ATCA Reference Pointing
IntroductionReference pointing involves performing a pointing pattern on a strong, compact source to update the antenna pointing models. Reference pointing is useful for measuring accurate fluxes, mosaicing or polarization at frequencies of 8GHz and higher. In good weather, reference pointing is recommended for all 12mm, 7mm, and 3mm observations.
This web page describes how to schedule, set-up and monitor reference pointing.
Reference pointing modesReference pointing is usually be performed in "self-cal" mode. In this mode, all the antennas perform the pointing pattern at the same time. This has the obvious advantage of being much faster, but (perhaps surprisingly) is also more accurate. However, self-cal mode requires at least four antennas to work.
If only four (or fewer) antennas are available, reference pointing must use a "holography-style" reference pointing system, in the pointing pattern is done for each antenna in turn. This works with a minimum of two antennas, but is slower and less accurate than self-cal mode pointing.
How often to point?The pointing errors will usually be different for sources at different azimuth and elevation. Corrections will become obsolete as sources move, and as environmental conditions change. As a general rule, determine pointing corrections every hour.
What is the usable radius of the pointing solution?At most elevations, pointing corrections can be used for target sources up to about 20 degrees away from the pointing source with no appreciable decrease in accuracy. Below 30 degrees elevation, and above 70 degrees elevation, try to keep the pointing source within 10 degrees of the target.
Reference pointing will not work very well for sources within a few degrees of zenith. Further, the pointing corrections will become obsolete very quickly because the azimuth rate is comparatively large near zenith.
The original pointing software was not sophisticated enough to store multiple pointing solutions, making reference pointing very inefficient for snapshots of isolated sources. In March 2007, a new pointing keyword was added in sched, OFFPNT, which is like OFFSET, but instead of selecting the most recent pointing solution, it selects the one nearest on the sky to the current position (in Az/El). This should allow you to do, e.g., reference pointing for snapshot observations or observations of several sources with large separation. The search is currently limited to at most 2h in time and 25 degrees in distance. These numbers could be changed (or made user selectable) in reponse to observer feedback.
How accurate will the pointing be?This depends on many parameters - the source strength, atmosphere, wind, frequency, azimuth/elevation, distance to target, global pointing solution accuracy, and antennas used, to name the things we know about! Probably 5 arcseconds in good conditions. This is a substantial improvement on the global pointing solution, which can easily be 20 arcsec out. See the reference section for more information.
Configuring Reference Pointing - OverviewThe ATCA reference pointing implementation requires the user to specify configuration settings in several programs.
The major steps required to configure reference pointing are:
- Set up pointing scans and offset pointing in SCHED.
- Check that CATAG is running.
- Set the pointing mode, IF(s), and number of cycles/point in CAOBS.
- If using a maser pointing source, select the channel range in CACOR.
To determine pointing corrections:Schedule an observation of a pointing calibrator. The observation must have SCTYPE=POINT, and will usually have POINTING=UPDATE at 3mm and POINTING=REFPNT for other bands. This scan will cause CATAG to determine the pointing offsets, and load a new pointing solution into the ACCs. It is recommended that you give your pointing calibrator a source name such as "pntcal1", rather than the IAU name, so that it is easy to separate pointing and phase calibrator data when you come to process it.
Set the SCAN length to 5 minutes. SCHED will probably warn about scans not being long enough regardless; ignore it. Unless you expect a particularly long drive time, 5 minutes is fine. CAOBS will move to the next source in the sched file as soon as the pointing is complete.
IMPORTANT: For pointing scans, you should set AVERAGING=1 in SCHED.
To use the new pointing solution for a target source:Schedule the target source with SCTYPE=DWELL (or unset it, as the default is DWELL) and POINTING=OFFSET.
Be sure to check when the scan is completed that it has successfully derived and applied a new solution (most easily done by checking the /point page in CAMON), and check that the solution is acceptable. If there are corrections larger than 10 arsec, you may wish to run another pointing scan.
