10.3.2 Instrument Settings

We are going for areal coverage here, so we choose ``Full Array'' readout mode, and make sure that both fields of view are checked, because we do indeed want all four wavelengths. We also ensure that ``HDR'' mode is selected. As with MIPS, all four detectors are always taking data, but the fact that we have checked both fields of view affects exactly how the map is executed, in exactly what pattern. For each pointing, we select 1 frame, with a 12 second frame time.

Figure 10.2: IRAC mapping parameters.

Now, we want to do both mapping and dithering. Clicking ``yes'' for mapping mode produces the window in Figure 10.2. We obtain more uniform coverage when mapping in array coordinates. We want to cover as much as we can of the $1^{\circ} \times 1^{\circ}$ patch approximately lined up with the MIPS AOR. We'll grid the region, stepping 85% the width of the array, $260^{\prime\prime}$. This will allow enough overlap to match up the tiles afterward, but still cover the area. In order to do this, we will need a $12\times 12$ grid.<sup>10.1</sup> We don't want any map center offset, and we should use array (rather than celestial) orientation; this is because we don't want funny gaps in the map (see the IRAC chapter of the SOM for more discussion). And, finally, we want to execute this map only once. After clicking ``OK,'' now that we have set some mapping parameters, note that the button ``set mapping parameters'' is now clickable (it used to be greyed out). To retrieve the window from Figure 10.2, click on this button.

Note: We avoided the use of ``repeats,'' which are tantalizingly available as ``number of frames'' on the IRAC AOT. This option results in multiple observations without moving the telescope and should be avoided. Instead, we want to ``dither'' to ensure good cosmic ray rejection, mitigation of scattered light, long- and short-term latents, and artifact masking. Do NOT set number of frames to anything other than 1!

Figure 10.3: IRAC dither parameters.

Clicking ``yes'' for dither pattern produces the window in Figure 10.3. We click on the button to enact ``Cycling,'' and choose the recommended minimum of three dither positions from the dither table. (We can let it start any place it wants, it doesn't matter.) Since there is only small overlap between map tiles, we chose a ``medium'' scale factor. Now we do want to keep the dithers small to keep the coverage as uniform as possible, but, conversely, we also need large dithers to separate large-scale photometric variations from sky variations. A reasonable compromise is ``medium'' scale dithers. As with the mapping parameters, after clicking ``OK,'' we have set some dither parameters, and the button ``set dither parameters'' is now clickable (it used to be greyed out). To retrieve the window from Figure 10.3, click on this button.