IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
mosaics rely on multiple (≥3) sightings of each sky pixel. In general, a coverage of at least five is
necessary to produce optimal results with the multi-frame (standard) outlier rejection.
A special outlier rejection scheme can be used for sparse (2−4x) coverage; this “dual outlier" mode can be
turned on using the namelist parameter file. Dual outlier rejection identifies pixels greater than a specified
threshold above the background, groups these pixels and adjacent pixels above a threshold into objects,
and compares the object to objects in overlapping frames. If the object overlaps with objects in other
frames (in celestial coordinates), then it is not a cosmic ray. If the object is not detected in a user-specified
fraction of overlapping images, it is flagged as a cosmic ray. This information is written into the Rmask
files used for mosaicking and source extraction. The dual outlier method should also be used in
conjunction with the multi-frame outlier rejection method. Multi-frame rejection may throw out data
around bright sources depending on the thresholding, due to pixel phase effects between BCDs. Using the
dual outlier rejection and the REFINE_OUTLIER=1 option in MOPEX will prevent this.
Additionally, a single-frame radiation hit detector is run and produces bit 9 in the imask, but this bit is not
used by the SSC post-BCD software and is not recommended because radiation hits cannot be uniquely
separated from real sources in single images.
Data Features and Artifacts 131 Cosmic Rays and Solar Protons