rerefant(vis, tablein, caltable='', refantmode='flex', refant='')¶
Re-apply refant to a caltable
The time-dependent phases for a specified caltable are re-referenced to a new antenna by the specified algorithm.
Fundamentally, the baseline visibility phases measured by a synthesis instrument are exclusively differences (between antennas), and so no absolute phase reference exists. Antenna-based calibration solutions thus have phases which are constrained only to satisfy the observed visibility phases; further, each solution in time is formally independent of all others, though stability (or at least continuity) in time is expected. Therefore, to assert phase continuity in time, it is conventional practice to assign a reference antenna whose phase will be held constant at zero in time. Any actual phase variation associated with the reference antenna will be transferred to the phase solutions of all others (consistent with satisfying the visibility phase differences), and if the time sampling is adequate, phase continuity for all antennas—and thus also for all visibility baselines—will be assured. Usually, the phase reference antenna is applied on the back-end of the calibration solving task, e.g., gaincal or bandpass.
However, it is sometimes the case that the referencing must be reapplied with a different reference antenna choice, or with a different algorithm, e.g., due to dropouts (from visibility flagging) or other problems in the nominally preferred reference antenna. The rerefant task provides a convenient mechanism to achieve this without having to re-run the calibration solving task with only a new refant setting.
An input MS, input caltable, and output caltable are specified in vis, tablein, and caltable, respectively.
Reference antenna choice: refant
Reference antennas are specified using the refant parameter. A list of antennas may be specified in decreasing order of preference. (See below for the conditions under which alternate reference antennas are used.)
Phase referencing mode: refantmode
The refantmode parameter controls how the refant is applied. Currently available choices are ‘flex’ and ‘strict’.
If the preferred or current refant drops out, switch to another. Alternate reference antennas will be chosen from the refant parameter (if a nontrivial list of antennas is specified), or according to proximity to the last-used refant. If and when the preferred refant returns, the referencing will switch back to it. Note that the preferred reference antenna may no longer be at zero at phase with respect to the alternate refant, and so it will return at a new phase value that will be kept constant from that point. Generally, this mode of referencing should ensure continuity at the available phase SNR (signal-to-noise ratio). However, under low SNR conditions, the effective cross-hand phase may not be assured stable at refant changes, since each hand of polarization is referenced independently, and the non-zero noise on the cross-hand phase difference will change. When doing polarimetry (which depends on stable cross-hand phase), it may be preferable to use refantmode=’strict’ (see below).
If the preferred or current refant is absent for a solution, flag all antennas at that solution. This mode ensures that the effective cross-hand phase will be maintained constant and equal to that of the chosen refant, and not subject to variation due to finite SNR when switching among reference antennas. If a list of antennas is specified in refant, only the first is used. Note that refantmode=’strict’ is not reversible, since the reason for flagging solutions is not preserved. Also note that a poor refant choice could lead to excessive data flagging when using this mode.
To apply a prioritized list of reference antennas using refantmode=’flex’:
rerefant(vis='n5921.ms', tablein='n5921.gcal', caltable='n5921_ea03ish.gcal', # Output caltable refant='ea03,ea05,ea23,ea01', # prioritized list of reference antennas refantmode='flex') # flexible use of alternates
To strictly apply a preferred reference antenna:
rerefant(vis='n5921.ms', tablein='n5921.gcal', caltable='n5921_ea03.gcal', # Output caltable refant='ea03', # the strictly preferred reference antenna refantmode='strict') # strict!
No additional development details
- Parameter Details
Detailed descriptions of each function parameter
vis (string)- Name of input visibility file (MS)
tablein (string)- Input calibration table
caltable (string='')- Output calibration table (overwrite tablein if unspecified)
refantmode (string='flex')- Refant algorithm to use
refant (string='')- Reference antenna name(s)