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We first summarize the coordinate systems used for formulating the ray tracing problem. For a more comprehensive description of the various coordinate systems of the mission, we refer to the ``ASC Coordinate System'' . The coordinate systems we will use below are:

- Chip Physical Coordinates (CPC), which give the physical
position of an event on the active surface of the CCD: X
_{CPC}, Y_{CPC}, Z_{CPC}, in mm. The Y_{CPC}, Z_{CPC}axes describe the plane of the CCD, with origin in its lower-left corner. The X_{CPC}axis completes a right-handed set; - Local Science Instrument (LSI). This system is fixed for each instrument in the SIM. The origin is in the instrument, the +X axis runs toward the mirror aperture, the +Z axis coincides with the upward translation direction of the instrument table. The Y axis completes a right-handed system.

Note that, since the output from SAOSAC is provided in the XRCF coordinate system, a transformation of the event coordinates into LSI will be necessary.

While the LSI is unique for the instrument, every CCD in both ACIS-I
and ACIS-S has its own CPC system, where the plane of the CCD defines
the (Y_{CPC}, Z_{CPC}) plane. This is illustrated in Figure 3 of
the ``ASC Coordinate
System''
. To
transform from CPC to LSI we need a rotation and a translation, so
that a generic point ** r** on the plane is described in the LSI
system by:

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11/20/1997