2.3.2 Focal Plane Assembly

**Figure 2.11:** ACIS-I Array in Isometric Projection
$\begin{figure} \centering\leavevmode\epsfxsize=4truein \epsfbox{ACISplots/acis-i-3d.eps}\end{figure}$

**Figure 2.12:** ACIS-S Array in Isometric Projection
$\begin{figure} \centering\leavevmode\epsfxsize=4truein \epsfbox{ACISplots/acis-s-3d.eps}\end{figure}$

The CCD chips are arranged into two arrays, optimized for wide field imaging (ACIS-I) and spectroscopy (ACIS-S). Please see Figs. 2.11 and 2.12 to visualize the chip layouts. Note that only the active area of the chips are indicated. In addition to the active area each chip has a small border of inactive silicon around the edges, and an area roughly equal to the active area which contains the framestore portion of the chip. The framestore area projects outward from each of the chip areas shown. Please note that the vertical scale on the ACIS-S figure is greatly expanded. The ACIS-S array is nearly flat (less than 0.75 mm rise from the center of the array to the outer edges).

ACIS is brought into the on-axis focus of the HRMA by a one dimensional translation carried out by the SIM. This motion is parallel to the spacecraft Z-axis. Although the SIM can move to intermediate Z translations, the operational plan is to operate ACIS for observations of the sky at three positions, called `Aim Points'. Two aim points are chosen at symmetric points on the I array and labelled IP#1 and IP#2. IP#1 falls on the I1 chip; IP#2 on the I3 chip. The other aimpoint falls on the S3 chip, and is used for gratings observations and narrow field imaging.

Table 2.4 contains the corner coordinates of each of the ten CCD chips, given in millimeters, in the spacecraft coordinate system. The origin of the coordinate system is the on-axis focal point of the HRMA, when the SIM is located at IP#1.

**Table 2.4:** ACIS-I & S Chip Coordinates
4cTable 2.4.1. ACIS-I Chip Coordinates
Chip #	X	Y	Z
	(HRMA)	(Dispersion)	(SIM)
I0	1.130	-1.939	23.088
	2.361	-26.484	23.088
	1.130	-26.546	-1.458
	-0.100	-2.001	-1.458

I1	2.361	23.024	23.088
	1.130	-1.521	23.088
	-0.100	-1.459	-1.458
	1.130	23.086	-1.458

I2	-0.100	-2.001	-1.997
	1.130	-26.546	-1.997
	2.361	-26.484	-26.543
	1.130	-1.939	-26.543

I3	1.130	23.086	-1.997
	-0.100	-1.459	-1.997
	1.130	-1.521	-26.543
	2.361	23.024	-26.543

4cTable 2.4.2. ACIS-S Chip Coordinates
Chip #	X	Y	Z
	(HRMA)	(Dispersion)	(SIM)
S0	0.744	-81.051	-59.17
	0.353	-56.478	-59.17
	0.353	-56.478	-34.59
	0.744	-81.051	-34.59

S1	0.348	-56.047	-59.17
	0.099	-31.473	-59.17
	0.099	-31.473	-34.59
	0.348	-56.047	-34.59

S2	0.096	-31.042	-59.17
	-0.011	-6.466	-59.17
	-0.011	-6.466	-34.59
	0.096	-31.042	-34.59

S3	-0.011	-6.035	-59.17
	0.024	18.541	-59.17
	0.024	18.541	-34.59
	-0.011	-6.035	-34.59

S4	0.026	18.972	-59.17
	0.208	43.547	-59.17
	0.208	43.547	-34.59
	0.026	18.972	-34.59

S5	0.208	43.978	-59.17
	0.528	68.552	-59.17
	0.528	68.552	-34.59
	0.208	43.978	-34.59