| BAT is a coded mask detector and consists of 32,768 pieces of 4×4×2 mm CdZnTe (CZT) form a 1.2×0.6 m sensitive area in the detector plane. Groups of 128 detector elements are assembled into 8×16 arrays, each connected to 128-channel readout Application Specific Integrated Circuits (ASICs). Detector modules, each containing two such arrays, are further grouped by eights into blocks. This hierarchical structure, along with the forgiving nature of the coded aperture technique, means that the BAT can tolerate the loss of individual pixels, individual detector modules, and even whole blocks without losing the ability to detect bursts and determine locations. The CZT array has a nominal operating temperature of 20 degrees C, and its thermal gradients are kept to within ±1° C.
BAT has a D-shaped coded aperture mask, made of ∼54,000 lead tiles (5×5×1 mm) mounted on a 5 cm thick composite honeycomb panel, which is mounted by composite fiber struts 1 meter above the detector plane. Because the large FOV requires the aperture to be much larger than the detector plane and the detector plane is not uniform due to gaps between the detector modules, the BAT coded-aperture uses a completely random, 50% open-50% closed pattern, rather than the commonly used Uniformly Redundant Array pattern. The mask has an area of 2.7 m2 |
| The XRT uses a grazing incidence Wolter 1 telescope to focus X-rays onto a state-of-the-art CCD.
The complete mirror module for the XRT consists of the X-ray mirrors, thermal baffle, a mirror collar, and an electron deflector. The X-ray mirrors are the FM3 units built, qualified and calibrated as flight spares for the JET-X instrument on the Spectrum X-Gamma mission. To prevent on-orbit degradation of the mirror module’s performance, it is be maintained at 20° ±5° C, with gradients of <1° C by an actively controlled thermal baffle. A composite telescope tube holds the focal plane camera, containing a single CCD-22 detector.
The CCD-22 detector, designed for the EPIC MOS instruments on the XMM-Newton mission, is a three-phase frame-transfer device, using high resistivity silicon and an open-electrode structure.
The CCD consists of an image area with 600×602 pixels (40×40 microns) and a storage region of 600×602 pixels. A special “open-gate” electrode structure gives the CCD-22 excellent low energy quantum efficiency (QE) while high resistivity silicon provides a depletion depth of 30 to 35 microns to give good QE at high energies. The detectors operate at approximately -100° C to ensure low dark current and to reduce the CCD’s sensitivity to irradiation by protons. |
| The UVOT optics consists of a 30 cm diameter modified Ritchey-Chrétien telescope with an f/2.0 primary that is re-imaged to f/13 by the secondary. The filter wheel includes a 4-times magnifier that results in 0.13 arcsec pixels for near diffraction limited imaging. The optics used for Swift/UVOT are the flight spares from the XMM-Newton/Optical Monitor (OM). The telescope structure, baffle, and thermal designs are also from the XMM-Newton project.
The detectors are copies of two micro-channel plate intensified CCD (MIC) detectors from the XMM-Newton/OM design. They are photon counting devices capable of detecting very low signal levels, allowing the UVOT to detect faint objects. The design is able to operate in a photon counting mode, unaffected by CCD read noise and cosmic ray events on the CCD. The UVOT can autonomously determine the spacecraft drift using guide stars in the FOV. The UVOT design includes the XMM-Newton/OM 11 position filter wheel in front of the detectors. The two grisms can be used to obtain low resolution spectra of bursts brighter than approximately mb > 15. |
