Note that for Mac OS X in some cases libraries required by ROOT
(libfreetype and libpng) may not be available in the expected location
or have the correct version. If plotting fails when looking for these
libraries you should install them using your favorite package manager
or have your system manager install them on your system. Please see
the Binary install instructions for more details. 

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gtbkg does not currently support the CALDB entry for the irfs. If you
see the error "Caught St13runtime_error at the top level:AppHelpers::checkCuts:"
then you will need to specify the irfs in the par file or on the
command line. You can determine the irfs if you have an irf dependent
product you have created, for instance, an exposure map
>fkeyprint ,<Your Source>_bExpMap2.fits"[0]" DSVAL6
 
# FILE: 3c279_100_100000_bExpMap2.fits[0]
# KEYNAME: DSVAL6
 
# EXTENSION:    0
DSVAL6  = 'P7REP_SOURCE_V15'

and you can use the string returned for your irf in gtbkg, e.g.

>gtbkg irfs=P7REP_SOURCE_V15



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This is not strictly an error but it is often reported as such.
gtexpcube2 needs to have a counts cube when creating the exposure
maps. Since the livetime cube is IRF independent the counts cube is
used to deduce the IRF's based on the cuts you've made.