Saturated star recovery problems in S18.5 CBCDs
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We installed a new saturated star recovery module into IRAC pipeline
version S18.5. The saturated star recovery module includes the
identification of candidate saturated stars, and the rectification of
the pixels of those stars that are lost due to saturation and/or
non-linearity near the saturation limit. It uses the 2MASS Point Source
Catalog to help identify stars that may be saturated, and then performs
some additional tests for the saturation signature. The saturation
recovery procedure itself entails fitting the wings of the star light
distribution with the appropriate PSF and determining which pixels require
replacement with the fitted function. Those "recovered" pixels are then
placed into the modified BCD image (cBCD) so that the resultant star is fully
recovered from saturation. Additionally, for IRAC channels 3 and 4, an
addtional replacement of pixels is carried out for the locations of the
so-called "bandwidth effect," which represent non-physical (i.e.,
artifact) contamination of the stellar profile. This module appears to work
very well in BCD images that have low background emission levels.

Recently we have found that the saturated star recovery module does not
always work reliably in BCD images that have complex backgrounds. Such 
backgrounds are typically found in the Galactic Plane and in areas of
high, spatially variable cirrus emission. The high background levels are
usually only seen in images at 5.8 and 8.0 microns, with IRAC channel 4 images
having the most significant problems.

The image 

http://ssc.spitzer.caltech.edu/irac/img/satcorr.jpg

displays instances where the module has mistakenly taken faint stars that
are surrounded by bright background emission as saturated stars, and has
replaced them with a PSF with a very high flux density. The PSFs that
were used do not correctly account for the high background diffuse
emission. These examples demonstrate that the saturated star recovery
module produces erroneous results when the stellar wings are poorly fit.

We are currently working on improving the fit to the stellar wings and
using less aggressive saturation recovery thresholds and more robust 
checking algorithms. In preliminary testing, these changes have 
eliminated the saturation recovery problems found in areas of high 
background emission. The examples at

http://ssc.spitzer.caltech.edu/irac/img/satcorr.jpg

also show the improved saturation correction (the images on the farthest
to the right). We will include the improved version of the saturated star
recovery module in the next IRAC pipeline version, which is expected to
be released later in spring 2009. An offline version of the updated
module is available at 

http://spider.ipac.caltech.edu/staff/jarrett/irac/tools/Jarrett_BCDimageworks_06mar2009.tar.gz

We strongly encourage users of IRAC data to check the cBCDs against the 
BCDs before proceeding with data analysis. In a few cases the BCDs provide
a better starting point for data reduction than the cBCDs.