MERGEFITS


MERGEFITS converts a standard MERGE-format VLB  visibility  file  into
AIPS-compatible  FITS  format.  The FITS format for visibility data is
defined in "An extension of FITS for groups of small arrays  of  data"
by  E.W. Greisen  and  R.H. Harten,  Astr.   Astrophys.   Suppl.  v44,
pp371-374 (1981).  The disk  file  produced  by  MERGEFITS  should  be
copied  to  tape following the procedure described below.  The dataset
can be read from tape or disk into AIPS using the AIPS task UVLOD.  It
can  be translated back into MERGE format with FITSMERGE.  FITS-format
tapes and disk files can be examined with program FITSLIST.


Example

$ MERGEFITS
INPUT  = "0108.MRG"
OUTPUT = "0108.UVF"
STOKES = -1
OBSERVER = "PEARSON"
WTPOWER = 1
TAI-UTC = 17
/


Parameters

Parameter INPUT must be specified; the others are optional.  Parameter
names may be abbreviated.  End the list of parameters with a slash (/)
or end-of-file.

INPUT = "filename" :  the name of the input MERGE-format  file,  which
          must exist.

OUTPUT = "filename" :  a name for the new FITS-format file.   If  this
          is  not  specified, a filename of the form xxxx.UVF is used,
          where xxxx is the first part of the input file name.

STOKES  =  n  :   as  the  MERGE-format  files  do  not  specify   the
          polarization  of the data, but AIPS does, you should specify
          the polarization with this parameter.  Possible  values  for
          circularly-polarized  feeds  are:  -1 (RR), -2 (LL), -3(RL),
          and -4 (LR).  See the AIPS  programmer's  manual  for  other
          options.   The  default  is  STOKES=-2  as  most VLB network
          experiments are observed in LCP.

OBSERVER = "name" :  may be used to give the  name  of  the  observer.
          The  default  is  the  login  name  of  the  person  running
          MERGEFITS.

USERNO = n :  may be used to specify the AIPS user number, if desired.
          This is rarely if ever needed.

IMNAME = "name"

MERGEFITS                                                         Page 2


IMCLASS = "class"
IMSEQ = n :  these three parameters can be specified  to  indicate  to
          UVLOD  what  the  AIPS  filename should be.  If they are not
          specified, their values are taken from the file  name,  file
          type,  and version number of the input file.  You can change
          these when you run UVLOD if you wish.

WTFACTOR = f
WTPOWER = p :  these parameters control how MERGEFITS computes weights
          for  each data point from the amplitude error.  The computed
          weight is  WTFACTOR/(err**WTPOWER)  or  1000,  whichever  is
          smaller.   If  WTPOWER  =  0 all points are equally weighted
          with weight given by WTFACTOR.  The defaults  are  WTPOWER=1
          and WTFACTOR=1.

TAI-UTC = m :  specifies the Delta-AT correction, the  difference  TAI
          minus UTC, where TAI is International Atomic Time and UTC is
          Universal Coordinated Time.  The time-tags on the data  from
          in  the  input  file are UTC, while those on the data in the
          FITS file are TAI, as required by AIPS.  The  default  value
          used by MERGEFITS is the correct value (see table on page K9
          of  the  Astronomical  Almanac),  but  you  can  provide   a
          different value in order to modify the time tags.


AIPS weights and error bars

The parameters WTPOWER and WTFACTOR  are  used  to  convert  amplitude
error bars into the "weights" used by AIPS:

weight = WTFACTOR/(err**WTPOWER)

For importing data to AIPS, you should usually set  WTPOWER=2  (weight
proportional  to inverse variance) and adjust WTFACTOR to give weights
if a convenient size.  Most AIPS  programs  do  not  (usually)  attach
meaning  to  the  absolute  values of the weights, only their relative
values.  For many applications,  WTPOWER=1  gives  a  more  reasonable
range of weights.

     The values of WTPOWER and WTFACTOR actually used are recorded  in
the  header  of  the FITS file.  This enables FITSMERGE to reconstruct
the errors from the weights, unless WTPOWER=0 was used.


Copying a FITS-format file to tape (VMS)

Use the following procedure, or a similar one:
$ ALLOCATE MTA0:
(mount your tape on MTA0:)
$ MOUNT/FOREIGN/DENSITY=1600/BLOCK=2880/RECORD=2880
$ COPY 0108.UVF MTA0:
$ COPY 3C273.UVF MTA0:
...
$ DISMOUNT MTA0:
$ DEALLOCATE MTA0:

MERGEFITS                                                         Page 3


Limitations (some of these may be lifted in due course)

1.  Up to 40 stations are accepted.
2.  Deleted data are not entered in the output file.
3.  Closure phases, if present, are not entered in the output file.
4.  The (u,v,w) coordinates are not precessed back to  1950,  so  AIPS
will  make  maps in RA/Dec of the epoch of observation, not of 1950.0.
This can affect position angles near the north pole.
5.  The output FITS file is sorted in 'T*' order, and it will  usually
have to be resorted using the AIPS task UVSRT before it can be used in
AIPS.


History

Version 1.0:  1984 Mar 19 - new program (TJP).
Version 2.0:  1986 Apr 22 - use FITS format for antenna table.
Version 2.1:  1988 Jul  9 - higher precision in time tags; add weight
                            and TAI-UTC parameters; correct time
                            calculation.
Version 2.2:  1988 Jul 20 - change sign convention for phase, uvw.
Version 2.3:  1988 Nov 22 - force IMNAME etc to upper case.
Version 2.4:  1989 Feb 14 - correct error in SORT ORDER; reduce amount
                            of history.
Version 2.5:  1990 Feb 6 - put additional info in "CIT" keywords.
Version 2.6:  1990 May 22 - use correct TAI-UTC by default; record
                            B1950.0 position in header.
Version 2.7:  1991 Feb 13 - increase to 30 stations.
Version 2.8:  1991 Apr 22 - increase to 40 stations.
Version 2.9:  1991 Aug 11 - add WT keywords for fitsmerge; default
                            WTPOWER=1.