IRAC: AOT Description: Map Grid

Map Grid Definition

Map grid dialog box

If you select "Yes" under "mapping mode," then you will be presented with a dialog box, shown above, wherein you specify the rectangular map grid. The map grid may be specified in either "array" or "celestial" coordinates. If you choose array coordinates, then the map grid will be aligned with the edges of the array, such that the map rows and columns will correspond to rows and columns of the array. Specifically, a "column" will be along a line of constant solar elongation, and a "row" will be along an ecliptic parallel [line of constant ecliptic latitude]. It is worth noting that the two IRAC fields of view are at approximately constant solar elongation, so that a map with 1 column and several rows will make a strip along the direction of the separation between the two fields of view and will yield 4-array coverage along part of the strip (if it is long enough). If you select celestial coordinates, then the rows and columns will correspond to J2000 Right Ascension and Declination. You may specify a position angle, degrees E of N, to orient the raster in Equatorial coordinates. Specifically, if the position angle is zero, a "column" will be along a line of constant Right Ascension, and a "row" will be along a parallel (line of constant Declination). The celestial option can be useful for regions whose shape is clearly defined on the sky. This option also allows two separate AORs, both in celestial coordinates, to have a fully specified location of map grids such that they can be made adjacent to one another independent of observing date (but see below, regarding gaps).

The most important advantage of mapping in array coordinates is that the sky coverage can be custom-tailored, independent of the date when the observation is scheduled. In general, we do not recommend using celestial map grids. A celestial map grid can yield highly non-uniform coverage (including gaps), unless the individual pointings are spaced closely together. There are cases where a celestial grid is preferred, specifically for mapping a highly elongated region. We recommend that all observers considering a celestial grid inspect their survey coverage for a range of possible spacecraft roll angles. This can be done by overlaying your AORs on a sky image in Spot. The roll angle itself can be obtained in the Target Entry/Visibility section of Spot, by choosing an observing date when the target is visible. The roll angle is essentially fixed if the source has an absolute ecliptic latitude smaller than 10 degrees (where the total roll angle variation as a function of observing date is less than 8 degrees). The Figure below shows two examples of 3x3 map grids, one in array coordinates and the other in celestial coordinates. For the celestial grid, the spacecraft roll at the time of observation had a position angle of 30 degrees east of north. Not only are there large gaps in the celestial grid, but the coverage is non-uniform in the observed region as well. To eliminate the gaps, you could tighten the spacing of the map grid positions (sacrificing some area covered or some dithering), or you could fix the date of the observation such that the roll angle will be equal to your desired celestial position angle. But be aware that away from the ecliptic poles, not all position angles are available over all positions on the sky.

Simulated IRAC coverage for a map in array coordinates (left) and celestial coordinates (right).

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