Reference pointing at the Compact Array

Reference pointing is now available at the ATCA. This involves using a nearby calibrator to make adjustments to the global pointing model to increase the pointing accuracy of the target observations.

What can you expect?

Tests carried out in June 1995, indicate that for reference pointing by 1934-638 on itself at C-band (4800 MHz), an rms pointing error of 1 arcsecond can be achieved in both azimuth and elevation.

The worst errors seen on 1934-638 using the same parameters as above showed that over a 3 hour period over dawn, the global model drifted 15" in azimuth and 5" in elevation. During this period, simulated reference pointing corrections would have held this data to an rms of 3".

Tests done on 1610-771 using 1549-790 as a reference (a separation of about 2.2 degrees) obtained pointing accuracies on 1610-771 of 5" rms at an elevation of 20 degrees during sunrise. The C-band fluxes of these sources are approximately 3 Jy (1610-771) and 5 Jy (1549-790).

Without reference pointing, the global model is good to 5-10" rms at the time of the solution, but degrades with time (mainly due to thermal effects). At other times, the rms is typically 20-30" but up to 60" absolute errors have been seen.

Because the reference pointing only moves the antenna pointing, leaving the phase centre unchanged, it is possible to use reference calibrators as phase calibrators. At this stage no tests have been done on data taken through off-line processing. We would recommend a separate phase calibrator scan (it may still be the same source) be used in addition to the reference pointing.

Specifying reference pointing in SCHED

In SCHED, there are two relevant fields: SCTYPE and POINTING. Each scan will normally have SCTYPE set to DWELL, FREQSW or MOSAIC, but there is also an option POINT. This option causes CAOBS to execute a pointing pattern on that particular source.

The other field ( POINTING) is always set to GLOBAL for normal observations. The other options are OFFSET and UPDATE (see the POINTING field documentation for details).

A few other notes:

You should set the AVERAGING field to 1 for sources that use POINTING=UPDATE.
The reference pointing offsets are governed by the first frequency.
You should use 128MHz bandwidths (that is RFI free!)

Observing with reference pointing

When you go to use the reference pointing, you should ensure that you are running CATAG. If you need to start it, use:

 $ run at$run:catag

CATAG must be run from NOEL, the main observing computer. If you get an error message such as

%DCL-W-ACTIMAGE, error activating image PNTCOM
-CLI-E-IMAGEFNF, image file not found [SYS.SYSCOMMON.][SYSLIB]PNTCOM.EXE;

then you are not running CATAG on NOEL. If the error persists (even on NOEL) then get some help from support (see the CATAG documentation for more information).

When it starts you will be asked which IFs you want to use for the pointing calculations.

What CATAG does is to watch the data coming in, and if it sees a POINT pattern performed on a source labelled update, then it will transfer the measured offsets from the POINT pattern into the global solution. Once that is done, it immediately loads them into the ACC.

Users can control how long they spend on a POINT pattern. Each POINT pattern is made up of six sub-scans. These correspond to the collection of data with the antenna at different offsets. These are:

  sub-scan 1     0     0
  sub-scan 2    +az    0
  sub-scan 3    -az    0
  sub-scan 4     0    +el
  sub-scan 5     0    -el
  sub-scan 6     0     0

Each sub-scan will be the same number of integration cycles. This number is specified in CAOBS before observations commence using the command:

  CAOBS> set point #

where # is the number of cycles per sub-scan. A typical value is 3, the minimum is 2.

The ammount of the offset is governed by the first frequency.

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 POINTING=UPDATE for that scan.

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 (expert mode).

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 stage it is up to the (expert) user to determine this.

Please remember to reload the global parameters when you finish.

References

D. J. McKay CATAG - Compact Array reference pointing program
D. J. McKay, ATCA pointing calibration, WWW document, 28 June 1995.
R. J. Sault, Accuracy of Reference Pointing, 12 September 1995.
D. McConnell & R. Wark, Pointing Calibration of Compact Array Antennas, Progress Report, AT/31.6.7/019, 8 June 1993.
D. J. McKay, ATCA: SCHED and associated scheduling programs, WWW document, 20 March 1995.
D. J. McKay, Pointing Techniques for Radiotelescopes, M.Sc thesis, University of New South Wales, 1997.

Original: dmckay (23-May-1996)
Modified: dmckay (4-Mar-1997)