|Space Telescope Science Institute|
|ACS Instrument Handbook|
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. An example of the dark current variation across the detector can be seen in Figure 4.13. Ground test measurements of the ACS MAMA showed count rates in the range of 10-5 to 10-4 counts per pixel per second as the temperature varied from 28 °C to 35 °C. The count rate increased by about 30% for one degree increase in temperature. In-flight measurements, taken weekly throughout June and July 2002, show count rates between 8x10-6 and 10-5. These measurements were taken as soon as the MAMA was turned on and were therefore at the lower end of the temperature range. A 10 hour observation in SMOV (SM3B), long enough for nominal temperatures to be reached, yielded a dark current of 1.2 x 10-5 counts per second per pixel. Monthly monitoring shows the in-flight dark current to be about 9x10-6 counts per second per pixel.A comparison of the dark rate as a function of temperature is shown in Figure 4.14. As a function of temperature the behavior of the dark current was essentially unchanged. No long-term increase in the dark rate was found. Due to the insignificant dark current, dark corrections are no longer performed in the ACS pipeline.The ACS MAMA has a broken anode which disables rows 600 to 605. There are three dark spots smaller than 50 microns at positions (334,977), (578,964), and (960,851), as well as two bright spots at (55,281) and (645,102) with fluctuating rates that are always less than 3 counts per second. The reference pixel has been moved to (512,400) to avoid these areas (see Table 7.9)Figure 4.13: MAMA dark image.Full frame MAMA dark image. The average dark rate in this image is 5 x 10-5 counts per second per pixel.Figure 4.14: SBC Dark Rate and Operating TemperatureUpper panel: SBC Dark Rate (counts s-1 pix -1) as a function of the operating temperature in Celsius. Lower panel: The operating temperatures vs. time elapsed since SBC was switched on. Red crosses and green circles are data from May 2002 and March 2012, respectively. Dark rates have remained stable since ACS activation in 2002 and are unaffected by SM4. For both epochs, dark rates are stable for operating temperatures below 28 Celsius (for about 4 hours after being switched on).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); Gilliland, 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 6% 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 4.15: 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 were derived by Bohlin & Mack (ACS ISR 2005-04) using the on-board deuterium lamp and were found to be independent of wavelength. The P-flat in Figure 4.15 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 (Mack, et al., 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. 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. The sensitivity appears to have leveled off after this time. A slight sensitivity loss as a function of temperature (~0.1%/degree) was also discovered. These effects were also observed for the STIS FUV-MAMA detector (see STIS ISRs 2003-01 and 2004-04). Following the repair of the ACS WFC in May 2009, no significant changes in the average SBC sensitivity have been found.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).