English: Swift's instrument : Burst Alert Telescope (BAT) the Coded aperture mask Coded aperture is a mask positioned in front of the gamma-ray detectors. Swift's coded aperture mask sits 1-meter from the detector plane, and is made from about 54,000 lead tiles arranged in a random half-open/half-closed pattern. Each tile measures 5 x 5 x 1 mm. While the overall tile pattern is random, computers know the position of each tile. When gamma-rays come through the BAT, the lead tiles stop some, while the open areas let others through to the detector. In this way, the mask casts a shadow on the detector plane. Using the position of the shadow, the computers can determine the direction of the gamma-ray source.