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A critical decision which must be made for each observation is to choose where the on-axis HRMA image will be placed. As the ACIS instrument is located on a translating structure called the SIM, there is one degree of freedom of motion.
Five different SIM positions are planned for normal operations. Each is characterized by the intersection point of the HRMA axis on the ACIS focal plane arrays, called the imaging point. These positions are:
IP2 is slightly preferable to IP1 and was principally used as the on-axis point during XRCF calibrations. Although the I array is equally symmetric about the IP1 and IP2 points, the S array is closer to IP2 than IP1, and so serendipitous imaging on S2 and S3 will be better for IP2.
Depending on the HRC operations policy there may be more HRC imaging points. In this notation we call HI the HRC prime imaging point and HS the spectroscopic imaging point. The HRC position used is not expected to have any significant impact on ACIS performance.
In the HI and HS positions the specific array which is to be analyzed must be specified (see following discussion of electronic array selection).
The ACIS on-board electronics consists of ten signal chains. Each signal chain connects to one chip, but the selection of array imaging point generally determines which signal chains are active. For the S1 imaging point, chips S0-S5 are used. For the IP1 and IP2 imaging points, chips I0-I3 and S2-S3 are used. For the HI and HS points, the choice of S0-S5 or I0-I3 and S2-S3 must be made depending on which array is intended to be calibrated. (Note that the selection of a set of chips does not necessarily require that data be processed from the entire set. See Section 188.8.131.52, under Chip Readout Pattern, available options for restricting the number of CCD chips processed.)