Australia Telescope Compact Array

Additional information on reference pointing

This page contains additional information which all users of reference pointing should read, but most won't.

1. SCHED - Additional information

The pointing source may be either a continuum source, or a strong maser. Sources stronger than 1 Jy (average flux density over the bandpass) are preferred, except at 3mm, where 5 Jy is more like it. See the ATCA calibrator search page.

There are two POINTING types which can be used for reference pointing:

UPDATE determines the pointing offsets with respect to the currently installed pointing solution.
REFPNT causes the global pointing parameters to be reloaded prior to determining the pointing offsets.

Both UPDATE and REFPNT are equivalent if the previous scan was POINTING=GLOBAL eg. if you have just finished setup.

When reference pointing at 3mm, it is possible that the global solution will be so bad that the reference pointing will not work. For this reason, it is preferable to use POINTING=UPDATE at 3mm, so that once you have the pointing approximately correct, you can continue to reference point.

WARNING! The problem with using UPDATE is that if the pointing gets badly corrupted, all further pointing solutions will fail, and the corrupted solutions will be retained. For this reason it is advised to use UPDATE only at 3mm. Also, KEEP A CLOSE WATCH ON YOUR CALIBRATOR AMPLITUDES!!

WARNING! There is currently a bug in CATAG, in that if you perform a REFPNT and then immediately after do another REFPNT, it will not reload the global solution.

In the even of solutions getting corrupted, reload the global pointing parameters.

A few other notes:

SCHED doesn't know how to calculate the drive time for pointing observations, and sets it to zero.
CATAG doesn't handle bin-mode observations.
Unless you are pointing at a maser with a line-width significantly narrower than 8MHz, use CONFIG=FULL_128_2.
When pointing on a continuum source, use CONFIG=FULL_128_2.
When pointing on a continuum source, make sure the dual frequencies are adjacent or the same. The reference pointing offsets are governed by the first frequency.

2. CATAG - Additional information

What the CATAG program does is to watch the data coming in, and if it sees a POINT pattern performed on a source, it will collect the data and evaluate the pointing offsets. If the scan has POINTING type UPDATE or REFPNT, then it will add the measured offsets, and create a new set of pointing parameters. It then loads the new pointing parameters into the ACCs.

CATAG now requires no direct user interation, and runs the whole time. Check the /CATAG (or equivalently, /POINT) page on CAMON to make sure it is running.

If you start CATAG, but when you do so it exits with the message

Checking for other CATAG processes...
ERROR: CATAG heartbeat detected.
Another CATAG process must still be about.
Abort

then someone else is running CATAG. Instructions on how to remove a CATAG session are available here.

CATAG also produces a diagnostic line for each integration.

3. CAOBS - Additional information

Each self-cal mode POINT pattern is made up of 6 sub-scans, and each holography-mode POINT pattern is made up of 11 sub-scans. The final sub-scan is used as a marker to tell CATAG that the pointing pattern is complete.

You can restrict the antennas which CAOBS uses for the solution using point_antennas.

CAOBS> set point_antennas 12345

would use only antennas 1-5.

WARNING! There is currently a bug in the CAOBS/CATAG antenna selection; if you don't want to use an antenna for pointing, you should stow it! If you are paranoid, set point_antennas as well.

4. CACOR - Additional information

Because the channel display range specified in CACOR is used for all observations, complications will arise if you have several reference pointing maser sources in the schedule, or if you are mixing maser and continuum sources. The only way to get around this without significant loss of sensitivity is to use a spectral line correlator configuration for the pointing sources, and carefully select the center frequency so that the maser fills the central 50% of the passband (the default CACOR channel range).

Original: Dave Rayner (2001)
Modified: Bob Sault (6-Sep-2003), Phil Edwards (21-Oct-2008)