SHOW PRIMARY [PLANE|FIELD Kn] [First Last]
Show some sub-cube of the (4-D) Primary Beams.
The 2nd argument indicates whether SHOW shows all Fields for a given
frequency plane (PLANE keyword), or all Frequency-dependent beams for a
given Field (FIELD keyword). The 2nd and 3rd arguments are optional if
the beam is only 3-D, which occurs for a single field or a frequency-in-
depedent beam in Mosaics (narrow band approximation).