MODFIX


MODFIX is a program for  manipulating  model  files.   It  can  shift,
rotate,  scale  and  window models.  It can also be used for squashing
model files  (consolidating  multiple  clean  components  into  single
components)  and  for sorting components into decreasing order of flux
density.   To  run  MODFIX,  use  the  command  MODFIX   (defined   in
VLB:LOGIN.COM).   The program is controlled by parameters in the usual
format.  Operations are in the order:  STRETCH, ROTATE, SHIFT, WINDOW.
To avoid confusion, specify only one of these in each run of MODFIX.

Parameters

End the list of parameters with a slash (/) or end-of-file (ctrl-Z  in
VMS, ctrl-D in Unix).  Parameters INPUT and OUTPUT must be specified.

INPUT = "file"      input model file name.

OUTPUT = "file"     output model file name.

NUMBER = n          read only the first n components  from  the  input
                    model file.

SHIFT = dx, dy      add (dx, dy) milliarcsec to the coordinates of all
                    model  components  (i.e.   shift left (east) by dx
                    milliarcsec and up (north) by dy milliarcsec).

ROTATE = angle      add angle (degrees) to the position angle  of  all
                    components  (i.e.   rotate  anticlockwise  through
                    angle degrees).

LRTB = x1,x2,y1,y2  select  only  the  components  whose  centers  lie
                    within  the window (specified in milliarcsec, same
                    convention as MODPLOT:  x1 < x2, y1 > y2).

STRETCH = factor    multiply the angular dimensions (radii  and  major
                    axes)  of  all  the  components  by  the specified
                    factor (default 1).

SCALE = factor      multiply the flux densities of all  components  by
                    the specified factor (default 1).

NONEG               if NONEG is specified (no value),  all  components
                    with  negative  flux  density will be removed from
                    the model.

SQUASH              if SQUASH is specified (no value), all  components
                    at  the  same location will be consolidated into a
                    single component (cf.   the  SQUASH  operation  in
                    program CLEAN).  MODFIX can only squash a model if
                    all the components are  delta-functions.   If  you
                    use  SQUASH, it is advisable to also specify SORT,
                    so that after squashing, the components are  still
                    in  decreasing  order of flux density.  Otherwise,

MODFIX                                                            Page 2


                    using the option in AMPHI to use only the first  n
                    components will not do what you expect.

SORT                if SORT is specified (no  value),  the  components
                    are  sorted  into decreasing order of flux density
                    (i.e., strongest component first).

Applications

     1.  Shift a model made at one frequency to match a model made  at
         another frequency:  use the SHIFT parameter.

     2.  Select a subset of  components  for  the  next  iteration  of
         hybrid mapping:  use the LRTB and/or NONEG parameters.

     3.  Sum the flux density in a rectangle:  use the LRTB  parameter
         (MODFIX  types  out  the  total  flux  density  in the output
         model).

     4.  Squash a model to  save  cpu  time  in  later  programs  like
         VISPLOT  and AMPHI which have to recalculate the model in the
         (u,v)-plane:  use the SQUASH and SORT parameters.   (This  is
         similar to the use of task CCMRG in AIPS.)


Limitations

     1.  Up to 10000 input model components are accepted.

     2.  When  using  SQUASH,  all  model  components  must  be  delta
         functions.


History

Version 1.0:  1983 Aug 24 - new program (T.J. Pearson).
Version 1.1:  1983 Aug 25 - add STRETCH option (TJP).
Version 1.2:  1983 Sep 22 - correct sign convention in SHIFT (TJP).
Version 1.3:  1985 Mar 27 - correct sign  convention  in  LRTB  (error
          introduced in v1.2) (TJP).
Version 1.4:  1985 Apr 16 - add SQUASH parameter; allow all  component
          types (TJP).
Version 1.5:  1986 Sep 18 - add SORT parameter (TJP).
Version 1.6:  1988 Apr  6 - changes for Convex (TJP).
Version 1.7:  1992 Feb 12 - add NUMBER option (TJP).