2 SXC instrumentation

Each SXC module consists of a pair of large-format CCDs behind a finely-ruled one-dimensional coded aperture; two such modules, with orthogonal localization capabilities, comprise the full SXC complement. An optical "boresight'' CCD camera is comounted with each SXC module to provide an absolute reference system for detected X-ray transients. Details of the SXC X-ray and optical systems can be found in Table 1.

The mask elements (slits) are oriented parallel to the CCD columns, so the mask shadow patterns fall along CCD columns. The CCD is clocked in a fashion where every row of data read out is the sum of 100 rows on the detector (similar to "fast'' mode on ASCA): the CCD thus acts as a one-dimensional detector. The positions of X-ray sources in the field-of-view are calculated by cross-correlating the mask pattern with a histogram of the number of events detected in each CCD column. The angular size of one cross-correlation element is 33''.

The mask is an electroformed gold sheet 30 $\mu$m thick, capable of blocking 99$\%$ of 14 keV photons. The slits in the mask have widths which are integral multiples of 45 $\mu$m. The mask is covered by an optical blocking filter of 5000A of polyimide and 1500A of aluminum.

The CCDs are frontside-illuminated MIT/Lincoln Laboratory CCID-20 devices. These CCDs share heritage with the ACIS CCDs being flown on the Chandra X-ray Observatory: they have good sensitivity to 500 eV photons and have a demonstrated energy resolution of 150 eV at 5.9 keV when clocked in the manner described above. One of the two CCDs in each SXC module is covered with a 25 $\mu$m sheet of beryllium as a hedge against the possibility of damage by micrometeorites.

In order to achieve good localization precision, we have placed stringent requirements on the manufacture and stability of the SXCs. The key requirements on the SXC hardware, which must be met over the full range of SXC operating temperature, are 1) that the mask slits are parallel to each other to within 5 $\mu$m over the length of the mask, and 2) that the mask slits are parallel to the CCD columns to within 5 $\mu$m over the length of the mask (5 $\mu$m skew adds $\sim5''$ to the 1$\sigma$ uncertainty in the burst localization). Thermal and mechanical models show that the SXC hardware should be isothermal and sufficiently stable to meet these requirements, and tests with a prototype SXC have demonstrated the stability of the mechanical system over a wide range of temperatures.

  

Table 1: SXC instrumental characteristics