The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa

D.2. BATCELLDETECT 105
D.2 BATCELLDETECT
D.2.1 NAME
batcelldetect - perform source detection using the sliding cell method
D.2.2 USAGE
batcelldetect infile outfile snrthresh
D.2.3 DESCRIPTION
batcelldetect performs source detection on a sky image. The sliding cell method is used to locate
regions of the image which are significantly different from the background.
This tool is more appropriate for coded aperture imaging because: (1) it assumes Gaussian
fluctuations, not Poissonian; (2) it measures the local background; and (3) it measures the local
background standard deviation. A source is detected at a pixel if that pixel’s value exceeds the
background by more than “snrthresh” times the background standard deviation. Users can increase
the significance of a detection and reduce false detections by requiring more than one adjacent pixel
exceed the threshold, using the “nadjpix” parameter. The “nullborder” and “bkgpcodethresh”
parameters can also be used to exclude false positives near the edge of the image.
The background value is estimated by using a sliding window. The shape of the window is
either circular or square, and the radius (or half-width) is specified by the bkgradius parameter.
In determining the background, a circular region at the center of the window is excluded, whose
radius is the srcradius parameter. Thus, the background does not include contamination from the
source region.
In a single iteration, the sliding cell algorithm is less sensitive in a region around bright sources,
because the background standard deviation becomes biased. To avoid this, the algorithm can be
run in multiple iterations. After each iteration, the detected pixels are removed, and thus the bias
can be significantly reduced.
A second stage fits a point spread function to regions of the image where sources are detected.
For new sources, this aids in refining the centroid of the source position, as well as in estimating
uncertainties.
The default output is a catalog list of detected sources, plus various statistics about them. The
output flux column is either COUNTS or RATE, depending on the input image units. The tool can
optionally output the background map, the background fluctuation map, and a significance map.
IMPORTANT NOTE: if the keepbadsources parameter is ‘YES’, then the output catalog
may contain some sources which failed processing for one reason or another. Information on the
success or failure on a source-by-source basis is recorded in the DETECT STATUS column.
The user can also supply an input catalog. Sources in the input catalog, which are within the
field of view of the image, are assumed to be fixed at their known positions, and fitted during