Re: DIFCAL

[<aah@nofs.navy.mil>]

Michael is right.  Difcal is the differential photometry
part of my Mark III pipeline code.  Full documentation and
Fortran code for these programs can be found at
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/tass/sources
This is a complete photometric reduction suite; I am surprised
no one looked at this for creating a pipeline for the Mark IV.
Difcal does the following things to a single starlist:
  (1) reads a master starlist.  This is generated by master,
      and contains the mean V&I magnitudes for all stars.  You
      do this step before doing differential photometry.
  (2) reads the input starlist.  This is a merged file generated
      by collate that includes V&I for every located star.
      That merged file is output from a previous part of the
      pipeline, and includes on each line (record) the ra and
      dec for the star, the hjd for each of the three cameras,
      the magnitude for each camera and the magnitude errors.
  (3) Only stars with at least V and one I measure are accepted.
  (4) Input stars are matched against the master starlist.
  (5) For each star in the input file, neighbors are found and
      a differential offset with respect to those neighbors is
      calculated.  Up to 100 neighbors can be used.  This is
      the heart of ensemble differential photometry.
  (6) For each input star, a line is written to the output file
      that contains the index number of the corresponding star
      in the master star list (the 'name' of the star), the ra and dec
      from the master star list, the mean hjd of the current observation,
      the star's V and I magnitudes, 4 errors, the number of comparison
      stars used in the calculation, and the name of the starlist file.
      The 4 errors are: Vsig/Isig for first pass; Vsig/Isig for second
      pass, where on the second pass the comparison stars with the
      largest deviation from the mean were eliminated.
 Arne