IRAC: AOT Description: Field of View

Selecting a field of view

When making an IRAC map (not allowed in subarray mode), you can use the field of view checkboxes to control the way the IRAC AOT performs the map. Whatever your selection, data will be collected with all 4 arrays, and the same sky will always be covered by a pair of arrays sharing an entrance aperture. If you select the 3.6/5.8 micron field of view as primary, then your specified map will be performed with the 3.6/5.8 micron field of view. A nearby portion of sky (significantly overlapping for large maps) will be covered with the other, 4.5/8 micron field of view. This is illustrated in the Figure below, where the sky coverage of a 3 column by 5 row map is shown. If you select the 3.6/5.8 micron field of view, the target position will be centered in the left-hand panel. The serendipitous sky coverage by the 4.5/8 field of view is shown in the center panel, and the sky covered by both fields of view is shown in the right-hand panel.

Schematic diagram of sky covered by the two IRAC fields of view during a 3x5 map. On the left is the sky covered by the 3.6/5.8 micron field of view; in the middle is the sky covered by the 4.5/8 micron field of view; and on the right is the sky covered by both fields of view. In the left and center panels, lighter shades indicate a higher depth of coverage, while darker shades indicate less coverage. The right-hand panel is white when both fields of view cover the sky.

If you have selected "array coordinates" and "both" fields of view, then the only special action that will be taken by the AOT is to center the map on the point directly between the two IRAC fields of view. If your map is one row by one column, the map center will not be imaged by any of the IRAC arrays. Therefore, if you want the same desired area to be covered by all four arrays, you will have to increase the number of rows in the map. For example, suppose that you want to cover an area with a diameter of D, and you want the spacing between map grid positions to be delta. The number of columns in your map is just 1+[(D-A)/ delta]. One IRAC field of view size is A=5.2 arcmin and the gap between the two fields of view is G=1.5 arcmin. The number of rows required to cover this area with both fields of view is therefore 1+(D+G)/ delta. (You should round the number of rows and columns up for any fraction, or else some area at the top and bottom of the desired region will not be covered by all 4 arrays.) For example, if your region has diameter 14 arcmin and you are using a 280 arcsec map grid spacing, then your map should have 3 columns and 5 rows.

There are special cases for which observers should be careful with their choice of field of view. For example, if you want to make a sparse map, with spacing greater than the array size (delta >A), then each of the 3 choices of field-of-view (3.6/5.8, 4.5/8, or both) will yield a significantly different map. To make a sparse map with both fields of view, ensuring that all 4 arrays cover the exact same sky, you should split the observation into two AORs, one per field of view. Also, for a very small map (delta*M~A, where M is the number of rows), be sure to make the map large enough that both fields of view will cover the target. If the number of extra rows required using the equations above is excessive, you may prefer to create two AORs, one per field of view.

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