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The Chandra X-ray Observatory was launched almost 19 years ago and has beendelivering spectacular science over the course of its mission. The Advanced CCDImager Spectrometer is the prime instrument on the satellite, conducting over90% of the observations. The CCDs operate at a temperature of -120 C and theoptical blocking filter in front of the CCDs is at a temperature ofapproximately -60C. The surface of the OBF has accumulated a layer ofcontamination over the course of the mission. We have been characterizing thethickness, chemical composition, and spatial distribution of the contaminationlayer as a function of time over the mission. All three have exhibitedsignificant changes with time. There has been a dramatic decrease in theaccumulation rate of the contaminant starting in 2017. The lower accumulationrate may be due to a decrease in the deposition rate or an increase in thevaporization rate or a combination of the two. We show that the currentcalibration file which models the additional absorption of the contaminationlayer is significantly overestimating that additional absorption by using thestandard model spectrum for the supernova remnant 1E 0102.2-7219 developed bythe International Astronomical Consortium for High Energy Calibration. Inaddition, spectral data from the cluster of galaxies known as Abell 1795 andthe Blazar Markarian 421 are used to generate a model of the absorptionproduced by the contamination layer. The Chandra X-ray Center calibration teamis preparing a revised calibration file that more accurately represents thecomplex time dependence of the accumulation rate, the spatial dependence, andthe chemical composition of the contaminant. Given the rapid changes in thecontamination layer over the past year, future calibration observations at ahigher cadence will be necessary to more accurately monitor such changes.

The Complicated Evolution of the ACIS Contamination Layer over the Mission Life of the Chandra X-ray Observatory