The Compact Array pointing system

The pointing system provides capability of

automated pointing parameter resetting,
automated rough pointing reduction,
improved handling of global pointing solutions,
automatic log keeping, and
reference pointing

To implement this there are programs and log files. The programs are CATAG In its final incarnation it will incorporate the subprograms PPLOAD and POINT_PROCESS.

Reference to global memory is that of PNTCOM, a global common holding pointing data. Like SACCOM and OBSCOM it is a DEC FORTRAN global memory, however it is only propagated on the node NOEL:: rather than the entire cluster. This means that the programs which use it must be run on that node.

Executable code is kept in AT$RUN. If there is no copy there, then OBS$1:[CATAG.WORK] will have it. Source code for the programs is kept in OBS$1:[CATAG.CODE.REF] under CMS for the PNTCOM and CATAG modules. The PNTCOM and CATAG software has extensive internal commenting and documentation.

To run CATAG, use $ run at$run:catag from a NOEL prompt. CATAG must be run on NOEL (Programmers note: it must also be linked on NOEL, but compiled on LEON because NOEL doesn't have the compiler and LEON doesn't have the global common)). On starting CATAG, the user will be asked which IFs are to be used for pointing calculations. With the exceptionof reference pointing, the default (all IFs) is fine.

Global pointing files are kept in:

  AT$ANTENNA:CA01_POINTING.DAT
  AT$ANTENNA:CA02_POINTING.DAT
  AT$ANTENNA:CA03_POINTING.DAT
  AT$ANTENNA:CA04_POINTING.DAT
  AT$ANTENNA:CA05_POINTING.DAT
  AT$ANTENNA:CA06_POINTING.DAT

and are transferred into the "current" parameters file using CAIN. The standby-ready-to-go pointing parameters are kept in:

  AT$ACC:PPARAMS.DAT

and are loaded using CAIN. The log of all pointing solutions is kept in:

  AT$LOG:PPARAMS.LOG

Individual reference pointing solutiosn are not currently logged.

I will now outline the various procedures used in the basic pointing operations.

Zeroing the pointing

Login as NOEL::OBSERVER. Ensure that RECONFIG.COM has completed and the new antenna positions are in the system. Watch out for duplicate entries in the file AT$LOG:CONFIG_HIST.LOG, which is where PPLOAD gets its previous configuration stations from.

$ set def OBS$1:[CATAG.WORK]
$ run ppload
    i         (initialise the pointing)
    at$acc:pparams.dat   (this is the default)

(PPLOAD will now zero the pointing for antennas that have moved and write out the new PPARAMS.DAT file and global pointing files)

$ cain
    e         (expert mode)
    y         (confirm)
    p         (load pointing parameters)
    y         (default - use the new global values)
    *         (all antennas)
    y         (do the initialisation)

Rough pointing

On xbones, ensure CATAG is running, then

CAOBS> set file setup_1934   (or any other appropriate sched file)
CAOBS> set point 2           (2 is fast and it is, after all, only rough)
CAOBS> set point_if 1234     (ensures that you are using all the antenna IFs)
CAOBS> set point_antenna 123456 (ensures that you are using all antennas)
CAOBS> point 3               (executes a 6cm pointing pattern)

When the scan has finished, start tracking on your calibrator again then on an xbones Xterm

xbones:~$ ppload
         r            (rough pointing)
         at$acc:pparams.dat   (this is the default)
         accept corrections > 10" for antennas that have moved

This will update /atomsexport/caobsexport/caobs_pparams/ca0#_pointing.dat
Then, in caobs

CAOBS> ppglobal

Iterate through this sequence until the az errors are < 10" on all antennas

Fine pointing

Create a pointing schedule using the SRCPAC. Select about 15 sources, getting _good_ AZ/EL coverage. Make sure CATAG is running... and remember that CATAG will only work on 4 or more antennas. If the array has 3 or less - give up!

CAOBS> set file point           (our schedule...)
CAOBS> track 1                  (the first source)

$ cacal                         (calibrate the array)

CAOBS> stop                     (at the end of calibration)
CAOBS> corr closef              (Let's get into the habit of neatness, eh?) 
CAOBS> start 1                  (commence observations)

At this point it is important to make a note of the first PNTCOM record that CATAG writes into (a number from 0-127). We'll need it later.

At the completion of observations make a note of the last PNTCOM record that CATAG wrote into. Process this data using the point_process procedure This produces a modified PPARAMS.DAT style file, written to params.dat i.e. only one P).
Then, on xbones

xbones:~$  cp params.dat /atomsexport/caobsonline/caobs_pparams/pparams.dat
xbones:~$  ppload
          e             (make it a global solution)
          pparams.dat   (again, this is the default)

Then, in caobs

CAOBS> ppglobal

Adding a pparams file to the log

There are times where you may want to add the current AT$ACC:PPARAMS.DAT file to the perpetual pointing log. To do this, use PPLOAD again

$ run ppload
    l         (select log file entry)
    This is the comment    (put in a suitable comment)

Recovering from disaster

If something goes horribly wrong and you want to go back to an old solution, there is a full log of solutions in AT$LOG:PPARAMS.LOG.

$ copy at$log:pparams.log at$acc:pparams.dat
$ edt at$acc:pparams.dat          (Isolate the solution you want)
$ run ppload
    e                (enter the solution into the global one)
    at$acc:pparams.dat

$ cain               (as per above - reload the global solution)

Reference pointing

This is in a somewhat embryotic form. To do reference pointing, you need a source close to your target source. This scan requires the scan type "UPDATE" in order to work. You should not use averaging on this source, nor should you use narrow bandwidths or RFI affected frequencies (although there is some control over this using COR_DISP).

CATAG will automatically pick up the UPDATE scan type and CAOBS will automatically do a pointing pattern on it. Use "set point n" to set the number of cycles CAOBS spends on each offset.

At the conclusion of the pattern, CATAG will automatically determine the offsets, take a median, write a new AT$ACC:PPARAMS.DAT file and DOWNLOAD THE RESULTS TO THE ACC.

If something goes wrong and you need to reload local reference pointing parameters to the ACC, answer "no" to the "Reload global pointing parameters?" question in CAIN.

If you want to get rid of the local corrections, you say "yes" to this question.

NB: CATAG does not detect when you no longer need a local solution. At this satge it is up the the (expert) user to determine this.

Please remember to reload the global parameters when you finish.

Reference pointing has only been used a few times. I would be very keen to be present for the first few attempts.

Interested observers might like to check the other reference pointing documents on this WWW server:

Original: Derek Mckay (28-Jun-1995)
Modified: Robin Wark (13-Jul-2006)