The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa

4 CHAPTER 2. BAT INSTRUMENT
Figure 2.1: Idealized view of the Swift optical bench, including a cut-away view of the Burst Alert
Telescope (BAT). The main BAT structures are the coded aperture mask (top, shown as a randomly
filled grid, and the detector array (bottom). The narrow field instruments are mounted to the side
of the BAT.
2.4 The BAT Instrument
The BAT instrument is shown in Figure 2.1. The Burst Alert Telescope (BAT) makes the initial
detection of the gamma-ray burst (GRB), calculates a position for that burst, makes an on-board
decision if the burst is worth an NFI follow-up observation, and sends that position to the spacecraft
attitude control system, if it is worthy. It does all this within 10-30 sec of the initial trigger of
the burst. To do this for a large number of bursts (∼100 yr-1), BAT has a large FOV (1.4 sr
half-coded & 2.2 sr partially-coded). The only way to image such a large FOV is to use the codedaperture
technique. The following sections describe the details of the design, the function of the
BAT instrument, and the data products that will be available to the world community.
2.4.1 Technical Description
The basic numbers describing the BAT instrument are listed in Table 2.1. The BAT instrument
consists of a detector plane of 32,768 CZT detector elements and front-end electronics, a coded
aperture mask located 1 m above the detector plane, a graded-Z fringe shield to reduce the instrumental
background event rate and cosmic diffuse background, and a thermal radiator and control
system to keep the detector plane at a constant temperature. The control of the BAT instrument
is done through the Image Processor and it also does the on-board event processing (burst trigger
detection, burst location calculations, and burst figure-of-merit calculation). While searching for
bursts, BAT also accumulates a hard x-ray survey of the entire sky over the course of the mission.
The energy range of 15-150 keV in Table 2.1 describes the energy range over which the effective
area is more than 50% of the peak value. The range is governed at the lower end by the electronic
discriminator threshold, and at the upper end by increasing transparency of the lead tiles in the