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| Chandra ACIS | |
|---|---|
| Name | Advanced CCD Imaging Spectrometer |
| Mission | Chandra X-ray Observatory |
| Operator | NASA/Smithsonian Astrophysical Observatory |
| Launch | STS-93 (1999) |
| Type | X-ray imaging spectrometer |
| Instruments | CCD arrays |
| Wavelength | X-ray (0.2–10 keV) |
Chandra ACIS
The Advanced CCD Imaging Spectrometer (ACIS) is a focal-plane instrument aboard the Chandra X-ray Observatory built to produce high-resolution imaging and moderate-resolution spectroscopy of astrophysical X-ray sources. Operated by teams at the Smithsonian Astrophysical Observatory, Massachusetts Institute of Technology, and partners including Lockheed Martin and NASA Goddard Space Flight Center, ACIS has observed targets ranging from the Crab Nebula to distant quasars and galaxy clusters. ACIS data underpin studies conducted by researchers at institutions such as Harvard University, Caltech, MIT Kavli Institute, and the European Space Agency community.
ACIS comprises two arrays of charge-coupled device detectors developed by the Pennsylvania State University and the Massachusetts Institute of Technology Lincoln Laboratory, designed to work with the Chandra X-ray Observatory optics delivered by prime contractor TRW Inc. and later supported by Raytheon. ACIS provides imaging comparable to observations by instruments like Hubble Space Telescope counterparts at X-ray wavelengths, enabling analysis of sources cataloged by surveys including the ROSAT All-Sky Survey, Sloan Digital Sky Survey, and follow-ups to discoveries from the Fermi Gamma-ray Space Telescope and INTEGRAL missions. ACIS operations are integrated with mission planning at facilities such as the Chandra X-ray Center and science teams at the Harvard-Smithsonian Center for Astrophysics.
ACIS contains two primary focal-plane arrays: the imaging array (ACIS-I) and the spectroscopy array (ACIS-S), each composed of multiple CCD chips manufactured using techniques developed by Janesville Semiconductor and test facilities at MIT Lincoln Laboratory. The arrays are mounted on a cold finger connected to a radiator and cryostat designed with heritage from missions like ASCA and XMM-Newton. Electronics for readout and processing were supplied by contractors including Ball Aerospace and Northrop Grumman, with flight software contributions from teams at SAO and MIT. ACIS uses an optical blocking filter, mechanical shutters, and graded-Z shielding similar to designs used in Suzaku and Swift instruments, while relying on calibration sources such as the on-board radioisotope sources and ground reference detectors at NASA Goddard and Lawrence Berkeley National Laboratory.
ACIS delivers angular resolution matched to Chandra's mirrors, providing on-axis point-spread functions comparable to the imaging achieved by Hubble Space Telescope instruments in different bands. Spectral resolution of the CCDs allows line studies of elements observed in remnants like the Cassiopeia A and Tycho remnants, and in hot plasmas of clusters such as Perseus Cluster and Coma Cluster. Calibration campaigns coordinated with International Astronomical Union working groups, the High Energy Astrophysics Science Archive Research Center, and laboratories at Los Alamos National Laboratory established response matrices and gain tables used by analysis tools like XSPEC and CIAO. Regular cross-calibration efforts compared ACIS performance with instruments on XMM-Newton, Suzaku, and NuSTAR.
ACIS supports timed exposure and continuous clocking modes enabling studies of fast transients such as emissions from Pulsar Wind Nebulae like the Crab Nebula and accreting systems including Cygnus X-1, Vela X-1, and GRO J1655-40. Observing programs coordinated through the Chandra X-ray Center and peer-reviewed proposals from investigators at Space Telescope Science Institute, European Southern Observatory, and national observatories allow multiwavelength campaigns with facilities like ALMA, VLA, Keck Observatory, and Very Large Telescope. ACIS telemetry and data products are processed into FITS files used by analysts at institutions such as Caltech, University of Cambridge, Max Planck Institute for Astrophysics, and archived in the Chandra Data Archive.
ACIS observations contributed to mapping shocks and element distribution in supernova remnants including SN 1987A, identifying cavities and bubbles in galaxy clusters such as the Perseus Cluster linked to feedback from M87, and resolving X-ray jets from active galactic nuclei like Centaurus A and 3C 273. ACIS enabled measurements of black hole spin and accretion physics in sources including NGC 1365 and constrained dark matter and cosmological parameters through cluster temperature profiles in samples including the Bullet Cluster. Studies by teams at Princeton University, University of Chicago, and Columbia University used ACIS spectra to detect chemical abundances revealing nucleosynthesis signatures tied to events like Type Ia supernovae and core-collapse supernovae.
ACIS performance has been affected by radiation damage from charged particles encountered in Earth’s magnetosphere, particularly during passages through the Van Allen radiation belts and solar particle events associated with Solar Cycle activity. Charge transfer inefficiency increased over time, necessitating correction algorithms developed by engineers at MIT and calibration scientists at SAO and NASA Ames Research Center. Thermal control degradations and contamination of the optical blocking filter produced low-energy response changes, prompting corrective strategies and updates from teams at Jet Propulsion Laboratory and Goddard Space Flight Center. Single-event upsets in electronics mirrored issues seen on missions like Hubble Space Telescope and required mitigation by operations teams at Lockheed Martin and Chandra X-ray Center.
The ACIS project evolved from proposals in the 1980s funded by NASA and shaped by contributions from university groups at Penn State, MIT, and industrial partners including Raytheon and Lockheed Martin. Flight hardware underwent environmental testing at facilities like NASA Johnson Space Center and integration with the Chandra X-ray Observatory in campaigns led by TRW Inc. before launch on STS-93 with Eileen Collins commanding the mission. Post-launch adjustments, software updates, and periodic recalibration activities have been coordinated by the Chandra X-ray Center, with science legacy programs led by personnel at Harvard-Smithsonian Center for Astrophysics and international collaborators including researchers at Max Planck Institute for Extraterrestrial Physics and University of Tokyo.
Category:X-ray astronomy instruments