Catherine Grant, for the ACIS Team
The Advanced CCD Imaging Spectrometer (ACIS) continues to operate nominally and produce high quality data after more than twenty-two 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 with bug fixes, improvements to on-board processing, and enhancements to its capabilities. All indications are that ACIS will continue observing for many more years to come.
The loss of HRC data to on-board radiation monitoring leaves the ACIS txingsalgorithm as the sole input to the autonomous radiation safing system on Chandra. The txings patch to the ACIS flight software was installed in 2011; it examines threshold crossing rates in the ACIS CCDs and signals the On-Board Computer (OBC) if the rates are above pre-set triggers levels and monotonically increasing. The OBC can then take action to protect the ACIS CCDs by moving ACIS out of the focal plane and turning off the radiation-sensitive video boards.
ACIS and the txings patch have limitations compared to a dedicated particle instrument, like EPHIN, the original Chandra particle detector, or the HRC anti-coincidence shield. The algorithm runs parasitically while ACIS is taking event data, not during biases or slews, and therefore includes signal from both astrophysical X-rays as well as particle radiation. Given these known limitations, it was decided after the HRC-B anomaly to impose additional radiation conservatism, at least in the short term. An update to the txings patch is also currently in the review stages, which makes it more sensitive to radiation and does not solely rely on a monotonically increasing profile. We aim to upload the updated patch sometime this summer.
As Chandra ages and the thermal insulation on the spacecraft continues to degrade, the ACIS thermal environment becomes more challenging. In addition to the long-term steady increases to all spacecraft subsystems, Chandra’s evolving orbit will reach a mission low perigee in 2023, increasing the solid angle of the Earth on the ACIS radiator which adds extra heat onto the focal plane during perigee passages. ACIS focal plane temperatures will continue to vary throughout observations and the current maximum temperatures may be raised to accommodate temperature limits from other systems and to retain overall observational efficiency. The calibration team is working to improve temperature-dependent calibration and is making plans for dedicated observations at temperatures above the current limits. Careful modeling and planning of temperatures will keep them below the limits established to maintain CCD health and prevent additional increase to charge transfer inefficiency.
As the temperatures of the ACIS electronics and the focal plane are dependent on many factors, including the number of CCDs in use, observers have been asked to thoughtfully choose which CCDs are required to accomplish the desired science and which CCDs are optional or unnecessary. After the HRC anomalies, an additional request has been to require one front-illuminated (FI) CCD during all observations, as particles produce a stronger signal in the FI CCDs. This improves the sensitivity of the ACIS radiation monitor. Observers that previously used only the BI CCD ACIS-S3 have been requested to add S2, an FI CCD, as a required CCD. For programs where pile-up is an issue and the additional CCD may have implications for the frame time, observers should discuss with their USINT contact how best to maintain high quality science while also protecting the instrument.
Finally, the contamination layer on the ACIS Optical Blocking Filter has continued to accumulate at a roughly linear rate since 2014. This degrades the low energy response and requires careful dedicated calibration observations, but has little effect above 2 keV. The calibration team is analyzing the recent observations used to characterize the accumulation rate of the contamination and may release an updated calibration file later this year, if warranted.