; UVBAS
;---------------------------------------------------------------
;! averages several channels and subtracts from uv data.
;# Task UV SPECTRAL
;-----------------------------------------------------------------------
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;;
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;-----------------------------------------------------------------------
;---------------------------------------------------------------
UVBAS     LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC
UVBAS     Averages several channels and subtracts from uv data.
INNAME                             Input UV file name (name)
INCLASS                            Input UV file name (class)
INSEQ             0.0     9999.0   Input UV file name (seq. #)
INDISK            0.0        9.0   Input UV file disk unit #
OUTNAME                            Output UV file name (name)
OUTCLASS                           Output UV file name (class)
OUTSEQ           -1.0     9999.0   Output UV file name (seq. #)
OUTDISK           0.0        9.0   Output UV file disk unit #.
BCHAN                              Lowest channel to write
ECHAN                              Highest channel to write
APARM                              1,2 & 3,4 channel ranges
----------------------------------------------------------------
UVBAS
Task:  This task does a spectral baseline subtractraction by
       averaging two ranges of channels in the input data set
       and subtracting them from all channels. The two ranges
       count equally in determining the amount to be subtracted.
Adverbs:
  INNAME.....Input UV file name (name).      Standard defaults.
  INCLASS....Input UV file name (class).     Standard defaults.
  INSEQ......Input UV file name (seq. #).    0 => highest.
  INDISK.....Disk drive # of input UV file.  0 => any.
  OUTNAME....Output UV file name (name).     Standard defaults.
  OUTCLASS...Output UV file name (class).    Standard defaults.
  OUTSEQ.....Output UV file name (seq. #).   0 => highest unique
  OUTDISK....Disk drive # of output UV file. 0 => highest with
             space for the file.
  BCHAN......Lowest channel number in the input file to write to
             the output file. 0=> 1.
  ECHAN......Highest channel number in the input file to write
             to the output file. 0=> highest in input data.
  APARM......Channels APARM(1) to APARM(2) and APARM(3) to
             APARM(4) are to be averaged and subtracted from
             all channels.
----------------------------------------------------------------
UVBAS: Task which subtracts continuum from channels in UV-plane
DOCUMENTOR: H.J. van Langevelde (Sterrewacht Leiden)

                          PURPOSE

    UVBAS will estimate the continuum visibilities and subtract
these from a specified range of channels, hopefully leaving only
the information about spectral features in the output UV-file.
In the channel ranges APARM(1)-APARM(2) and APARM(3)-APARM(4)
the data is averaged. Both complex numbers are then averaged to
get a first order accurate estimation for the continuum
visibility in the midpoint between these ranges. This value is
subtracted from the channels specified in BCHAN - ECHAN,
performing what in single dish is known as frequency switching
or baseline subtraction.

    This can only work properly if the UV coverage is the same
for all spectral line channels. It can, however, deal with
frequency dependent flagging.


                          COMMENTS

    This tasks has proved to be powerful in problems where
bandwith is small and extended continuum emission has to be
removed. Its accuracy is limited by the fact that the
visibilities in the uv plane change over the bandwith. The
errors made in the approximation of the continuum visibility
--and thus in the subtraction-- can be estimated as:

               d V      d u             d V      L
       D V  =  ---  *   ---  *  D v  =  ---  *  ---  *  D v
          L    d u      d v             d u      c

Where D V is the error, D v the bandwith over which we try to do
this and u a coordinate in UV-plane. L is the baseline length
specified in the same units as those used for c, the speed of
light.  The formula tells us that UVBAS should do an accurate
job on short baselines and/or small bandwith, provided there is
a reasonably smooth signature in the UV-plane. That means that
it will generally not work for a field dominated by discrte
(point)sources.

    In comparison with the method of cleaning the background and
UVSUB the components from the spectral line channels, this
method has the main advantages that it is 1) much faster, 2)
will work when the background is difficult to model with clean
components. When point- sources are the main source of continuum
emission the UVSUB method is perfectly suited. In some cases a
hybrid method may be advantageous.

    In comparison with averaging maps to estimate the continuum
this method is again faster and more reliable, since there will
be no sidelobs of the continuum in the map.

    The output data set can also be a powerful diagnostic tool.
In principle, the data can be used to apply selfcal on your
spectral line data.
