Updates to the SBC Dark Rate Monitor

1. Problem Statement

The Solar Blind Channel (SBC) of the Hubble Space Telescope's (HST) Advanced Camera for Surveys (ACS) utilizes a Multi-Anode Microchannel Array (MAMA) detector. While this detector typically exhibits low dark current at temperatures below 25.5°C, anomalies have been observed where the dark rate increases rapidly before the detector reaches this thermal threshold.

• Historical Context: Prior to Cycle 28, dark rates were measured annually and remained low. However, elevated rates were detected in Cycles 26 and 27, raising concerns about instrument stability.
• The Anomaly: Specific visits showed exponential increases in dark current starting as early as one hour after the detector was powered on, well below the 25.5°C temperature limit.
• Uncertainty: The cause of these premature dark rate spikes was unknown, and it was unclear if they were linked to the South Atlantic Anomaly (SAA) or specific detector regions.

2. Methodology

To investigate these anomalies, the ACS team expanded the calibration monitoring program starting with Cycle 30 and continuing into Cycle 31 and beyond.

• Observation Strategy:
  • Frequency: Increased from annual measurements to 24 orbits per year, distributed across eight visits.
  • Visit Types:
    • Long Visits: 20 exposures of 1,000 seconds each, taken over four orbits (scheduled for Dec, Mar, Jun, Sep).
    • Short Visits: 11 exposures of 1,000 seconds each, taken over two orbits (scheduled for Oct, Jan, Apr, Jul).
  • Pre-conditioning: Ideally, the detector is powered off for at least 10 hours prior to a visit to ensure a consistent starting temperature, though some visits did not meet this criteria.
• Data Processing:
  • Sources: RAW and SPT (Science Planning Tool) files retrieved from the Mikulski Archive for Space Telescopes (MAST).
  • Calibration: Images were processed using CALACS to generate FLT products. Pixel values were converted from integer to floating-point, and temperature data (measured in the enclosure tube via MDECODT1/MDECODT2 keywords) was extracted.
  • Metric Calculation: Dark rate was calculated as the mean value of the science frame (excluding bad pixels flagged in the DQ array) divided by exposure time.
• Analytical Approaches:
  • Temporal Analysis: Plotted dark rate vs. time, temperature, and elapsed time since power-on.
  • SAA Correlation: Used SPT orbital parameters to map HST locations during dark exposures relative to the South Atlantic Anomaly (SAA) boundaries for both stable and elevated visits.
  • Spatial Analysis: Compared dark rates in two specific regions:
    1. SBC-LODARK Aperture: A low-dark region (~390x390 pixels) in the lower-left corner.
    2. High-Dark Region: An elevated region (~750x750 pixels) in the center/top-right.

3. Key Contributions

• Enhanced Monitoring Protocol: Established a rigorous, high-frequency monitoring schedule (8 visits/year) to capture transient dark rate anomalies that annual monitoring would miss.
• SAA Hypothesis Testing: Systematically tested and refuted the hypothesis that proximity to the SAA boundary causes premature dark rate increases.
• Spatial Characterization: Validated the stability of the SBC-LODARK aperture as a distinct, reliable region for scientific observations, even during periods of global detector instability.
• Thermal Threshold Refinement: Confirmed that while 25.5°C is the standard threshold for high dark current, the onset of elevated rates can occur earlier and is not strictly linear with temperature in all cases.

4. Results

• Anomaly Frequency: Since 2018, five distinct "elevated" visits have been identified where dark rates increased exponentially before the detector reached 25.5°C.
• SAA Correlation: Analysis of HST orbital paths during both stable and elevated visits revealed no significant correlation between proximity to the SAA boundary and dark rate increases. The telescope's distance from the SAA was similar in both scenarios, and no decrease in dark rate was observed as the telescope moved away from the SAA.
• Temperature vs. Time:
  • Stable Visits: Dark rates remain low (~8.52 x 10⁻⁶ counts/s/pixel) until the temperature hits 25.5°C (approx. 2 hours after power-on), after which they rise rapidly.
  • Elevated Visits: Dark rates begin increasing either before or approximately one hour after power-on, independent of the specific "turn-on" time duration.
• Spatial Stability (SBC-LODARK):
  • The SBC-LODARK aperture (lower-left) maintains consistently low dark rates across all visits, including those where the rest of the detector is elevated.
  • Even during the most extreme elevated visit (2023-03-03), the dark rate in the LODARK region remained an order of magnitude lower than the overall detector average.
• Spatial Instability (High-Dark Region): The center/top-right region consistently exhibits high dark current, with elevated visits showing a sharp increase in dark rate starting around 22.5°C.

5. Significance and Recommendations

• Operational Guidance: The report confirms that the SBC-LODARK aperture is the optimal location for observing small sources. Observers are strongly advised to position targets within this region to avoid elevated dark noise, regardless of the global detector temperature or visit status.
• Calibration Pipeline: The findings reinforce the decision not to perform dark frame subtraction for MAMA detectors in the CALACS pipeline, as the spatial distribution of dark counts is random and frame-dependent.
• Instrument Health: The expanded monitoring program provides the data necessary to distinguish between thermal anomalies and other potential instrumental issues, ensuring that science data quality is maintained despite the detector's aging characteristics.
• Future Planning: The identification of the LODARK region as a stable "safe zone" allows for continued high-quality science operations with the SBC, even as the detector experiences increasing dark current in other areas.