6.9.2 Choosing the Best Peak-up Option

``Peaking up'' is at the heart of Spitzer's target acquisition process for the IRS, and partly determines the basic feasibility of a particular AOR. Configuring the peak-up for an IRS AOR should be done with care, as a failed or imprecise peak-up can lead to the subsequent science exposure not meeting the desired accuracies for target placement and photometric flux calibration. During the peak-up process, on-board software views either a software-limited area of $43^{\prime\prime} \times 43^{\prime\prime}$ (for the IRS peak-up imaging arrays) or a $40^{\prime\prime} \times 40^{\prime\prime}$ area (for the PCRS imaging array), whichever is chosen, and determines the position of peak brightness in this field of view. The software then assumes that the position of peak brightness corresponds to the ``peak-up target'' that the proposer has specified via Spot. (There is, of course, no separate target acquisition if blind pointing - i.e., ``No Peak-Up'' - is requested.) The peak-up target may be either the science target itself (not recommended for extended objects) or an offset target. In either case, after the software has identified the point of peak brightness, it then directs the spacecraft to move so that the science target is precisely placed in the desired IRS slit. The peak-up arrays occupy different parts of the Spitzer focal plane from the IRS slits and from each other (see Fig. 4.5 of the SOM). One peak-up array is integrated into the IRS SL module, while Spitzer's Pointing Calibration and Reference Sensor (PCRS) is used as the other peak-up array. The IRS peak-up array is actually composed of two sub-arrays that are sensitive to slightly different IR wavelength regions (see ``Option 1'' below). An IRS peak-up images the peak-up target in one of these two IR bands, while a PCRS peak-up images the peak-up target in an optical band (see ``Option 2'' below). The peak-up process introduces an overhead of several minutes; the exact length depends on the requested accuracy (with higher accuracy requiring longer overhead). For an IRS peak-up, Low, Moderate, and High accuracies yield target placements within $2\hbox{$.\!\!^{\prime\prime}$}0$, $1\hbox{$.\!\!^{\prime\prime}$}0$, and $0\hbox{$.\!\!^{\prime\prime}$}4$, respectively. Each of these peak-up accuracies corresponds to a level of uncertainty in the photometric flux calibration of the resultant IRS spectrum (with a higher accuracy peak-up corresponding to smaller photometric uncertainty; see the IRS chapter of the SOM for more details). The nominal initial pointing accuracy of Spitzer, as measured during the In-Orbit Checkout (IOC), is $\approx 1^{\prime\prime}$; thus, the use of a peak-up in situations requiring Low accuracy is not strictly necessary if the target coordinates are well-known and if the set of peak-up images is not needed. Use of the Moderate accuracy peak-up for observations that require a pointing accuracy of $1^{\prime\prime}$ (in order to obtain the associated photometric uncertainty) is still advisable. This is especially true when multiple IRS modules are selected for use. PCRS peak-up accuracy is comparable to a High accuracy IRS peak-up. It is also possible to request that no peak-up be done. When filling out an IRS AOT with Spot, unless the ``No Peak-Up'' option is chosen, the proposer always needs to consider the following:

• Peak-up Mode: Either IRS peak-up or PCRS peak-up?
• Is the peak-up object a point source or an extended source?
• What is the peak-up target brightness in the relevant wavelength band?
• What is the offset distance between the peak-up and science targets?

Additionally, if ``IRS Peak-Up'' is selected, then the proposer must consider:

• Peak-up Filter: Red or Blue? This determines which of the two IRS peak-up sub-arrays is actually used.
• Peak-up Accuracy: Which pointing accuracy is needed (Low, Moderate, or High)? Each of these carries a successively better photometric calibration accuracy and longer associated overhead time.

An IRS peak-up has at least one advantage over a PCRS peak-up. During an IRS peak-up, 3 images of the peak-up target at the nominal spacecraft pointing, and an additional 3 images centered on the peak-up array ``sweet spot,'' are obtained. These images are delivered to the observer (along with 2 combined, ``cleaned'' images constructed from each set of 3 raw images). Thus, if the science target is used as the IRS peak-up target, then ``free'' IR images of the science target are gained. Note that although useful for viewing, e.g., the source's morphology, these images are not calibratible. If you require calibratible images, then you should use the IRS peak-up imaging AOT.