NAME:
  linkobj
 PURPOSE:
  Cross check three source lists from one field and identify moving objects.
 DESCRIPTION:

 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  linkobj,tag,fna,fnb,fnc,CANDIDATE=cand,NODISPLAY=nodisplay
 INPUTS:
  tag - name of the field, should not contain any blanks
  fna - file name for the earliest image (Frame A)
  fnb - file name for the middle image (Frame B)
  fnc - file name for the last image (Frame C)
 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:

  BADCOL    - Array of column numbers, any source found within 2.5 pixels of
                these columns will be discarded.  Default = no bad columns.

  CANDIDATE - Flag, used for testing, when set will plot all detected sources
                in all images.  If you think a triplet should have been picked
                up, set this to see if all three images were actually in the
                original source list.

  KEYLIST=  - Name of a file containing a correspondence list. This list
                 associates a set of standard names with the actual keyword
                 names found in a FITS file header. If this keyword is
                 omitted, a default list is used, as if a file with the
                 following contents had been supplied:
                  AIRMASS   K  AIRMASS
                  DATE      K  DATE-OBS
                  DATETMPL  T  DD-MM-YYYY
                  EXPDELTA  V  0.0
                  EXPTIME   K  EXPTIME
                  FILTER    K  FILTERS
                  FILENAME  K  CCDFNAME
                  OBJECT    K  OBJECT
                  UT        K  UT 
                 The middle column is a flag. It may be K, for Keyword,
                 T, for Template, or V, for Value. If it is V, the contents
                 of the third field on that line should make sense for the
                 name in the first field.

  MAXRATE   - Maximum motion rate permitted for a tripet, in pixels/hour.
                The default is 50.0 pixels/hour.

  MINRATE   - Minimum motion rate permitted for a tripet, in pixels/hour.
                The default is 1.0 pixels/hour.

  NODISPLAY - Flag, when set will suppress all image display allowing program
                to be run in background or batch mode.  This will be somewhat
                faster as well.  The display steps take a small but non-trivial
                amount of time.

  PATH      - Optional path for original image directory.
                If not specified, the current directory is used.

 OUTPUTS:

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
   98/03/11, Written by Marc W. Buie, Lowell Observatory
   99/03/22, extracted frmdxdy.pro