The ACIS focal plane contains ten MIT Lincoln Laboratory CCID17 detectors. The CCID17 is a three-phase, frame transfer imager with 1026 rows of 1024 columns in each of the imaging and framestore areas. The pixels in the imaging area are 24 microns square. The device is served by four output nodes which are (usually) operated in parallel. ACIS flight detectors are fabricated of high-resistivity silicon () to maximize depletion depth and, therefore, high-energy X-ray detection efficiency. Flight CCDs have been fabricated in both front-illuminated and back-illuminated configurations. In this paper, we confine our discussion to the front-illuminated devices.
The engineering model ACIS focal plane is shown in Figure 1. The focal plane consists of ten detectors, organized into two arrays. The devices in the 2-chip-by-2 chip ACIS-I array are intended primarily for high-angular resolution, wide-field imaging, and are therefore tilted to conform as nearly as possible to the highly-curved focal surface of the AXAF High-Resolution Mirror Assembly. The linear, 6-chip-long ACIS-S array is intended primarily for use with the AXAF High-energy Transmission Gratings (HETG) . The ACIS-S detectors are oriented to conform to the Rowland circle of the HETG. The gaps between adjacent CCDs are typically 300 microns wide, and are in no case more than 500 microns wide. In orbit, the focal plane is radiatively cooled to a temperature of -120 C. This relatively low temperature minimizes the sensitivity of the detectors to damage from energetic protons encountered on-orbit.
Figure 1: The ACIS CCD focal plane. The ten CCD detectors are arranged in two subarrays. The 2-by-2 detector subarray supports wide-field imaging. The 1-by-6 detector subarray supports transmission grating spectroscopy and narrow-field imaging. The engineering model is shown here; the flight detector configuration is identical.
The ACIS focal plane is served by a low-noise ( RMS) detector electronics assembly which contains a dedicated signal chain for each of the 40 CCD output nodes in the focal plane. Each signal chain operates at a rate of 100 kpixels s, allowing a full detector readout in less than 3 seconds. The clock sequences produced by the detector electronics are programmable, so a variety of CCD readout modes are available.
Raw digital data from the detector electronics passes to a high-throughput digital processing assembly which can service as many of six operating CCDs simultaneously. The digital processing hardware and software determine the CCD bias (energy zero-point) level on a pixel by pixel basis, extract X-ray event information from the incoming data stream, and format and buffer that data for telemetry to the ground.
ACIS requires optical blocking filters to reject out-of-band radiation (mainly optical and UV light). Calibration of these filters, which is a critical part of the ACIS calibration program, is described elsewhere. [9,10]