2.3.2.4 S Array Imaging Performance

In addition to its utility as a readout device for grating dispersed spectra, the S array has important, complementary imaging functions. As previously explained the ideal grating image resolution occurs on the Rowland Circle of the grating, which is far less steeply curved than the optimum imaging surface. Nevertheless for targets close to the HRMA on-axis image the S array has just as good imaging properties as the I array. The imaging properties of the S array are shown in Fig. 2.15.

Possible reasons to use the S array for imaging include:

• The seams between chips in the I array are closer to the on-axis target point, so if this cosmetic effect is undesired or a tight cluster of sources are observed the S array may be preferable.
• The S array chips are more nearly flat and the characterization of the PSF across the array may be simpler and more accurate than the I array. Astrometric accuracy might be better for this array for the same reason.
• The S array contains the only Back Illuminated (BI) CCD chips. The BI chip located at S3 provides a nearly flat (i.e. orthogonal to the incident HRMA beam) detector for imaging studies. The advantage of using a BI chip is a roughly two times higher counting rate for most sources, and even more for soft spectrum sources. The other BI chip is located at the S1 position, and is not expected to be used for direct imaging (but does provide three times higher effciciency for the important O K$\alpha$ line). Note, however, that the BI chips are less uniform, have significant charge transfer inefficiency, and have worse intrinsic energy resolution than the Frontside Illuminated (FI) chips.
• The S3 chip is the AXAF Launch-Lock detector. If the SIM should fail on launch the S array may be forced to perform both roles of imaging and grating spectroscopy.