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The Chandra X-ray Observatory (CXO) was launched over 23 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°C and the optical blocking filter (OBF) in front of the CCDs is at a temperature of approximately −60°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. In this paper, we evaluate the performance of the current contamination model using the most recent calibration observations conducted in 2021 and 2022. 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. We show that the current calibration model underestimates the additional absorption of the contamination layer by using the standard model spectrum for the supernova remnant 1E 0102.2-7219 developed by the International Astronomical Consortium for High Energy Calibration (IACHEC), spectral data from the cluster of galaxies known as Abell 1795, and high resolution x-ray spectra of Mrk 421 and other active galaxies. The current model of the contamination layer under-estimates the optical depth by ∼17% at 0.66 keV and ∼10% at 1.49 keV. We suggest that the model may be improved with a change to the temporal component of the model only. This revised model is expected to be included in a future release of the CXO calibration database.
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