Figure 3.15 shows the actual location of the pick-off mirrors that feed the science instruments as viewed from above (looking down the boresight). The Y- and Z-axis directions are the same as the telescope coordinate system shown in Figure 3.4; the +X direction comes out of the page. Figure 3.15 shows the Spitzer entrance apertures as projected onto the sky and appears inverted compared to Figure 3.1, due to the combined effects of looking out from behind the focal plane and the projection of the sky onto the focal plane through the telescope optics. (To understand this inversion, recall the 3-D geometry and the fact that a Cassegrain telescope inverts its image.)
Figure 3.15: Schematic view of Spitzer focal plane from above, looking down the boresight. The solar panel is on the IRS side of the spacecraft. This figure shows the region of the focal surface where the pick-off mirrors for each instrument are located. This is in contrast to where the apertures project onto the sky. See Figure 3.1 for comparison.
Table 3.5: Spitzer focal plane layout: approximate offsets from boresight to aperture centers.
Aperture
Z (´)
Y (´)
IRAC 3.6 µm
2.86
3.67
IRAC 3.6 µm subarray
4.97
5.88
IRAC 5.8 µm
2.86
3.61
IRAC 5.8 µm subarray
0.72
5.71
IRAC 4.5 µm
2.72
-3.05
IRAC 4.5 µm subarray
4.80
-0.92
IRAC 8.0 µm
2.75
-3.07
IRAC 8.0 µm subarray
0.65
-0.97
IRS SL 1st order
12.03
-2.79
IRS SL 2nd order
11.91
-4.09
IRS LL 1st order
4.41
-14.00
IRS LL 2nd order
1.21
-14.07
IRS SH
10.56
10.01
IRS LH
10.42
-10.23
IRS Red Peak-Up
11.63
2.00
IRS Blue Peak-Up
13.52
1.91
MIPS 24 µm
-6.72
4.25
MIPS 70 µm default
-6.53
-8.06
MIPS 70 µm fine
-7.10
-6.81
MIPS 70 µm default side A
-7.95
-8.11
MIPS 70 µm fine side A
-6.39
-6.71
MIPS 160 µm
-7.35
12.11
MIPS SED
-6.06
-9.44
Table 3.5 gives the measured offsets from the center of the field of view to the centers of the main science apertures. The values were determined post-launch based on the results from the focal plane mapping survey during IOC+SV.
Each of the fields of view that are used has a code that specifies the position in the focal plane. The uplink software uses the field-of-view index for commanding the target position, and the pipelines similarly use it to reconstruct where the telescope was pointing. These indices and field-of-view names appear in the headers of Spitzer data, and are listed in Table 3.6.
