IRAC Instrument Handbook - IRSA - California Institute of Technology

8 Introduction to Data Analysis
8.1 Post-BCD Data Processing
IRAC Instrument Handbook
All processing of IRAC data beyond the individual, calibrated, 256 x 256 images produced by the BCD
science pipeline is called “post-BCD." This includes combining all images from an observation (AOR)
into a mosaic, detecting sources, and any cosmetic corrections, e.g., cosmic ray hits, to the images
(individual or mosaic) that are not based on understood instrumental artifacts or detector physics. Two
important post-BCD processes are performed routinely by the pipeline and generate results that are placed
in the science archive. These include pointing refinement, wherein a set of point sources are identified in
the images and are astrometrically matched to the 2MASS catalog, and mosaicking, wherein the
individua l images in an AOR are robustly combined into celestial coordinate mosaics for each IRAC
waveband. The post-BCD processing can (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. Here we discuss some IRAC-specific issues. The post-BCD software consists of a series of
modules linked by Perl wrapper scripts and controlled by namelists. Namelists need to be placed in a
subdirectory called cdf and have filenames ending .nl. A (MOPEX) GUI is available as well. The
namelist controls which modules are called, contain the names of the input file lists and output directories,
and detailed parameter sets for each module. Input file lists should not have any blank lines, otherwise the
programs will look for non-existent files. The following subsections deal with each part of the post-BCD
pipeline in turn, starting with pointing refinement, then overlap correction, mosaicking and finally point
source extraction.
8.1.1 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 are combined and applied to all four channels to produce the
default pointing via the “superboresight." 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.
8.1.2 Overlap Correction
The post-BCD software contains an overlap correction module which matches the background leve ls of
overlapping frames in a mosaic. Generating new mosaics by running MOPEX with overlap correction
Introduction to Data Analysis 134 Post-BCD Data Processing