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

D.8. BATFFTIMAGE 137
D.8 BATFFTIMAGE
D.8.1 NAME
batfftimage - make sky image from detector plane image by FFT deconvolution
D.8.2 USAGE
batfftimage infile outfile
D.8.3 DESCRIPTION
batfftimage constructs a sky image by deconvolving the observed detector plane image with the
BAT mask aperture map. This tool is used to estimate the positions and intensities of previously
unknown sources on the sky. Typically this tool may be run on a detector image after background
cleaning with batbkgclean, but this step is not required.
batfftimage also has a secondary usage, which is to compute the partial coding map of the sky.
This map represents the fractional exposure of the sky for a given detector/aperture configuration,
and is similar to the vignetting profile for imaging telescopes. To compute the partial coding
map, users should the pcodemap parameter. Users must also provide an input map. Although
the contents of the map are ignored, the FITS keywords are used to select the proper calibration
files from CALDB. A partial coding threshold can also be specified using pcodethresh, which is a
fraction between 0.0 and 1.0. Partial coding values below pcodethresh are set to zero.
If an attitude file is specified, then a celestial coordinate system will be attached to the primary
WCS descriptors of the image. If no attitude file is specified, then the BAT instrument tangent
plane coordinates are assigned instead.
batfftimage will operate on single or multiple images. Multiple input images may be stored as
multiple extensions in a single file, or as a FITS table containing one image per row. Individual
images may be selected by using the ‘rows’ parameter. If a table of input images is supplied,
then the column specified by ‘countscol’ is selected. If a column is not specified, then the first 2D
column is selected. Output images are always stored as FITS image extensions. Output images
have extension names of BAT IMAGE n and BAT PCODE n for sky and partial coding maps,
respectively.
If a background detector map is available, batfftimage can automatically subtract it. It automatically
recognizes rate vs. count images, and applies an exposure correction to the background
if necessary. If multiple images are operated on, then the background file must either have a single
image (which is applied to all inputs); or the same number of images as the input file.
batfftimage can also calculate theoretical Poisson variance and significance maps. Note that
the input file must not be cleaned for these maps to be correct.
By default, batfftimage will quantize the image into discrete intensity levels. For sky images, the
quantization steps are adaptive, depending on the local noise level. Quantization usually reduces
compressed image sizes significantly. The degree of quantization is governed by the ‘keepbits’