6.2 Summary of the Observing Program

In this section, we summarize the basic steps followed to design a Spitzer observing program that uses the SH and LH modules of the IRS to obtain spectra of 3C 273, which is a bright ( $V \approx 12.9$ mag), nearby ($z=0.158$) quasar, located at RA = 12:29:06.7, DEC = +02:03:08.6 (J2000), ecliptic latitude = $+4.7^{\circ}$ (this will affect the signal-to-noise ratio estimate below). The design process for this example is presented in more detail in the remainder of this chapter.

• Target Selection and Duplication Check: Checking the Spitzer Reserved Observations Catalog shows that the field around 3C 273 already has observations planned with the IRS. In particular, GTO observations in program PID 105 will use both the SH and LH modules. If we were considering to write a new Spitzer observing proposal, then this could preclude obtaining new observations at the specified target position. According to the Spitzer Duplicate Observations Rules, new observations would need to be designed in such a way that: (1) the integration time per pixel for each observation differs by more than a factor of three in sensitivity, and (2) the target positions are separated by more than one-half of the slit length of the relevant IRS module. See §6.3 for additional details.
• Exposure Time Estimate: Requesting exposures of $1\times120$ seconds and $1\times240$ seconds for the SH and LH slits, respectively, will yield a signal-to-noise ratio of ${\rm S/N} \approx 8$ in each spectrum from both modules for a target line flux of $5 \times 10^{-18} \; {\rm W \; m}^{-2}$. (Note that because two spectra with the requested exposure time are automatically obtained at two different positions of the source within the slit in IRS Staring Mode observations, the signal-to-noise ratio can be increased by combining these spectrum pairs, to yield ${\rm S/N} \approx 12$ in the final combined spectrum from each module.) See §6.7 for additional details. Although in this particular example we have used 1 cycle per slit, it is strongly recommended that the observers provide a minimum of two cycles per slit. This provides greater redundancy against cosmic rays and variability in the pixels.
• Filling in the IRS Staring Mode AOT: We select one cycle using the 120-second and 240-second ramp durations for the SH and LH slits, respectively. We present examples of configuring offset IRS and PCRS peak-ups for 3C 273, using stars selected from the 2MASS Point Source Catalog or the Pointing Calibration and Reference Sensor (PCRS) Guide Star Catalog, respectively. (In addition, the procedure for configuring a peak-up on the science target itself is discussed.) See §6.9 for additional details.