B solutions

NRAO Home > CASA > CASA Cookbook and User Reference Manual

4.4.2.2 B solutions

Calibration type ’B’ differs from ’G’ only in that it is determined for each channel in each spectral window. It is possible to solve for it as a function of time, but it is most efficient to keep the ’B’ solving timescale as long as possible, and use ’G’ or ’T’ for rapid frequency-independent time-scale variations.

The ’B’ solutions are limited by the signal-to-noise ratio available per channel, which may be quite small. It is therefore important that the data be coherent over the time-range of the ’B’ solutions. As a result, ’B’ solutions are almost always preceded by an initial ’G’ or ’T’ solve using gaincal (§ 4.4.3). In turn, if the ’B’ solution improves the frequency domain coherence significantly, a ’G’ or ’T’ solution following it will be better than the original.

For example, to solve for a ’B’ bandpass using a single short scan on the calibrator, then

default(’bandpass’)

vis = ’n5921.ms’
caltable = ’n5921.bcal’
gaintable = ’’                   # No gain tables yet
gainfield = ’’
interp = ’’
field = ’0’                      # Calibrator 1331+305 = 3C286 (FIELD_ID 0)
spw = ’’                         # all channels
selectdata = False               # No other selection
gaincurve = False                # No gaincurve at L-band
opacity = 0.0                    # No troposphere
bandtype = ’B’                   # standard time-binned B (rather than BPOLY)
solint = ’inf’                   # set solution interval arbitrarily long
refant = ’15’                    # ref antenna 15 (=VLA:N2) (ID 14)

bandpass()

On the other hand, we might have a number of scans on the bandpass calibrator spread over time, but we want a single bandpass solution. In this case, we could solve for and then pre-apply an initial gain calibration, and let the bandpass solution cross scans:

gaintable = ’n5921.init.gcal’    # Our previously determined G table
gainfield = ’0’
interp = ’linear’                # Do linear interpolation
solint = ’inf’                   # One interval over dataset
combine = ’scan’                 # Solution crosses scans

Note that we obtained a bandpass solution for all channels in the MS. If explicit channel selection is desired, for example some channels are useless and can be avoided entirely (e.g. edge channels or those dominated by Gibbs ringing), then spw can be set to select only these channels, e.g.

spw = ’0:4~59’ # channels 4-59 of spw 0

This is not so critical for ’B’ solutions as for ’BPOLY’, as each channel is solved for independently, and poor solutions can be dropped.

If you have multiple time solutions, then these will be applied using whatever interpolation scheme is specified in later tasks.

The combine parameter (§ 4.4.1.5) can be used to combine data across spectral windows, scans, and fields.

[next] [prev] [prev-tail] [front] [up]

More information about CASA may be found at the CASA web page

This code is available under the terms of the GNU General Public Lincense