Penn State Dept. of Astronomy & Astrophysics


Advanced X-ray Astrophysics Facility (Chandra)
CCD Imaging Spectrometer (ACIS)

(by John Nousek)


Penn State is a leader in developing and flying detectors of astronomical X-radiation. The cornerstone activity is the program building a CCD Imaging Spectrometer for Chandra, a billion dollar class satellite to be launched by NASA in August of 1998. Evan Pugh Professor Gordon P. Garmire is in charge of the collaboration building this instrument, which includes scientists and engineers at MIT, the Jet Propulsion Laboratory, and Lockheed Martin Aerospace.

The Chandra X-ray Observatory promises to be the premier X-ray observatory of the 1990's. As a result of major redesigns forced by economic and political considerations the mission of the Chandra satellite has centered on the extremely high spatial resolution of the incomparable X-ray telescope mirrors. The High Resolution Mirror Assembly (HRMA) is being characterized by the Chandra Mission Support Team.

[ACIS Focal Plane] The critical active element in ACIS is the CCD chip which converts incident X-ray events into electronic signals which record the location of the event and its energy. The spatial resolution of the CCD is limited by the physical dimensions of the discrete charge collecting locations (called pixels). These pixels are 24 microns square with no spacing or dead area between pixels. The ACIS instrument has four primary imaging chips and 6 spectroscopic chips as shown here (200 Kbytes).

In most operating modes the CCDs integrate the X-ray generated electron signal for a period of time of order seconds. The CCD pixel charges are then transferred very rapidly into a section of the CCD shielded from incident radiation, where they can be slowly read out and the signals processed. In most applications the incident X-ray flux will be low enough that only a single X-ray will be detected for every 100 or more pixels sampled. If this is true then the electronic signal is related to the energy of the X-ray with reasonably high accuracy ((E/delta E) ~50), and the instrument simultaneously functions as a high spatial resolution detector and moderate spectral resolution non-dispersive spectrometer.

When a transmission grating is introduced into the light path between the X-ray mirrors and ACIS the image of sources becomes dispersed. In this case even higher spectral resolution is possible ((E/delta E) ~500).

Because the optimal focal surfaces for imaging and spectroscopy are sufficiently different it is necessary to include separate CCD arrays optimized for each application. The ACIS-I array offers the largest field of view with best imaging performance across the field. The ACIS-S array follows the Rowland Circle defined by the gratings, and gives the best spectroscopic energy resolution. The ACIS-S array also serves as the detector locked into the focal plane at launch (representing the best compromise instrument if the SIM translation should fail), and offers narrow field imaging without field losses due to intra-chip gaps near to the desired target.

ACIS Optical/UV Filters The ACIS CCDs are sensitive to optical and ultraviolet photons as well as x-ray photons. As such, an optical/UV blocking filter must be placed in front of the CCDs. Leisa Townsley is characterizing the spatial properties of the ACIS Optical/UV Blocking filters, while George Chartas is characterizing the spectral properties of the ACIS Optical/UV Blocking filters.

Integrated Chandra calibration (HRMA, gratings, and flight detectors) was performed  at the X-ray Calibration Facility (XRCF) in Winter 1996 and Spring 1997.  To learn more about ACIS calibration at XRCF, see our XRCF Calibration Web Page. We have compiled a list of Chandra Links which might also be of interest.

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Penn State's High Energy Astrophysics group web pages are maintained by Scott Koch. Last update: July 19, 1999