4.6.3 Mapping

The choices for mapping and field of view are co-dependent. Selecting one of the fields of view as primary ensures that the map you define will be centered on the target coordinates in your primary field of view. All four fields of view collect data, however, so an offset map will be imaged by the other field of view.

For example, for a single pointing observation (i.e., no mapping), by selecting the $3.6/5.8 \; \mu {\rm m}$ field of view as primary, the specified target will be imaged with the $3.6/5.8 \; \mu {\rm m}$ field of view, while a nearby portion of sky will be covered with the $4.5/8.0 \; \mu {\rm m}$ FOV. By selecting both fields-of-view (still with no mapping), we request that the same piece of sky be covered by each array, and in this case, there will be `offset' fields that are imaged only by either the $3.6/5.8 \; \mu {\rm m}$ or $4.5/8.0 \; \mu {\rm m}$ FOVs.

In mapping mode, selecting both fields of view does not guarantee that the same sky is covered at all four wavelengths. If you select the celestial coordinates map, the map grid is executed once with the 3.6/5.8 field of view centered on the map positions, and then again with the 4.5/8.0 field of view. This can lead to significant redundancy, as the same area of sky is imaged more than once by the same aperture. If you select array coordinates and both fields of view, then the map will be centered on the point midway between the two IRAC fields of view.

Confused? A few examples are in order. From the main IRAC AOT entry window (see Fig. 4.7), under the ``Mapping and Dithering'' section, clicking the ``Yes'' radio button under ''Mapping Mode'' brings up the mapping entry dialog in Figure 4.8.

Figure 4.8: The IRAC Mapping AOT map parameter entry dialog.

We have choices of the number of rows and columns of the map, the map spacing, and the coordinate system for the offsets: array or celestial. As alluded to above, how maps are executed depends upon the field of view selection, as we illustrate below. For this example, all choices will be done with the rectangular grid.

$\bullet$ Case 1: no mapping. The easiest case first. If both fields of view are selected, the observations are executed with the target in each aperture successively. With either the 3.6/5.8 or 4.5/8.0 field of view selected, the target is centered in the chosen aperture only. A simple illustration of these three cases is shown in Figure 4.9, where the `X' marks the target location, and the two apertures are shown as the squares.

Figure 4.9: No mapping. The dependence of the sky area covered in no-map mode on the field of view selection. The ``x'' marks the target position in each case, and the squares show the 3.6/5.8 and 4.5/8.0 micron apertures. In the first panel, the second pointing has been shown as dashed lines, to aid in the visualization.

$\bullet$ Case 2: single position (1x1) map. A tricky thing can happen if you choose both fields of view, and do a 1 row, 1 column map in array coordinates. In this case, the target is placed midway between the two fields of view. Choosing one or the other field of view only centers the target coordinates at the center of that field of view. This is illustrated in Figure 4.10.

Figure 4.10: Single position map. The dependence of the sky area covered with a single position (1 column, 1 row) array coordinates map. The ``x'' marks the target position in each case, and the squares show the 3.6/5.8 and 4.5/8.0 micron apertures.

$\bullet$ Case 3: simple map. Let's consider a map consisting of 2 rows, 1 column, with offsets = 260 $^{\prime \prime }$ in array coordinates. This should be sufficient to illustrate more complex mapping strategies. Choosing both fields of view centers the target in the middle of the overlap region of both fields of view. Conversely selecting one of the 3.6/5.8 and 4.5/8.0 micron fields of view centers the map in the center of the overlap region for that field of view. The maps are illustrated in Figure 4.11.

Figure 4.11: Simple map. The dependence of the sky area covered with a simple map (1 column, 2 rows with offsets = 260 $^{\prime \prime }$ in array coordinates). The ``x'' marks the target position in each case, and the squares show the 3.6/5.8 and 4.5/8.0 micron apertures.

So, what does this all mean? You should choose a map scheme such that you obtain sky coverage in the passbands you wish, obviously. What about if you want exactly the same sky to be covered in both apertures? This is possible, although at the added expense of constraining when your observations can be performed. The Spitzer Space Telescope has a restricted roll angle, such that it must be positioned so that the solar shield offers protection from the Sun. Hence to `flip' the relative orientations of the 3.6/5.8 and 4.5/8.0 micron apertures, effectively we must wait $\simeq$ six months for the observatory to be on the other side of the Sun. You can do this with Timing Constraints, or, equivalently, by creating two AORs with position angle constraints. However, do so at your own risk! The more constrained a program is, the less likely it can be scheduled. The preferred method is to make a map large enough so that there is sufficient overlap in both fields of view without constraining the observations.

For the purposes of the example we are developing in this chapter, we could do one of the following (which are equivalent). The first way is the simplest: do the observations with no mapping, but select both fields of view. The sky coverage is given as in the left panel of Figure 4.9.

An equivalent way to implement this using mapping is to perform a 1 column, 2 row array coordinates map. We adjust the offsets to be one array width + the spacing between the arrays $\simeq 310^{\prime\prime} + 90^{\prime\prime} = 400^{\prime\prime}$, and select both fields of view. This will mean that the second pointing places the 4.5/8.0 micron aperture at the position of the 3.6/5.8 micron aperture in the first pointing. The end result is uniform coverage over a $5\hbox{$.\!\!^{\prime}$}2 \Box$ FOV in 3.6, 4.5, 5.8, and 8.0 microns, with flanking fields imaged for half as much time in either 3.6/5.8, or 4.5/8.0 microns. The resulting sky-coverage map is illustrated in Figure 4.12.

Figure 4.12: The sky area covered either with no mapping, selecting both fields of view, or a simple map: 2 columns, 1 row, offsets 410 arcseconds in array coordinates, and both fields of view selected as primary. The ``x'' marks the target position, and the squares show the 3.6/5.8 and 4.5/8.0 micron apertures. The second pointing in the map has been shown as a dashed line, and offset slightly in the x-direction, to aid in visual recognition of each pointing.