Spitzer’s payload was capable, but simple. Among the three science instruments there were only two moving parts: the shutter on IRAC (which was not used until the final days of the mission) and the scan mirror on MIPS. The observatory schedule was organized into instrument campaigns. Only one of the three instruments could be powered on at any one time, but all of the apertures (or observing modes) of that instrument were available whenever the instrument was on (with the exception that only one of the four IRS modules could be read out at a time). As shown in Figure 3.1, the apertures pointed toward different portions of the sky at the same time; switching apertures and staying on the same target in general required re-pointing the telescope. A few things to notice are that the IRS slits covered a wide range of orientations and that the MIPS fields of view were larger than the array size; this is because they show the range covered by the motion of the MIPS scan mirror.
A central concept in Spitzer science operations was that of the Astronomical Observation Templates (AOTs). The three science instruments are operated in nine discrete observing modes that offer the observer a selected number of choices in configuring and operating the instrument. These observing modes are known as AOTs. The AOT concept and the nine Spitzer AOTs are described in Chapter 4, and in greater detail in the Instrument Handbooks.
The SSC planned and executed calibration activities to maintain the instrument calibration. The instrument calibration for each instrument is discussed in the relevant Instrument Handbook. Information about cross-calibration is discussed in Chapter 5.
Figure 3.1: Science instrument apertures projected onto the sky. Because of the optical inversion in this projection, the section of sky closest to the projected Sun is on the MIPS side of the focal plane, e.g. to the right in this view. Because the spacecraft did not rotate about the line of sight, this vector is fixed relative to the focal plane on the sky. The IRAC sub-array fields are shown by the small boxes in the lower corners of both IRAC arrays. (The 8.0 and 5.8 µm sub-arrays are on the right and the 4.5 and 3.6 µm sub-arrays are on the left.) Note that for figure clarity, the widths of the IRS slits as shown are rendered substantially larger than their actual scale. The PCRS, or Pointing Control Reference Sensor, is a small visible wavelength array used initially to monitor the boresight alignment of the telescope and the spacecraft star sensors and, subsequently, to position targets on spectroscopy slits and photometry arrays.