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The Advanced CCD Imaging Spectrometer (ACIS), formerly the AXAF CCD Imaging Spectrometer, is an instrument built by a team from the Massachusetts Institute of Technology's Center for Space Research and the Pennsylvania State University for the Chandra X-ray Observatory.

ACIS is a focal plane instrument that uses an array of charge-coupled devices. It serves as an X-ray integral field spectrograph for Chandra. The instrument is capable of measuring both the position and energy of incoming X-rays.^[1]

The CCD sensors of ACIS operate at −120 °C (−184 °F) and its filters at −60 and −50 °C (−76 and −58 °F). It carries a special heater that allows contamination from Chandra to be baked off; the spacecraft contains lubricants, and the ACIS design took this into account in order to clean its sensors. Contamination buildup can reduce the instrument's sensitivity.^[2] Radiation in space is another potential danger to the sensor.^[3]

As of 2014, after 15 years of operation, there was no indication of a limit to the lifetime of ACIS. Another design feature of the instrument was a calibration source that can be used to understand its health. This allows for a measurement of the level of contamination, if present, as well as any degree of charge transfer inefficiency.^[3]

• ACIS website Archived 10 October 2006 at the Wayback Machine by the Massachusetts Institute of Technology
• ACIS website by Pennsylvania State University
• The Chandra Proposers' Observatory Guide by the Smithsonian Astrophysical Observatory

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