Details of the detector assembly are shown in Figure 12.
The assembly is entirely fabricated from aluminum for low mass and low background, with the exception of the following items: CCD detectors (ceramic packages), thermal mounting blocks (niobium to match the thermal expansion coefficient of the CCD packages), indium foil (between the Nb mounting blocks and Al cold plate to provide good thermal contact), filters (nickel mesh), frame-store masks (Cu/Al), standoffs (G-10), and screws (stainless steel). The assembly holds the CCDs in close thermal contact against the TEC and supports the optical/UV filters, calibration sources, and frame-store masks.
In order to prevent photons from reaching the frame-store portion of the CCD, it is covered by a mask consisting of a 10 mil foil of 99.99% pure Cu epoxied to a 10 mil foil of 99.997% pure Al. The Cu foil provides a transmission of over the energy range of the instrument ( at 8.979 keV, just below the Cu K-edge). The Al foil, which is located on the side nearest the CCDs, absorbs any Cu-K or Cu-L fluorescent emission from the Cu foil. The transmission of this Al foil to Cu-L lines is about . The transmission to Cu-K is about 0.04, and the transmission to Cu-K is about 0.09. Since the photon flux at these high energies is small already, and since the fluorescent X-ray lines are emitted into 4 steradians, we do not expect the Cu-K fluorescence to be much of a problem. These Cu-K fluorescence lines can also produce Al-K fluorescence, but the rate is so small (only Cu-K lines absorbed in the last 1 mil of the Al foil have any possibility of getting to the CCD) that this effect should be negligible.
The 94 rows of the CCD adjacent to the unused serial register are masked off to permit measurement of the particle background rate and to provide room for Fe calibration sources (see §4.2.6). The open area of the CCD is about pixels (Y Z). This corresponds to linear dimensions of 1.129 cm 2.074 cm. The chips are centered on the aperture in the Z direction, but are offset in the Y direction, with the edges of the open area of each chip located and from the center of the aperture in the Y direction as shown in Figure 13.
Because the framestore mask and calibration mask are not perfectly aligned with the CCD rows, the open area of each chip is actually slightly skewed. The actual open areas, as measured during ground calibrations, are given in Table 4 in raw pixel units (RAWX, RAWY; see below).
Table 4: Open Area of CUBIC CCDs. Corner coordinates are (RAWX, RAWY).
The total open area of the instrument is 4.722 cm.
We have defined several coordinate systems for defining the positions of X-ray events. The raw detector coordinates are RAWX and RAWY: these count from pixel (1,1), which is the first guard pixel digitized, to pixel (832,512). In the CUBIC flight camera, pixel (1,1) is always the pixel closest to amplifier A3.
For compatibility with ASCA and ACIS coordinates, we define a tiled coordinate system, DETX and DETY, which provides unique coordinates to all pixels from both CCDs, as:
Finally, we define a ``focal plane'' coordinate system, FOCX and FOCY, which specifies the physical location of each pixel in the focal plane in mm, with the origin centered on the focal plane array. (This is a right-handed coordinate system looking down at the focal plane detectors, as shown in Figure 13.) The FOCX axis is parallel to the SAC-B +Y axis, and the FOCY axis is parallel to the SAC-B +Z axis. These coordinates are given by: