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

5.8. MAKING BAT SKY IMAGES 49
5.8.5 Making a Background Detector Plane Image
Making a background detector plane image is optional, but it will increase the sensitivity of the
image when bright sources are present. If possible, the background interval should be made from
data with similar background levels and for a longer duration than the source image.
Use the same procedure as above to create a DPI file during a time range when you know there
is no source emission. Let us call that file “bkg.dpi”.
NOTE: if the background or bright sources are known to have varied significantly between the
time of the source and background DPIs, there may be little or no benefit to making a background
file.
Advanced usage: if bright sources are known to be present in the image, it is possible to use
the batclean task to create a background map which removes many systematic artifacts. This is
usually not necessary for images with exposures less than 100 seconds.
5.8.6 Making a Sky Image
A sky image is created from a DPI using the ‘batfftimage’ task. Issue the following command,
batfftimage infile=flare.dpi outfile=flare.img attitude=../../auxil/sw00145675000sat.fits.gz \
bkgfile=bkg.dpi detmask=../hk/sw00145675000bcbdq.hk.gz clobber=YES
where
• infile is the DPI file name
• bkgfile is the background DPI file name (‘NONE’ can also be used if no background file is
available)
• outfile is the output sky image file name
• attitude is the spacecraft attitude file name
• detmask is the detector quality map
The result is a BAT sky image. If valid spacecraft attitude data was used, then the sky image
should automatically have celestial coordinates (WCS keywords) attached.
You may use your favorite FITS image viewer to examine the image (for example FV or DS9).
Figure 5.6 shows the appearance of a typical bright point source in a BAT sky image.
5.8.7 Caveats
The spacecraft attitude may have changed during the accumulation interval of your DPI. You must
carefully filter out spacecraft slew data and use only steady pointed attitude information, or the
celestial coordinate system will be erroneous.