Spitzer Documentation & Tools
IRAC Instrument Handbook
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8.4                  Post-BCD Data Processing

All processing of IRAC data beyond the individual, calibrated, 256 x 256 pixel square images produced by the BCD science pipeline was called “post-BCD.” This included combining all the images from an observation (AOR) into a mosaic, detecting sources, and flagging outliers, e.g., cosmic ray hits, in the images that are not based on understood instrumental artifacts or detector physics. Two important post-BCD processes were performed routinely by the pipeline and generated results that were placed in the science archive. These included pointing refinement, wherein a set of point sources were identified in the images and were astrometrically matched to the 2MASS catalog, and mosaicking, wherein the individual images in an AOR were robustly combined into celestial coordinate mosaics for each IRAC waveband. The post-BCD mosaic generation could (and should) be performed in different ways for different observing strategies and scientific goals. The post-BCD pipeline processing was performed with a specific, conservative, set of parameters. Observers and archival researchers will very likely need to do post-BCD processing on their own. Most common will be generating mosaics from data in multiple AORs.


The following subsections deal with each part of the post-BCD pipeline in turn, starting with the pointing refinement, then overlap correction, then mosaicking. Point source extraction is discussed briefly, though it is not done in the pipeline. The post-BCD software is available in the MOPEX package at


https://irsa.ipac.caltech.edu/data/SPITZER/docs/dataanalysistools/tools/mopex/ .


This consists of a series of modules linked by Perl wrapper scripts and controlled by namelists. Namelists are placed in a subdirectory called cdf and have filenames ending in .nl. The namelist controls which modules are called, along with the parameters each uses. Input file lists can also be provided in the namelists. The file lists should not have any blank lines, otherwise the programs will look for non-existent files.


While the discussion of overlap correction and mosaicking here is tied to the MOPEX package, other software can be used as long as it understands the -SIP distortion of the (C)BCDs. Also note that the discussion of the tools here is far from a complete description, but just touches on some issues relevant to IRAC.

8.4.1        Pointing Refinement

Pointing refinement corrects the pointing of each frame to the 2MASS sky. In the pipeline, the pointing refinement solutions for channels 1 and 2 were combined and applied to all four (two) channels to produce the default pointing via the “superboresight” in the cryogenic and warm mission data, respectively. However, if they wish to try to improve on the supplied pointing, users may rerun the pointing refinement themselves using scripts that come with the MOPEX software package. Pointing refinement may not always be successful in channels 3 and 4, in which there are few 2MASS stars per image. Note that each run of pointing refinement overwrites any previous solutions (in the header keywords RARFND, DECRFND, etc.), so users should make copies of the BCDs before rerunning the pointing refinement, if they wish to retain the old corrections.  

8.4.2        Overlap Correction

The post-BCD software contains an overlap correction module that matches the background levels of overlapping frames in a mosaic. Generating new mosaics by running MOPEX with overlap correction turned on can remove the “patchiness” often seen in mosaics due to bias fluctuations in the array (due both to the first-frame effect and bright source effects). Note that background matching does not always work well, especially between areas of unequal depth.

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