4.1 Overview of the Program

In this section, we summarize the steps followed to design the deep imaging IRAC program. This will serve both as a one-stop resource for those requiring only an overview of how to do such a program, and also as a roadmap of where to find more detailed information later in this chapter.

IRAC is a four-channel camera that provides simultaneous $5\hbox{$.\!\!^{\prime}$}2 \times 5\hbox{$.\!\!^{\prime}$}2$ images at 3.6, 4.5, 5.8 and 8 microns. Two adjacent fields of view are imaged in pairs (3.6 and 5.8 microns; 4.5 and 8.0 microns) using a dichroic beamsplitter. All four detector arrays in the camera are 256 x 256 pixels in size, with a pixel size of $1\hbox{$.\!\!^{\prime\prime}$}2$.

Observing programs with IRAC are designed with the IRAC Mapping AOT, supporting both mapping and dithering schemes, a choice of full, subarray, stellar or high dynamic range modes, and a selection of frame times, ranging from 2-100 sec in full-array mode, 0.02-0.4 sec in subarray mode, and 12-100 sec in high dynamic range mode. Users can design single pointing observations, or a mosaic, subject only to the constraint that each AOR must have a duration of less than 8 hours; longer duration programs must be split into separate AORs.

• Target Selection: For this example, we observe in the fields of rich, low- to moderate-z galaxy clusters. After a duplication check using Leopard (http://ssc.spitzer.caltech.edu/documents/leopard/) we see that these fields already have scheduled observations with Spitzer, and hence we would need to carefully design observations so as not to violate the Spitzer Duplicate Observation Rule. See §4.2.

• Background Estimate: We use Spot to obtain a background estimate at each of the target positions. In general, the background is characterized qualitatively by three levels (low, medium and high), and for these targets, the IR background is low/medium/high for the targets. We compare these estimates with what we see in IRAS Sky Survey Atlas images of the target field. See §4.4.

• Exposure Time Estimate: Armed with the background characterization above, we look at the IRAC sensitivities for low, medium and high backgrounds, respectively. This enables an estimate of the required exposure time to reach a desired signal-to-noise ratio for point source detection. In this case, for medium background, to reach $\simeq 1.7 \; \mu {\rm Jy}\; (1 \sigma)$ in the 8.0 micron band, we require $t_{\rm tot} \simeq 50 \times 100 \; {\rm sec.}$ See §4.5.

• Filling out the IRAC AOT: The observations themselves are designed within Spot. We complete the IRAC Mapping AOT, and a screen capture of the completed AOT is shown in Figure 4.14. For each target, we observe the cluster central region in all four passbands, and hence select both the $3.6/5.8 \; \mu {\rm m}$ and $4.5/8.0 \; \mu {\rm m}$ fields of view. The objects we wish to detect are intrinsically faint, so we select bright object avoidance, and read-out in full array mode; see §4.6.1. To reach the desired depth with the integrations, we require $\simeq 5000 \; {\rm sec}$ total integration. We select a frame time of 100 sec., and a 50-point dither using the cycling dither pattern, with medium scale factor; see §4.6.4. To image the central region, we explore the possibility of using either a simple map (1 column, 2 row array coordinates map, with row/column spacing of $410\hbox{$.\!\!^{\prime\prime}$}2$), or no mapping with both fields-of-view selected; see §4.6.3.

• Duration: The total time required, per target, to execute these observations is 11125 seconds, which is less than the limit of 8 hours for an IRAC AOR. See §4.7.

• Visualization: We visualize the observations via an overlay of the IRAC fields-of-view on Digital Sky Survey images of the clusters; see §4.8.