chgcentre

The ‘chgcentre’ tool can be used to change the phase centre of a measurement set. It will recalculate the uvw-values (from the antenna locations, phase centre and time) and phase-rotate the visibilities. We found that the casa task ‘fixvis’ has a bug (as of March 2014) that causes it to malfunction for some arrays (e.g. MWA, LOFAR).

Since wsclean 2.10, chgcentre is compiled and installed when wsclean is installed. See the general installation instruction for help.

Execute chgcentre without parameters to get help on the syntax:

A program to change the phase centre of a measurement set.
Written by André Offringa (offringa@gmail.com).

Syntax: chgcentre [options] <ms> <new ra> <new dec>

The format of RA can either be 00h00m00.0s or 00:00:00.0
The format of Dec can either be 00d00m00.0s or 00.00.00.0

Example to rotate to HydA:
        chgcentre myset.ms 09h18m05.8s -12d05m44s

Some options:
-geozenith
        Will calculate the RA,dec of zenith for each timestep, and moves there. This make the set non-standard.
-flipuvwsign
        Flips the UVW sign. Necessary for LOFAR, for unknown reasons.
-minw
        Calculate the direction that gives the minimum w-values for the array.
-zenith
        Shift to the average zenith value.
-only-uvw
        Only update UVW values, do not apply the phase shift.
-shiftback
        After changing the phase centre, project the visibilities back to the old phase centre. This is useful
        in WSClean for imaging with minimum w-values in a different projection.
-f
        Force recalculation, even if destination is same as original phase direction.
-datacolumn <name>
        Only phase-rotate the visibilities in the given column. Otherwise, the columns
        DATA, MODEL_DATA and CORRECTED_DATA will all be processed if they exist.
-from-ms <ms>
        Rotate the measurement set to the same direction as specified
        in the provided measurement set.

When a measurement set contains multiple data columns (e.g., DATA, MODEL_DATA, CORRECTED_DATA), each column will be updated (as long as they have a standard name).

If you do not provide a new RA and dec, chgcentre will give you some info about the measurement set:

$ chgcentre myobs.ms
A program to change the phase centre of a measurement set.
Written by André Offringa (offringa@gmail.com).

Current phase direction:
  -00h59m31.7s -16d46m19s
Zenith is at:
  -01h00m00.8s -26d46m44s
  (-01h00m49.0s -26d46m44s - -00h59m12.7s -26d46m44s)
Min-w direction is at:
  -00h59m31.7s -26d46m19s

You can specify ‘-zenith’ or ‘-minw’ as option to rephase to the local array zenith or the direction orthogonal to the best-fit plane to the antennas. The latter is close to zenith, but provides slightly lower w-terms. This has not been tested on telescopes other than the MWA. The syntax for this is:

$ chgcentre -minw myobs.ms

This can be used in combination with WSClean’s -shift parameter for w-snapshot imaging. In that case, the original phase centre should be specified with the -shift parameter. The net effect is that the measurement set is phase rotated to the sky direction with minimal w-terms, and shifted back along the tangent plane to the direction of interest.

In WSClean 3.0, this approach replaced the -shiftback option of chgcentre for shifting the visibilities along the tangent plane. See the w-snapshot algorithm page for more info.

Legacy data with -shiftback applied

Before WSClean 3.0, it was possible to prepare w-snapshotting with the -shiftback option, e.g.:

$ chgcentre -minw -shiftback myobs.ms

A shifted measurement set uses wsclean-specific keywords. Support for this was removed in WSClean 3.0 (see the changelog for details), and any observation for which these shifting keywords are detected will produce an error in WSClean 3.0.

In case archival data to which -shiftback is applied needs to be imaged with WSClean 3.0, the option should be undone. A shifted measurement set can be restored by phase rotating it to its original RA/dec: chgcentre will detect the keywords in the measurement set, undo the shift and update the keywords.

A LOFAR bug

For unknown reasons, the uvw value needs to be flipped for LOFAR sets. As far as I know, this is not necessary for other telescopes, but LOFAR requires you to specify -flipuvwsign.