CATAG was once exited/restarted for each observation, but it now usually runs continuously. Before observing, you should check that the CATAG program is running on xbones (or skull). In the control room at Narrabri, the CATAG window traditionally has a light blue background and is located behind the SPD display on the left-hand-most screen. Alternatively, check the /CATAG (or equivalently, /POINT) page on CAMON. If the CATAG status is really "NOT RUNNING" (note that the status may initally be reported as "NOT RUNNING" but within 10 seconds will change to "operational" if CATAG is running), start CATAG by
(ii) Point_antennaThe parameter point_antenna sets the antennas to be used in the POINT pattern. Clearly, as there is no 3mm receiver on CA06, when observing at 3mm, this should be set to
CAOBS> set point_antenna 12345.
(In this case you would usually detach CA06 before starting observing.) This may also apply at other bands in compact configurations if CA06 is not being used. If switching to 7mm or 12mm from 3mm, or taking over from a 3mm observer, it may be necessary to set
CAOBS> set point_antenna 123456. Note that there is at present a known bug within CAOBS which results in the set point_antenna command being accepted but not being applied at the next pointing scan. This is being investigated, however, in the meantime, the advice is to execute the set point_antenna scan while CAOBS is tracking, and to carefully check the \POINT or \CATAG page in CAMON when the pointing scan starts.
(ii) Point_patternThe parameter point_pattern in CAOBS has two functions; it sets the number of cycles to spend on each sub-scan of the POINT pattern, and it sets the pointing mode.
With point_pattern +ve, self-cal mode is used. To select holography-mode pointing (e.g., if four or fewer antennas are available), set point_pattern to be negative in CAOBS:
CAOBS> set point_pattern -#
where# is the number of cycles to spend on each sub-scan.
CAOBS> set point_ifflag 12
would select XX and YY for the first frequency. Note that, currently, if you select CAOBS> set point_ifflag 1234 but there is a difference in frequencies between IFs of 10% or more, CATAG will default to using "point_ifflag 12".
Other than setting point_antenna, point_pattern and point_ifflag, a schedule containing reference pointing scans is run pretty much like any other schedule.
IMPORTANT: In order to start a reference pointing scan, you must use start, not track, in CAOBS.
IMPORTANT: Use at least two cycles/point. For some reason, using one cycle/point intermittently fails.CACOR and SPD.
Track the maser which is to be used for pointing. In SPD, look at the spectrum of the maser, as a function of channel:
SPD> mode b
SPD> arr 12345 (or as appropriate)
SPD> sel aa bb cc dd (or as appropriate)
SPD> x (if necessary to change from frequency to channels on the x-axis)
Note in which channels the maser appears. Then in the CACOR command-entry box:
CACOR> tvch 14 16 14 16
Where, in this case, the maser line is present in channels 14, 15 and 16.
Resetting reference pointingIt is possible to get a bad reference pointing solution into the system. At this stage, you are likely to want to backtrack to either a previous good solution or to the global solution. The commands within CAOBS (historically, on the VAXes this was a separate program) are
CAOBS> ppfix nwill restore the pointing scan results from CATAG record (i.e., PNTCOM number) n, and
CAOBS> ppglobalwill restore the global pointing solution into the ACCs, rewriting pparams.dat.
Other Links:The /CATAG page on CAMON - The /CATAG CAMON page shows the current pointing offsets and reference pointing results.
Reloading the global pointing parameters - For when you're done, or if you muck up, it's easy to go back to the global pointing solution.
Antenna pointing patterns - A list of the azimuth and elevation offsets applied for each point in the selfcal and holog pointing patterns.
The diagnostic line - Explanation of the columns of data produced by CATAG; useful for monitoring the pointing.
Holography mode pointing in action - "You can really see the difference!"
References - Collection of old and archival pages and papers.
Original: Dave Rayner (15-Mar-2001)
Modified: Bob Sault (15-Mar-2005), Phil Edwards (21-Oct-2008)