2 Instruments

  

Figure 1: The fan-beamed field of views of MAXI which scan the entire sky in one orbit

The FOV of MAXI slit cameras is defined by the combination of slat collimator and a slit aperture. The slat collimator defines a narrow and long field of view (1.5$^\circ$  FWHM $\times 90^\circ$ for a single collimator). The 1-D position-sensitive X-ray detectors are employed to map the X-ray sky within the narrow fan-beam FOV. Three collimated detectors are combined together to cover a half circle FOV (1.5$^\circ \times 180^\circ$) simultaneously. The ISS will rotate synchronously with its orbit so that one side will always points towards the center of the Earth and the opposite side will see the sky all the time. Figure 1 shows how it scans across the sky in the course of an orbit. Since observations are not possible at particular regions of the orbit with high charged particle flux, two fan beams are used to simultaneously cover two perpendicular fields of view, one centered on the zenith ("zenith view''), and the other centered at the forward direction of motion of the space station ("horizon view''). This will enable MAXI to achieve a more uniform coverage of the sky. To obtain the maximum scientific outputs within the constraints of the platform (JEM-EF), we designed the layout of instruments on MAXI as shown in Fig. 2. The size is 1.85 m (L) $\times$ 0.8 m (W) $\times$ 1.0 m (H) and the mass is about 500 kg. It is equipped with two scientific instruments, GSC (Gas Slit Cameras) and SSC (Solid-state Slit Cameras) (Mihara et al. 1999).

  

Figure 2: A layout of the instruments

GSC consists of an 1-D position sensitive proportional counter with a slat collimator and a slit on top. The proportional counter is based on the WXM/HETE2 counters with the gas of 1.4 atm composed of 97% Xe and 3% CO₂. It is sensitive to X-rays in 2-30 keV. Three identical GSC detectors are combined to cover a fan-beam field of view with 180$^\circ$ $\times$ 1.5$^\circ$.There are four such detector sets, of which two cover the zenith view, and the other two cover the horizon view. The combined geometrical area is $\approx 6000$ cm² with the angular resolution of better than 1.5$^\circ$ for 3-8 keV photons. SSC uses X-ray CCD to achieve soft X-ray response. The CCD chips will be developed by Osaka University and Hamamatsu Photonics. It is the full-frame transfer type, with pixel size of 25 $\mu$m square, and 1024 $\times$ 1024 pixels per chip. The energy range is 0.5-10 keV. Each CCD chip is employed as one-dimensional position sensitive detector with the fast readout. In the present design, we use 32 chips in total with an effective area of 200 cm².