MODELFIT

Program MODELFIT is an interactive program for fitting a model to VLBI
data  (in the u,v-plane) by non-linear least-squares.  Data files with
either MERGE-format 1 or 2 are accepted.

To  run  the  program  type  the  command  MODELFIT  (defined  in  the
VLB:LOGIN.COM file).  It will ask various questions (see notes below):

     1.  Input data file?

     2.  Underlying model?  (see below)

     3.  Input model file?

     4.  Model acceptable?  (if not, you can edit model)

     5.  Parameters to VARY for each component (0-6 integers)

     6.  Do a gradient fit?

     7.  Do a brute force fit?

     8.  Try again?  (if yes, goes to question 2)

The program is limited to a maximum of 15 stations, 200 data  records,
and 20 model components.  You can vary as many model parameters as you
wish.

Discussion:

Most questions about how to run this program can be answered when  you
actually run the program; what follows are a few hints.

     To speed up the program, average your data as  much  as  you  can
using  AVERAGE,  creating  a  temporary  file.   If  you wish to avoid
fitting some data, use VLBEDIT to create a temporary file.  MERGE  can
be  used  to  delete  one or more stations.  Note that such editing or
averaging is a one way process, so be sure to save your original file!

     The gradient fit is a fit where the  gradient  of  the  agreement
factor  (AF;  for  a definition, see QFIT) is found, then all variable
parameters are changed at  once  to  move  a  little  ways  along  the
gradient  towards  a  minimum in the AF.  It seems to be the method of
choice when you are a long ways from any local minimum in the AF.

     The brute force fit minimizes the AF for each parameter, one at a
time.   It works well if you are within striking distance of a minimum
in the AF.

     If you wish to halt  iteration,  type  a  single  control-C;  the
program  will then stop at the next reasonable chance and preserve the
current model.  In addition, if you  are  running  interactively,  the
program   will  ask  you  if  you  want  to  continue  iterating  when
approaching a local minimum.

MODELFIT                                                          Page 2


     Underlying model:  It is possible to in effect subtract  a  model
from  your data and fit the remainder.  This is useful in at least two
cases.  First, if you have  a  hybrid  map  which  fails  to  fit  the
shortest  baselines,  this  program  will  allow you to modelfit a few
components for the largest structure while using  the  inner  part  of
your  hybrid  map  for the smallest structure.  Second, if you wish to
measure the size of, say, the core in your map, you may  subtract  the
core  from your map and the modelfit with a single component.  This is
much better than counting contours from a plot!

     Model files are text files that can be typed or printed directly;
they  can  be  modified or created using the standard text editors.  A
model file consists of one line for each component of the model,  with
up to 7 numbers on each line (in free format):

     1.  Component flux density (Jy)

     2.  Distance of center of component  from  origin  (milliarcsec),
         "radius"

     3.  Position angle of center of component  with  respect  to  the
         origin (degrees, North through East), "theta"

     4.  Major axis of component (milliarcsec), "axis"

     5.  Axial ratio (minor/major, i.e.  < 1), "ratio"

     6.  Position angle of major axis (degrees, North  through  East),
         "phi"

     7.  Type:
             0 or 1:  elliptical Gaussian  (major  axis  is  FWHM)  or
             delta-function (major axis = 0)
             2:  uniform elliptical disk (major axis is diameter)
             3:  optically thin spheroid or tapered disk  (major  axis
             is diameter)
             4:  elliptical ring (major axis is diameter)
             5:  line (major axis is length)


For Gaussians and  delta-functions,  the  Type  can  be  omitted;  for
delta-functions, the major-axis, axial-ratio, position- angle and type
can be omitted.  (Not all the  programs  understand  types  2-5.)  The
"origin" is an arbitrary phase-reference point.  The maximum number of
components varies from 600  to  10000,  depending  on  the  individual
program.


History

Version 1:  1981 Jul 28 (Richard S. Simon).
Version 7.0 1983 July 7:  Extend to include MERGE-format 2  files  and
                            orbiting  VLBI  stations;  increase  to 15
                            stations (D.L.  Meier).
Version 8:  1985 Feb 7 [TJP] Fix bug found by VMS4.0:   attempting  to

MODELFIT                                                          Page 3


                            read SYS$OUTPUT.
Version 9:  1988 Apr 18 [TJP] Convex version.
Version 10:  1989 Mar 29 [TJP] Improved performance (vectorized).
Version 11:  1990 Nov 5 [TJP] Fix serious  bug:   it  would  give  the
                            wrong   answer   on  files  containing  an
                            incomplete set of baselines or triangles.