Polcal
From CASA Guides
Help on polcal task:
Determine instrumental polarization calibrations
The instrumental polarization factors (D-terms), the calibrator polarization,
and the R-L polarization angle can be determined using polcal. The solutions
can be obtained for each antenna/spwid and even individual channels, if desired.
Previous calibrations of the total intensity data should be applied on the fly
when running polcal, since polcal uses the 'data' column, not the 'corrected'
column.
After calibrating the total intensities, the simplest way to calibrate the
polarization data is:
a) Run polcal with poltype = 'D+QU' on the main 'calibrator' source. The D terms
and polarization (QU) of the calibrator will be determined. Relatively good
parallactic angle coverage is needed.
b) If there is little parallactic angle coverage, place the known calibration of
the main calibrator (or 0) using setjy with the appropriate fluxdensity. Then
run polcal with poltype = 'D'. Run plotcal with xaxis = 'real'; yaxis ='imag'
to view solutions.
c) To determine R-L polarization angle, use setjy to put the fluxdensity of the
polarization calibrator [I,Q,U,0.0] in the model column. For resolved sources
put in values associated with an appropriate u-v range. Polarized models are
not yet available for the major polarization standard sources, so very
resolved polarized sources should not be used.
d) Run polcal with poltype = 'X' and include polarization standard. Make sure to
include all previous calibrations, especially the D results. Run plotxy with
correlation = 'RL LR' and make sure polarization angles are as expected.
e) Run applycal with all calibration table, include the D and X tables. Make sure
that parang = T
NOTE: For very high dynamic range, add 'f' after D to determine D terms for each
channel.
Keyword arguments:
vis -- Name of input visibility file
default: none; example: vis='ngc5921.ms'
caltable -- Name of output gain calibration table
default: none; example: caltable='ngc5921.dcal'
--- Data Selection (see help par.selectdata for more detailed information)
field -- Select field using field id(s) or field name(s).
[run listobs to obtain the list id's or names]
default: ''=all fields.
Most likely, the main calibrator source should be picked.
If field string is a non-negative integer, it is assumed a field index
otherwise, it is assumed a field name
field='0~2'; field ids 0,1,2
field='0,4,5~7'; field ids 0,4,5,6,7
field='3C286,3C295'; field named 3C286 adn 3C295
field = '3,4C*'; field id 3, all names starting with 4C
spw -- Select spectral window/channels
type 'help par.selection' for more examples.
spw='0~2,4'; spectral windows 0,1,2,4 (all channels)
spw='<2'; spectral windows less than 2 (i.e. 0,1)
spw='0:5~61'; spw 0, channels 5 to 61, INCLUSIVE
spw='*:5~61'; all spw with channels 5 to 62
spw='0,10,3:3~45'; spw 0,10 all channels, spw 3, channels 3 to 45.
spw='0~2:2~6'; spw 0,1,2 with channels 2 through 6 in each.
spw='0:0~10;15~60'; spectral window 0 with channels 0-10,15-60
NOTE ';' to separate channel selections
spw='0:0~10^2,1:20~30^5'; spw 0, channels 0,2,4,6,8,10,
spw 1, channels 20,25,30
selectdata -- Other data selection parameters
default: True
timerange -- Select data based on time range:
default = '' (all); examples,
timerange = 'YYYY/MM/DD/hh:mm:ss~YYYY/MM/DD/hh:mm:ss'
Note: if YYYY/MM/DD is missing dat defaults to first day in data set
timerange='09:14:0~09:54:0' picks 40 min on first day
timerange= '25:00:00~27:30:00' picks 1 hr to 3 hr 30min on next day
timerange='09:44:00' data within one integration of time
timerange='>10:24:00' data after this time
uvrange -- Select data within uvrange (default units meters)
default: '' (all); example:
uvrange='0~1000'; uvrange from 0-1000 kilo-lamgda
uvrange='>4';uvranges greater than 4 kilo-lambda
antenna -- Select data based on antenna/baseline
default: '' (all)
If antenna string is a non-negative integer, it is assumed an antenna index
otherwise, it is assumed as an antenna name
antenna='5&6'; baseline between antenna index 5 and index 6.
antenna='VA05&VA06'; baseline between VLA antenna 5 and 6.
