ACIS Update

John ZuHone, for the ACIS Team

The Advanced CCD Imaging Spectrometer (ACIS) is in good health and continues to produce superb science results after twenty-one years in orbit. All ten ACIS CCDs are performing well, and the electronics are functioning without any failures or degradations. All hardware continues operating on the primary sides with no redundancy failovers. ACIS continues to be the workhorse for Chandra science, being used in 90% of science observations.

This past year, the ACIS Operations Team has responded to the COVID-19 pandemic by carrying out operations almost entirely with work from home practice without any pandemic-related interruptions, with one notable exception. The ACIS engineering unit (EU), a hardware simulator of the in-orbit instrument, is used to produce and test science instrument modes (SIMODEs) and new flight software patches, and is physically housed at MIT. Beginning in mid-March 2020, MIT restricted access to campus buildings due to the pandemic. It was decided that powering down the EU was the safest long term unsupervised configuration. While the EU was switched off, a moratorium was placed by the ACIS operations team on the production of new SIMODEs (though twenty years after launch, the number of required new SI modes each cycle is low, usually less than 10). Power to the EU was restored in August 2020, and it has since been possible to build new SIMODEs for science observations. The ACIS instrument team has also been developing a software simulator which emulates all functions of the Digital Processor Assembly and the digital part of the Digital Electronics Assembly. The goal is to use this emulator in conjunction with the EU to test new patches and SIMODEs. In the future it could replace a non-functional EU or support testing of a damaged flight instrument when the damage cannot (or should not) be replicated on the EU hardware.

As Chandra ages and the reflective outer surfaces continue degrading, ACIS along with many other subsystems run hotter each year. Managing the thermal load to operate safely while preserving the Focal Plane (FP) temperature to within reliable and well-calibrated limits becomes more demanding with time. Accordingly, since Cycle 20, we require observers to select 4 or fewer required CCDs unless the science case strongly demands additional chips. Working with fewer CCDs generates less heat in ACIS, allowing for longer and cooler observations, also reducing the likelihood of observations being split into multiple pointings. Having observers identify only the requisite number of CCDs is very helpful in keeping the focal plane (FP) and electronics temperatures within the desired operating ranges for as long as possible.

When more CCDs are desired but not required, the observer indicates this by specifying optional chips. This designation allows Mission Planning to turn on or off optional chips in priority order as allowed by the thermal stresses throughout the orbit. The optional chips are numbered in order of increasing importance to the science (i.e., “OFF1” is the least important optional chip and will be the first chip to be dropped or equivalently, the last optional chip to be turned back on).

The orbit of the Chandra spacecraft is continually evolving from year to year, and currently the altitude of perigee is decreasing. In 2023, Chandra will reach a perigee minimum of ~1000 km. For context, Chandra launched with a perigee of ~10,000 km and the previous minimum in 2021 went down to ~3500 km. The solid angle of the Earth in the radiator FOV increases as perigee altitude decreases, resulting in a larger impact on the focal plane temperature. This effect, while it always has been present, will be particularly significant during the perigee minimum. Steps are being taken to model the expected increase in temperature during perigee and to mitigate any health and safety effects on the instrument, as well as ensure that the focal plane is cold enough by the time Chandra has cleared the radiation zone for ACIS to resume science.

The Chandra operations team continues to perform monthly verification of the hard-wired EEPROM version of the flight software onboard ACIS, as has been routine since 2016. There are no corruptions or errors in this memory after twenty-one years in orbit.

The contamination layer continues to accumulate on the ACIS optical blocking filters (OBFs) at a rate that has become approximately linear over the last several years and is approximately consistent between the ACIS-I and ACIS-S filters.

As discussed in the Calibration article, ACIS has been acquiring routine calibration data in the science orbit, including observations of the galaxy cluster Abell 1795, supernova remnants Cas A and E0102-72.3, and the (cold) Fe-55 External Calibration Source (as of July 2020).