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

20 CHAPTER 4. INTRODUCTION TO BAT ANALYSIS
Figure 4.2: Top view of the BAT mask. The cut-outs at the edge correspond to portions of the
mask where support structures were mounted.
For example, consider Figure 4.1. This figure shows a gamma-ray source which illuminates the
aperture. The portions of the aperture which are blocked by lead tiles absorb the gamma-rays,
and the portions which are not blocked allow the rays to pass through. The result is a distinctive
shadow pattern which is measured by the detector array below.
Coded aperture imaging is a logical extension of the pinhole camera concept. For a pinhole camera,
there is a single small aperture. Each source at a different position on the sky casts a different
narrow-beam shadow onto the detector array, and thus each source can be uniquely distinguished
from the other by directly examining the intensity in the detector plane. One disadvantage to a
pinhole camera is that light collection efficiency is low: nearly 100% of the aperture is blocked.
A coded aperture system improves the efficiency situation by placing more than one opening in
the aperture. The disadvantage to this approach is that the shadow patterns of different sources
can overlap, so the measured intensities no longer directly reflect the distribution of sky intensities.
On the other hand, the aperture pattern is known, and each distinct source casts a unique shadow
pattern on the detector array. Thus, it is possible to disentangle the fluxes of individual sources
using special software. Figure 4.2 shows the aperture pattern of the entire BAT mask which is used
in the processing.
Figure 4.3 shows the mechanical layout of the BAT, as seen from the side. The top horizontal
structure supports the mask tiles, and the bottom horizontal structure contains the detector array.
The diagonal structures support the mask, but leave the central volume between the two planes
essentially empty. The focal length of the instrument – the distance between the two planes – is
approximately 1 meter. This distance, coupled with the sizes of the aperture and array, determine
the basic imaging properties of the instrument such as its field of view and point spread function.
Extracting light curves and spectra involves performing the coded aperture analysis in multiple
time and/or spectral bins. The BAT software is designed to produce standard light curve and
spectral FITS file products that can be used with other standard analysis software (although see
below).