2.1 Overall Description

ACIS is the AXAF CCD Imaging Spectrometer. It is comprised of two CCD arrays, one for imaging, the other for grating spectroscopy. Each CCD in each array is tipped to approximate the relevant focal surface. In conjunction with the High Resolution Mirror Assembly (HRMA), the ACIS imaging array will provide simultaneous time-resolved imaging and spectroscopy. With the High Energy Transmission Gratings (HETG) and the Low Energy Transmission Gratings (LETG), the ACIS spectroscopic array will acquire high resolution (up to $E/\Delta E = 1000$) spectra of point sources. The bare CCD arrays themselves have an intrinsic energy resolution of about $E/\Delta E = 50$ at 6 keV. The high quantum efficiency of the CCDs enables spectroscopy with much better counting statistics than can be achieved with other X-ray detectors.

 

Figure 2.1:   Schematic flow of information through the ACIS experiment.

Fig. 2.1 illustrates the schematic flow of X-ray signals from the incident X-radiation through to the telemetry of science data to the ground. The incident X-rays are collected by the HRMA and focused onto the CCD chips after passing through a UV/optical light blocking filter. In the CCD chip, the X-ray is converted into an analog charge signal which is processed by the DEA (Detector Electronics Assembly) signal chains, resulting in a digital number proportional to the charge induced in each pixel. These digital signals are further processed by the DPA (Digital Processing Assembly), which, depending on the mode of the instrument, selects the most important signals for telemetry to the ground. The DPA provides these science data and engineering housekeeping data to the RCTU (Remote Command and Telemetry Unit), which interfaces the experiment to the rest of the spacecraft, eventually resulting in telemetry of these data to the ground for scientific and engineering analysis.

From an organizational point of view, the ACIS experiment is divided into two parts: the Detector & Processor Subsystem (DPS), and the Power & Thermal-Control Subsystem (PTS) developed at LMA. The DPS consists of the Detector Assembly (DA), DEA and DPA. The PTS consists of the Power Supply and Mechanism Controller (PSMC) and the Thermal Control System.

 

Figure 2.2:   ACIS Detector Assembly Layout

The entire ACIS experiment is mounted on the Science Instrument Module (SIM) which translates in the spacecraft Z direction to allow selection of ACIS aim point, or to allow ACIS to move to a stow position (while the HRC is collecting data). A side view of the DA is shown in Figure 2.2.

The functional duties of each DPS subassembly are as follows: The DA includes the CCD arrays, fiducial lights and an X-ray calibration source used primarily for ground test. The DA also includes the mechanical structures of the detector housings, focal plane support structures, mechanical door, radiation shielding and long collimator/stray light baffle. Signals from the CCDs are amplified and digitized in the DEA. The DPA controls instrument operation, extracts valid X-ray events and processes data for downlink. The TRW Government-furnished Remote Command & Telemetry Unit (RCTU) provides the interface between the Observatory and the ACIS experiment for transmission of commands and data.

The PTS subassembly functions are: The PSMC supplies conditioned power to all assemblies. The Thermal Control System provides thermal control for the DA (via warm and cold radiators) to allow operation of the CCD chips at a nominal temperature of - 120 C. Heaters provided by the SIM will maintain minimum temperature levels on ACIS electronic assemblies.

The ACIS experiment contains two ancillary subsystems. These are the Calibration Sources installed at the SIM stow position to allow calibration activities in-flight while the ACIS is not in the focal spot of the HRMA, and the Door Assembly, which is expected to open just once, and remain in the open position for the rest of the spacecraft lifetime. (If necessary the Door Assembly can be closed and reopened.)