The Evolution of the ACIS Contamination Layer on the Chandra X-ray Observatory from 2010 to 2026

Abstract: The Chandra X-ray Observatory (CXO) was launched over 27 years ago and has been delivering spectacular science over the course of its mission. The Advanced CCD Imaging Spectrometer (ACIS) is the prime instrument on the satellite, conducting over 90% of the observations. The CCDs operate at a temperature of $-$120$<sup>\circ$C and the optical blocking filter (OBF) in front of the CCDs is at a temperature of approximately $-$60$<sup>\circ$C. The surface of the OBF has accumulated a layer of contamination over the course of the mission, as it is the coldest surface exposed to the interior to the spacecraft. We have been characterizing the thickness, chemical composition, and spatial distribution of the contamination layer as a function of time over the mission. The contamination model has required several revisions over the course of the mission as the properties of the contamination layer have changed and our understanding of the layer has improved. In this paper, we evaluate the performance of the current contamination model (N0016 released in CalDB 4.12.3 on 16 December 2025) using the most recent calibration observations conducted from 2023 to 2026 by using the standard model spectrum for the supernova remnant 1E 0102.2-7219 (E0102) developed by the International Astronomical Consortium for High Energy (IACHEC), spectral data from the cluster of galaxies known as Abell 1795, and high resolution X-ray spectra of Mrk 421. This evaluation has been complicated by the decreasing observed counts at low energies, especially the O VII He$α$ line complex and the O VIII Ly$α$ line from E0102 which are no longer useful for this purpose. The analyses of the E0102, Abell 1795, and Mrk 421 data show that the current model of the contamination adequately predicts the additional absorption through mid-2026.