated count rates exceeding 200,000 counts/second. The MAMA detector and processing software are also unable to count reliably at rates exceeding 285,000 counts/second. For these reasons, and to protect the detectors from over-illumination, observations yielding global count rates above 200,000 counts/second are not allowed (see Section 4.6).The MAMA pixels are linear to better than 1% up to ~22 counts/second/pixel. Nonlinearity at higher count rates is image-dependent such that the nonlinearity of one pixel depends on the photon rate affecting neighboring pixels. Consequently, it is impossible to reliably correct for the local nonlinearity in post-observation data processing. MAMA detectors are also subject to damage at high local count rates, so observations yielding local count rates above 50 counts/second/pixel are not allowed (see Section 4.6).