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 several bugs (as of March 2014), and cannot be used to phase rotate a measurement set properly.

The compilation of chgcentre is very similar to wsclean's compilation. See the general installation instruction for help. The chgcentre tool is available as a separate download. See the files section for downloading chgcentre.

Execute chgcentre without parameters to get help on the syntax:

$ chgcentre
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

When a measurement set contains multiple data columns (e.g., DATA, MODEL_DATA, CORRECTED_DATA), each column will be updated.

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

Finally, you can provide option '-shiftback' to prepare the measurement set for w-snapshot imaging with WSClean, using the recentring technique. See the w-snapshot algorithm page for more info. You can specify any phase centre with -shiftback, but the common way to do this would be:

$ chgcentre -minw -shiftback myobs.ms