Advanced CCD Imaging Spectrometer (ACIS)
on the Chandra X-ray Observatory

ACIS, the Advanced CCD Imaging Spectrometer, is one of two focal plane instruments on NASA's Chandra X-ray Observatory. ACIS was built by a team from the MIT Kavli Institute for Astrophysics and Space Research, Lockheed Martin, and the Pennsylvania State University. It was launched into high Earth orbit on a space shuttle on July 23, 1999, and continues to operate to this day.

Our group at MKI helped develop, test, and calibrate the ACIS instrument and continues to monitor and pursue exciting science with it. MKI provides support to the Chandra mission through instrument calibration, software development, and observer support. This includes maintaining the on-board ACIS flight software and the suite of ACIS command generation scripts.

The ACIS focal plane utilizes 10 charge-coupled devices (CCDs), arranged into two arrays: a two by two ACIS-I imaging array tilted to match the mirror focal surface, and the six CCD ACIS-S spectroscopy array tilted to match the grating Rowland circle. In principle, any combination of up to six CCDs can be operated simultaneously. ACIS-I is used for a larger field of view, 17 arcmin square, and ACIS-S for reading out spectra from the High-Energy Transmission Gratings (HETG). In practice, the ACIS-S aimpoint CCD is often used for imaging of smaller fields.

The figures below show examples of monitoring we perform to understand variability in the particle background and its effect on science observations.

The high-energy reject rate on Chandra ACIS-S3 over the course of the mission. This metric traces variability in the particle background, which occurs on a range of timescales. The effects of the 11-year Solar cycle are clearly seen, as are smaller timescale changes due to solar storms and heliospheric changes.	Movie of full frames from the Chandra ACIS-I3 CCD. Each frame is a 3.3-sec integration and virtually all the visible signal is due to high-energy cosmic rays.