MRGSUB


MRGSUB subtracts the contribution of a model from the visibility data in
a  MERGE-format  file.  The primary applications of this program are (a)
plotting the difference between a dataset and  a  model  (use  VPLOT  or
VISPLOT  on  the  output file); and (b) "difference mapping:" subtract a
model after self-calibration but before making an image.

     Algorithm:  The model visibility is computed at  each  (u,v)  point
specified  in  the  input  file.   If  OPCODE=VECTOR  (the  default)  is
specified, the complex visibility in the  output  file  is  the  complex
visibility  from  the input file minus the complex model visibility.  If
OPCODE=SCALAR is specified, the amplitude in  the  output  file  is  the
amplitude  from  the input file minus the model amplitude, and the phase
in the output file is the phase from the  input  file  minus  the  model
phase  (modulo  360  degrees).  The closure phases in the input file are
discarded; if you want to compute  the  difference  closure  phase,  run
program ADDCLO on the output from MRGSUB.

     For  OPCODE=SCALAR,  the  errors  on  the  data  points  are   left
unchanged.   For  OPCODE=VECTOR,  the errors on the phases in the output
file are computed as (amplitude error / new amplitude) [radians].


Example

$ mrgsub
INPUT  = T1.MRG
OUTPUT = T2.MRG
MODFILE = T1.MOD
OPCODE = V
/


Parameters

Parameters INPUT, OUTPUT, and MODFILE must be specified; the others  are
optional.   The  parameters  names  can  be  abbreviated  to  uniqueness
(minimum match).  All the parameters are  character  strings,  but  they
need  not  be  enclosed in quotation marks unless they include commas or
blank spaces.  End the list of parameters with a slash (/).

INPUT = "filename" :  the name of the  input  merge-format  file  (e.g.,
          INPUT   =  CITSCR:[TJP.B2]T1.MRG).   This  parameter  must  be
          specified; there is no default.

OUTPUT = "filename" :  a name for the output  file  created  by  MRGSUB.
          This parameter must be specified; there is no default.

MODFILE = "filename" :  the name  of  the  input  model  file  (e.g.,  a
          clean-compnents  file  from  CLEAN  or a model from MODELFIT).
          This parameter must be specified; there is no default.

HISTORY = "text" :  a character string to be written  in  the  "history"

MRGSUB                                                            Page 2


          section of the output file; this is optional.

OPCODE  =  "VECTOR"  or  "SCALAR"  (minimum  match,   case-insensitive):
          specify   VECTOR  for  vector  subtraction  of  complex  model
          visibility from complex input visibility; specify  SCALAR  for
          scalar subtraction of model amplitude from input amplitude and
          model phase from input phase.  The default is VECTOR.


Limitations

1.  The maximum number of stations is 40.

2.  Only MERGE format 1 files are acceptable (at present).

3.  The difference closure phases are not computed (use ADDCLO).


History

Version 1.0:  1989 Mar 20 - new program (T.J. Pearson).
Version 1.1:  1990 Nov 13:  remove unused variables.
Version 2.0:  1991 Jul 17:  add OPCODE for vector/scalar subtraction.
Version 2.1:  1993 Apr  1:  accept all model component types;  calculate
                            new phase errors for OPCODE=VECTOR.