BEAM_STEP Integer
Number of channels per synthesized beam plane.
Default is 0, meaning only 1 beam plane for all channels. N (>0) indi-
cates N consecutive channels will share the same dirty beam.
A value of -1 can be used to compute the number of channels per beam
plane that ensures that the angular scale does not deviate more than a
fraction of the map cell at the map edge. This fraction is controlled by
variable MAP_PRECIS (default 0.1)