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Knowledgebase: IRAC
What is different about taking IRAC data with the subarray mode (as opposed to full array or HDR mode) and what are the main differences in data processing and data products for IRAC subarray?
Posted by Elena Scire on 10 March 2010 10:39 AM
Subarray mode is useful for observing very bright sources and/or observing with high temporal resolution.

In subarray mode, only one corner, 32×32 pixels offset by 8 pixels from the edges, is read out from one array. Pixels (9:40, 9:40) of the array are read out. This corresponds to pixels (9:40,217:248) in channel 1 and 2 BCD images since the raw frame orientation is mirrored on the y-axis with respect to the BCD orientation in these channels. In channels 3 and 4, the raw and BCD pixel coordinates of the subarray location are the same (9:40, 9:40). The subarray pixel size is the same as the full array pixel size (roughly 1.2 arcseconds x 1.2 arcseconds). Fowler sampling is performed (as in full array mode): the subarray area is read (with Fowler sampling) 64 successive times and the data stored, in order, in the image data. In a 256x256 raw image the data appear scrambled. Changing the image dimensions to 32x32x64 will render them readable in a FITS viewer.

The subarray BCDs are actually 3-dimensional data cubes, with 64 32x32 pixel planes. Note that there is no post-BCD processing of subarray data through the IRAC pipeline, and pointing refinement of any type is not performed on subarray data as the number of astrometric truth sources visible in most subarray observations is too low for the refinement to work. The 64 subarray reads are performed with the spacecraft holding a fixed attitude. Although a small level of drift exists, for all but the most demanding observers the 64 subarray reads during a single exposure are at the same pointing. Note especially that NO CBCD FILES AND NO MOSAICS ARE GENERATED FOR SUBARRAY DATA. This can result in somewhat unexpected Spitzer Heritage Archive (SHA) behavior: you can be looking at AORs in the AOR window, but have "no data to display" in the Level 2 (PBCD) tab. If you want to use MOPEX to generate your own mosaics from subarray data, see this memo which explains in detail how to do it.

The IRAC BCD pipeline produces collapsed two-dimensional images, made by taking a median of the 64 pixels at a given location in the subarray field of view. These products are called *sub2d.fits. The corresponding collapsed 2D uncertainty files are called *unc2d.fits, imask files *msk2d.fits and coverage files *cov2d.fits. These are available in the BCD directory of downloaded subarray data, together with the 3D BCD data cubes (*bcd.fits), 3D uncertainty files (*bunc.fits) and 3D imask files (*bimsk.fits).

If the subarray data were used to obtain high-cadence observations of a target, you definitely want to reduce each of the 64 images separately. You can manipulate the original BCD images as described in the memo above, and pass them individually to MOPEX, or use your own favorite data reduction package. Note that you will have to handle any image artifacts on your own as well. The IRAC Instrument Handbook describes many of the common artifacts, and the Contributed IRAC Software Page contains several routines for removing artifacts, should they be in the data.