diff --git a/cscripts/csphagen.py b/cscripts/csphagen.py
index ac36f80..9a99874 100755
--- a/cscripts/csphagen.py
+++ b/cscripts/csphagen.py
@@ -287,6 +287,15 @@ class csphagen(ctools.csobservation):
             self._has_exclusion = True
         elif self['inexclusion'].is_valid():
             inexclusion         = self['inexclusion'].filename()
+            # If the user has not specified the extension to use
+            # and there is an extension called 'EXCLUSION' ...
+            if not inexclusion.has_extname()\
+                    and not inexclusion.has_extno()\
+                    and gammalib.GFits(inexclusion).contains('EXCLUSION'):
+                # ... choose it for the exclusion map
+                extname = inexclusion.url() + '[EXCLUSION]'
+                inexclusion = gammalib.GFilename(extname)
+            # Otherwise will pick the default (primary) HDU
             self._excl_reg      = gammalib.GSkyRegionMap(inexclusion)
             self._has_exclusion = True
         else:
diff --git a/doc/source/users/reference_manual/csphagen.rst b/doc/source/users/reference_manual/csphagen.rst
index 61a5659..ce40906 100644
--- a/doc/source/users/reference_manual/csphagen.rst
+++ b/doc/source/users/reference_manual/csphagen.rst
@@ -25,9 +25,13 @@ respect to the pointing direction that are smaller than the value specified by
 the ``maxoffset`` parameter. If less than ``bkgregmin`` reflected regions can be
 found because the offset angle between source and pointing direction is too small,
 the observation will be skipped. Regions to be excluded for background
-determination can be specified using a mask file that is specified using the
-``inexclusion`` parameter. A mask file is a sky map where all pixel values that
-differ from zero will be used as exclusion region.
+determination can be specified using a FITS mask file that is specified using the
+``inexclusion`` parameter. A mask is a sky map where all pixel values that
+differ from zero will be used as exclusion region. If the FITS file
+contains multiple extensions and the user does not specify which one
+to use, the extention ``EXCLUSION`` will be used if available (e.g.,
+if you use a skymap generated by :ref:`ctskymap`).
+Otherwise the primary extension will be used.  
 
 Alternatively, if ``bkgmethod=CUSTOM`` is specified, the source and background
 regions will be specified using the ``srcregfile`` and ``bkgregfile`` parameters,
@@ -240,4 +244,7 @@ Standard parameters
 Related tools or scripts
 ------------------------
 
-None
+:doc:`ctskymap`
+:doc:`csphasecrv`
+:doc:`cslightcrv`
+