8.5.1 Mapping Abell 2218

In this region of low background, we use the longest exposure time (30 seconds). We check only the box for $16 \mu {\rm m}$ imaging. We observe only one cycle per position, and dither for artifact rejection; see Figure 8.1.

We want to map the $5^{\prime}\times 5^{\prime}$ region to match the IRAC/MIPS coverage. Using the PUI mapping function requires $6\times 4$ positions. Because the array is not square ( $54\times 80$ arcseconds), we are not using the same number of rows as columns. We need very little overlap between map positions, so we use step sizes of 50 and 75 arcseconds. At each position, we want to dither. We choose the overlaps such that we maintain contiguous coverage regardless of position angle and therefore observing date. In the previous section, we determined that it would take 7 exposures per position to reach the required depth. We could settle for five position and use the fixed pattern dithers, or we can use the cycling pattern and get the 7 positions we really want. We choose the latter.

Figure 8.1 shows the completed AOT page and Figure 8.2 shows the overlay map on a DSS image. We can use a similar map for the other low background target. Together they require $\sim$5 hours to complete.

Figure 8.1: The completed PUI AOT for mapping Abell 2218.

Figure 8.2: The overlay of the PUI observation of Abell 2218 on the IRAC overlap region. This illustration assumes constant roll angle for the two observations, which is unlikely to be the case.