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**Up:** Focal plane geometry
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Lets assume the trajectory of the emergent ray is described by the following equation:

(57) |

We will distinguish two cases:
**A: The CCD plane is orthogonal to the HRMA axis.** Let P
be the intersection of the ray with the CCD plane, and (X_{CCD},
Y_{CCD}, Z_{CCD}) its coordinates in the LSI system on the
CCD. The geometry is summarized in Figure 6.10.
By imposing that P lies
on the line, i.e., its coordinates satisfy eq. (6.3), we derive *t*:

(58) |

(59) |

(60) |

One needs to know *a priori* X_{CCD}, which is just the distance
of the CCD from the LSI coordinate origin (Fig. 6.10).

**B: The CCD plane is tilted with respect to the HRMA
axis.** In this case we will use the general expression of a point on
the CCD plane in LSI system, given by eq. (6.2), and impose
that the point
belongs to the ray trajectory, eq. (6.3). Figure 6.11
visualizes the situation.

By equating eqs. (6.2) and (6.3), we derive:

(61) |

We now take the dot product of both sides of the equation with the
normal of the CCD (i.e., we project the vector along the tilted X_{CCD} axis) and derive *t*:

(62) |

Indeed, as Figure 6.10 shows, in the case of no tilt, , , and , so that eq. (6.7) becomes eq. (6.4). To derive the coordinates of the point in the LSI system we project P on the CCD axes, i.e.,

(63) |

(64) |

The ASC program SAOSAC will provide the necessary information about the ray trajectory, specifically the three coordinates of and the direction cosines defining . Since these coordinates are in the XRCF system, we will need to do the appropriate transformation into LSI coordinates before using the above formulas.

We are now working on incorporating the above alghorithms in an IDL program. The output (a FITS file) will be used directly for the CCD simulator and, eventually, for comparison with the calibration data. We can use the complete simulation system (SAO-sac + ray projection + CCD simulator) to estimate the orientation of the ACIS focal plane in XRCF coordinates, by fixing the chip spacing and tilts and other measured geometric quantities of the instrument/FAM combination, then comparing the photon positions inferred from the simulation with actual XRCF data. We can also use this system to obtain better estimates of the spacing and tilt of individual chips in the ACIS focal plane, iterating between XRCF data and the models to improve our picture of ACIS geometry. Ultimately, we will use this exercise to guide us in designing the most effective on-orbit tests to fix the geometry of ACIS in the spacecraft coordinate system, to be performed in the early calibration phase of on-orbit operations.

11/20/1997