ACIS Update

Paul Plucinsky, for the ACIS Team

The Advanced CCD Imaging Spectrometer (ACIS) continues to operate nominally and produce high quality data after more than twenty-three years in orbit. All ten CCDs are fully functional and all electronics are nominal, operating on the primary units. ACIS flight software is operating well and has been patched multiple times since launch (including a patch in the last year), including bug fixes, improvements to on-board processing, and enhancements to the flight software capabilities. All indications are that ACIS will be capable of observing for many more years to come.

The ACIS flight software (SW) was patched on 11 August 2022 to include revisions to txings, the algorithm for the ACIS radiation monitoring process. Changes were necessary in response to the loss of the HRC anti-coincidence signal as a radiation monitor. The previous version of the txings algorithm was designed to trigger only on rapidly and monotonically increasing count rates. However, since the ACIS txings process is only sensitive when ACIS is collecting event data, if ACIS were to begin collecting event data after a radiation event arrived at Chandra, a radiation shutdown would not trigger. To provide more protection for ACIS, additional tests were added to the txings algorithm to trigger on high and roughly stable count rates and high but declining count rates. This new algorithm has been operational since its uplink in August 2022, but, as of the writing of this article, it has not triggered yet. The next Solar Cycle is ramping up with an expected peak in 2024 (or perhaps late 2023), and there have been three manual shutdowns in the past year due to high radiation. During a radiation shutdown, ACIS is moved out of the focal position of the High Resolution Mirror Assembly (HRMA)—such that damaging, low-energy protons cannot reach the CCDs—and radiation sensitive components in the electronics are powered down. Given the expected increase in Solar activity, we anticipate that the txings algorithm will trigger several times over the coming year, protecting ACIS from preventable radiation damage.

A consequence of using ACIS as the primary radiation monitor for Chandra is that at least one front-illuminated (FI) CCD must be active for every observation, since the txings algorithm is often significantly more sensitive for data from FI chips than from back-illuminated chips. Chandra General Observers (GOs) should be aware that they will be required to have at least one FI CCD on for their observations. The impact on the total count rate and minimum frametime with at least one FI CCD on is small; for example, the typical total background rate in the 0.3–12.0 keV band for all grades is ∼7.3 cts/s for a FI CCD compared to the telemetry limits of 68.8 and 170.2 cts/s in Very Faint and Faint modes, respectively. And the minimum frametime with no dead time increases from 0.4 s to 0.5 s for a 128 row subarray if two CCDs are used instead of one. As always, Chandra GOs are encouraged to discuss the configuration for their observations with their Chandra Uplink Support Scientist.

As a reminder, Chandra GOs are discouraged from using Very Faint mode with 5 or 6 CCDs due to the risk of saturating telemetry. The frequency of 5 and 6 CCD observations has been declining over the mission due to the thermal constraints on the Chandra spacecraft, ACIS electronics, and focal plane (FP) temperature. Nevertheless, ACIS does still execute some observations with 5 or 6 CCDs on; in such cases, Faint mode should be used instead of Very Faint mode. If the GO wishes to use Very Faint mode to possibly reduce the background, the observation should have 4 or fewer CCDs on. GOs can control which CCDs they want to be on during the observation by setting them to “Y” or “O#” in the OBSCAT.

Chandra GOs should also be aware that the ACIS FP temperature will be, on average, higher for their observations than in previous cycles. The CXC now identifies observations that do not require the best spectral response from ACIS and schedules those observations with predicted FP temperature values up to −109 °C. Observations that do require the best spectral response from ACIS are scheduled for predicted FP temperatures of −111 °C for ACIS-S observations and −112 °C for ACIS-I observations. These limits are the maximum temperature that the FP is predicted to reach during the observation; most of the observation will be executed at temperatures below these limits. This is a consequence of the fact that, depending on the Solar pitch angle of the spacecraft, the ACIS temperature is usually rising or falling and is never stable at one of these limits.

In the coming year, the ACIS flight SW team is preparing another set of patches. None of these patches are as high a priority as the revised txings algorithm was, but they will improve the behavior of the flight SW. The patches include: ensuring that a front-end processor (FEP) that has been powered down and back up again will compute a new bias map, providing additional diagnostic information in the case of a unexpected back-end processor (BEP) reboot, eliminating a possible infinite loop in the bias calculation for alternating exposure mode science runs, and adding a test of the video board power state to the existing test of the FEP power state to ensure that the flight SW reports that the science run has completed after unexpected power downs. The expectation is that these patches will be ready for uplink later this year (2023).

The ACIS operations team welcomed Jim Francis from MIT into the weekly rotation of scientists who ensure that ACIS is operated safely and efficiently. Jim wrote a significant fraction of the ACIS flight SW and is therefore intimately aware of how it works. Jim also wrote flight SW for the TESS mission and is part of their operations team. We are extremely fortunate to have someone with the knowledge and experience of Jim join the Operations team at this point in the mission. Welcome aboard, Jim!