Description

fixvis recalculates (u, v, w) in an MS based on antenna positions, time and source position. fixvis can also change the phase center of visibilities.

If the phase center is changed, the corresponding modifications are applied to the visibility columns given by the parameter datacolumn which is by default set to all (DATA, CORRECTED, and MODEL).

ALERT: fixvis uses the small angle approximation and may be incorrect for large phase shifts. This may result in sources shifting position if large phase shifts are being applied (shifts up to a few beam sizes have been reported). Please use tclean for phase center shifts during imaging when applicable.

See also the description in section UV Manipulation

 

Parameter descriptions

vis

Name of the input visibility MS. 

outputvis

Name of the output MS. Default: same as vis

field

The field selection string. Default: '' = all.

refcode

The UVW coordinates are calculated based on a celestial position, the phase direction of the given field. This direction has a reference frame. If that frame is not the desired one, one can use the parameter refcode to convert to a different frame when the UVW coordinates are recalculated. Default: the refcode of PHASE_DIR in the FIELD table. Example: refcode='B1950'

reuse

If True and the given refcode is different from the original, the present UVW coordinates are just rotated to the new refcode. If False, the coordinates are re-calculated from the phase direction. Default: True; The parameter is ignored when the parameter phasecenter is set.

phasecenter

If set to a valid direction: change the phase center for the given field to this value. Example: phasecenter='J2000 9h25m00s -05d12m00s'. If given without the equinox, e.g. '0h01m00s +00d12m00s', the parameter is interpreted as a pair of offsets in RA and DEC to the present phase center. 

NOTE: The RA offset can be given in units of time or angle. If given as a time (i.e. as a single number with a time unit as in, e.g., 12s or in the XXhXXmXXs or XX:XX:XX.XXX formats), it is applied as is. If given as an angle (e.g., 0.01deg), it is divided by the cos(DEC) before it is applied.

distances

(experimental) List of the distances (as quanta) of the fields selected by field to be used for refocusing. If empty, the distances of all fields are assumed to be infinity. If not a list but just a single value is given, this is applied to all fields. Default: [ ].  Examples: distances=['2E6km', '3E6km'], distances='15au'

datacolumn

When applying a phase center shift, modify visibilities only in this/these column(s). Default: 'all' (DATA, CORRECTED, and MODEL). Example: 'DATA,CORRECTED' (will not modify MODEL).