3.1.1 Imaging Point Selection

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:

Spectroscopic Array: S1

This imaging point is located about 6.25 mm from the launch-lock position. It is the position which causes the grating dispersed HRMA image to fall onto the ACIS-S array. It is the position of choice for spectroscopic grating support (if the gratings are inserted) or for narrow field imaging where absence of inter-chip gaps near the center of the field of view is important, or if symmetric PSF telescope response is required. The deliberations of the AXAF SWG concluded that the best compromise, most flexible instrument configuration would be the ACIS-S array. In case the SIM translation mechanism fails, AXAF shall be launched with the ACIS-S array at the HRMA focal point.

Imaging Array: IP1 & IP2

For wide field imaging the imaging point will be directed near the center of the ACIS-I array. Two symmetrically placed imaging points are placed on chips I1 (IP1) and I3 (IP2). The points are slightly displaced to avoid having the target image fall into the inter-chip gaps (see the description of the I Array in Section 2.3.2). IP1 and IP2 are intended to provide redundancy in case of failure of a single CCD chip or signal chain.

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.

Stow Position: HI & HS

When the HRC is in the focal plane ACIS will be placed in the `stow' position. In the HS position (when the HRC is positioned for the HRC-S detector) the calibration sources (see Section 2.3.5) illuminate the ACIS-I and S arrays. During this time the ACIS will not have access to the 24 kbs science telemetry stream and will only be able to use the 0.5 kbs `next-in-line' stream.

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 3.1.3.3, under Chip Readout Pattern, available options for restricting the number of CCD chips processed.)