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

24 CHAPTER 4. INTRODUCTION TO BAT ANALYSIS
Figure 4.6: BAT partial coding map in sky coordinates, assuming all detectors are enabled. The
units are degrees on both axes.
the center of its field of view which is 100% coded. However, as a source moves farther off axis,
only a portion of the aperture shadow will illuminate the detector array, and the rest are blocked
by the shield which surrounds the BAT instrument. The partial coding effect is analogous to the
off-axis vignetting effect for classical telescopes. The fractional illumination is called the partial
coding fraction. For example, the source in Figure 4.5 is about 98% partially coded, since about
2% of the detectors in the lower right corner are not illuminated by the source. Eventually the
source is so far off axis that it can illuminate no detectors, and so the source becomes completely
undetectable using coded aperture techniques. Figure 4.6 shows various partial coding contours in
sky coordinates.
The BAT field of view which contains at least 10% partial coding is about 2.2 steradians (>50%,
1.5 steradians; >90%, 0.5 steradians). Of course, as the partial coding decreases, the instrument
sensitivity also decreases. This is why gamma-ray burst light curves are typically noisier at their
start, when the source is detected off-axis; and less noisy by the end, when the spacecraft has slewed
to put the source in the fully coded region.
The BAT software automatically corrects for partial coding effects by default.
4.6 BAT Coordinates
Various coordinate systems are used in analyzing BAT data. More detailed information can be
found in the document ”BAT Coordinates Definition and BAT Detector Layout,” available from
the Swift CALDB documentation area. Here is a short summary.
BAT detector plane images and detector plane histograms (DPIs and DPHs) have spatial dimensions
DETX and DETY. These refer to the individual pixel positions on the detector array
(DETX is numbered from 0-285; DETY from 0-172). Positions of detector gaps are filled with
zeroes.
Most users will be interested in images with celestial coordinates. By default, a standard WCS
celestial coordinate system is be attached to each BAT sky image, which should be readable by
any standard FITS image viewer.
BAT sky images are most naturally expressed in tangent plane coordinates. This is because the
coded mask image analysis translates directly into a tangent plane projection of the sky. All BAT
pixels have the same spacing and size in these coordinates; in any other projection BAT pixels will