The Burst Alert Telescope (BAT; Barthelmy, et al. 2005, SSRv, 120 143) is a highly sensitive, large FoV instrument designed to provide critical gamma-ray triggers and 4-arcminute positions. It is a coded aperture imaging instrument with a 1.4 steradian field-of-view (half coded). Figure 4 shows the field of view of the BAT. The energy range is 15-150 keV for imaging with a non-coded response up to approximately 500 keV. Within approximately the first ten seconds of detecting a burst, the BAT will calculate an initial position, decide whether the burst merits a spacecraft slew and, if worthy, send the position to the spacecraft.
In order to study bursts with a variety of intensities, durations, and temporal structures, the BAT has a large dynamical range and trigger capabilities. The BAT uses a two-dimensional coded aperture mask and a large area solid state detector array to detect weak bursts, and has a large FoV to detect a good fraction of bright bursts. Since the BAT coded aperture FoV always includes the XRT and UVOT FoVs, long duration gamma-ray emission from a burst can be studied simultaneously in the X-ray and UV/optical regimes. The data from the BAT will also produce a sensitive hard X-ray all-sky survey over the course of Swift's mission. Table 5 lists the BAT's parameters.