1.5 Instrument Overview

The AXAF (Advanced X-ray Astrophysics Facility) 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 AXAF satellite has centered on the extremely high spatial resolution of the incomparable X-ray telescope mirrors.

In order to utilize that resolution, high spatial resolution instruments are required, namely the ACIS (AXAF CCD Imaging Spectrometer) and the HRC (High Resolution Camera). ACIS offers superior high energy quantum efficiency and simultaneous imaging and spectroscopic capability, while the HRC offers low energy quantum efficiency, high time resolution and a wider field of view. The two instruments can also be used in conjunction with transmission gratings, with the ACIS primarily used with the HETG (High Energy Transmission Gratings) and METG (Medium Energy Transmission Gratings) and the HRC with the LETG (Low Energy Transmission Gratings).

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 picture elements or pixels). These pixels are 24 $\mu$m square with essentially no spacing or dead area between pixels. Two types of chip are used in ACIS. Both are fabricated at the MIT Lincoln Laboratory in Lexington, MA. One type is used with the gate structures, on the front of the CCD, facing the X-ray beam of the AXAF mirrors (hence `Frontside Illuminated' or FI). The other type has additional processing to remove material from the backside of the CCD, and it is deployed with the backside facing the X-ray beam (hence `Backside Illuminated' or BI). FI chips have superior energy resolution (especially at low energy) and somewhat better high energy quantum efficiency than BI chips. BI chips have far superior low energy quantum efficiency, but poorer cosmetic quality. Both types contain 1024x1024 pixels.

In most operating modes, each pixel of the CCD stores the signal generated in it by an incident X-ray for a period of time of order seconds. The electric charges accumulated in each CCD pixel are then transferred in less than 41 msec into a section of the CCD shielded from incident radiation, where they can then be 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. When this is true, the electronic signal is related to the energy of the X-ray with reasonably high accuracy ($E/\Delta E \sim 50$), and the instrument simultaneously functions as a high spatial resolution detector and moderate spectral resolution non-dispersive spectrometer. For higher flux sources, either special operating modes are required, or the analysis must allow for the possibility of multiple photons per pixel (``pile-up'').

When a transmission grating is introduced into the light path between the X-ray mirrors and the ACIS, the image of a source becomes dispersed. In this case even higher spectral resolution is possible ($E/\Delta E \sim 1000$).

Because the optimal focal surfaces for imaging and spectroscopy are considerably different, it is necessary to include separate CCD arrays optimized for each application. The ACIS-I array (2x2) offers the largest field of view with best imaging performance across the field. The ACIS-S array (6x1) follows the Rowland Circle defined by the gratings, and gives the best spectroscopic energy resolution. The center detector of the ACIS-S array is the position locked into the focal plane at launch (representing the best compromise instrument if the SIM translation should fail), and offers reduced field imaging without field losses due to intra-chip gaps near the desired target.

As a CCD does not produce a prompt signal when it is struck by an X-ray, but instead must wait for the CCD to be commanded to be read out, the timing resolution is limited by the time between successive readouts. This results in a typical time resolution on photon arrival times of 3.3 seconds. If better time resolution is required, however, ACIS can be operated in several special modes which reduce the time between readouts but also drastically reduce the field of view. See the discussion of observing modes in Section 3.1.

A more detailed overview describing the instrument can be found in Chapter 2.