Gregg Germain, Catherine Grant, Paul Plucinsky, Jack Steiner, John Zuhone
The Advanced CCD Imaging Spectrometer (ACIS) is in good health and continues to produce superb science results after twenty years in orbit. All ten ACIS CCDs are performing well, and the electronics are functioning well without any failures or degradations. All hardware is still operated off the primary sides with no redundancy failovers. This year, the ACIS Operations Team navigated the transition of the Chandra OCC from its long residence on Hampshire Street in Cambridge, MA to its new location on Wayside Road in Burlington, MA, where operations are running smoothly. ACIS continues to be the workhorse for Chandra science, being used in 90% of science observations.
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 it. Working with fewer CCDs dissipates 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 FP and electronics temperatures within the desired operating ranges for as much time 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).
For the first time in six years and for the seventh time since launch, the ACIS flight software team uploaded a new set of patches to the instrument. This patch adds new safing capability to protect the ACIS instrument while also fixing a minor bug in the ACIS response to an unexpected shutdown. This software update was uploaded from the new OCC on January 23, 2020 and has been running successfully since that time. The updated software allows ACIS to monitor the temperatures of the electronics in the ACIS Digital Processing Assembly and to signal the Chandra on-board computer (OBC) if any temperatures exceed definable limits. A future patch to the OBC will power down ACIS components in response. Thermal modeling by the ACIS team and careful scheduling by Mission Planning should prevent the ACIS electronics from ever reaching the on-board limits, but this patch provides an extra level of protection.
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 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 originally announced in last year’s newsletter article, Richard (Dick) Edgar retired from the Chandra project in May 2019. Dick was a member of the Chandra team at SAO for over twenty years and made numerous contributions to the project in a variety of roles ranging from pre-flight calibration of the HRMA, to post-flight calibration of ACIS and finally to ACIS operations. Dick still chimes in every now and then with helpful reminders and comments. We thank Dick for his many years of service and we will miss him greatly. The ACIS Operations Team welcomed James (Jack) Steiner to the team in April. Jack is no stranger to CfA having been a graduate student at Harvard and Fellow at CfA. He has also held positions at the University of Cambridge and the Massachusetts Institute of Technology. Jack is an expert on X-ray binaries that harbor a black hole. His expertise is greatly appreciated since these sources can reach high flux levels that present significant challenges for observations with Chandra. Jack is rapidly gaining experience with the details of ACIS operations and will be taking a leading role in some of the new initiatives in the coming year. We are delighted to have Jack as a member of the team.