antenna='5&6;7&8'; baseline 5-6 and 7-8
antenna='5'; all baselines with antenna index 5
antenna='05'; all baselines with antenna name 05, i.e. VLA ant 5
antenna='5,6,10'; all baselines with antennas 5, 6 and 10
scan -- Scan number range
msselect -- Optional complex data selection (ignore for now)
--- Solution parameters
poltype -- Type of instrumental polarization solution
'D+QU' (or 'Df+QU') solve also for apparent source polarization (channelized D)
Need relatively good parallactic angle coverage for this
'D' (or 'Df') solve only for instrumental polarization (channelized). The
I, Q, U flux density of the source can be placed in the model column using
setjy. Use for poor parallactic angle coverage.
'X' = solve only for position angle correction. The source must have its
I, Q, U flux density in the model column. If the source is resolved, use
a uvrange that is appropriate.
'D+X' (or 'Df+X') = solve also for position angle offset (channelized D) as
well as the D-term. Not normally done.
default: 'D+QU'
The solution used the traditional linear approximation. Non-linearized options
will be avaible soon.
solint -- Solution interval (units optional)
default: 'inf' (~infinite, up to boundaries controlled by combine);
Options: 'inf' (~infinite), 'int' (per integration), any float
or integer value with or without units
examples: solint='1m'; solint='60s', solint=60 --> 1 minute
solint='0s'; solint=0; solint='int' --> per integration
solint-'-1s'; solint='inf' --> ~infinite, up to boundaries
enforced by combine
combine -- Data axes to combine for solving
default: 'scan' --> solutions will break at field and spw boundaries,
but may extend over multiple scans (per field and spw) up
to solint.
Options: '','scan','spw',field', or any comma-separated combination
example: combine='scan,spw' --> extend solutions over scan boundaries
(up to the solint), and combine spws for solving
preavg -- Pre-averaging interval (sec)
default=300
Interval to apply parallactic angle.
refant -- Reference antenna name
default: '' => refant = '0'
example: refant='13' (antenna with index 13)
refant='VA04' (VLA antenna #4)
Use 'go listobs' for antenna listing.
USE SAME REFERENCE ANTENNA AS USED FOR I CALIBRATION.
minblperant -- Minimum number of baselines required per antenna for each solve
Antennas with fewer baaselines are excluded from solutions. Amplitude
solutions with fewer than 4 baselines, and phase solutions with fewer
than 3 baselines are only trivially constrained, and are no better
than baseline-based solutions.
default: 4
example: minblperant=10 => Antennas participating on 10 or more
baselines are included in the solve
minsnr -- Reject solutions below this SNR
default: 0.0 (accept all attempted solutions)
append -- Append solutions to the (existing) table
default: False; overwrite existing table or make new table
--- Other calibrations to apply on the fly before determining gaincal solution
gaintable -- Gain calibration table(s) to apply
default: '' (none); BUT I CALIBRATION TABLES SHOULD GENERALLY BE INCLUDED
examples: gaintable='ngc5921.gcal'
gaintable=['ngc5921.ampcal','ngc5921.phcal']
gainfield -- Select a subset of calibrators from gaintable(s)
default:'' ==> all sources in table;
same syntax as field
example: gainfield='0~3'
gainfield=['0~3','4~6'] means use field 0 through 3
from first gain file, field 4 through 6 for second.
interp -- Interpolation mode (in time) to use for each gaintable
default: '' --> 'linear' for all gaintable(s)
example: interp='nearest'
interp=['nearest','linear']
Options: 'nearest', 'linear', 'aipslin'
spwmap -- Spectral windows combinations to form for gaintable(s)
default: [] (apply solutions from each spw to that spw only)
Example: spwmap=[0,0,1,1] means apply the caltable solutions
from spw = 0 to the spw 0,1 and spw 1 to spw 2,3.
spwmap=[[0,0,1,1],[0,1,0,1]]
gaincurve -- Apply internal VLA antenna gain curve correction (True/False)
default: False;
Use gaincurve=True ONLY for VLA data
opacity -- Opacity correction to apply (nepers)
default: 0.0 (no opacity correction)
example: opacity=0.051
Typical VLA values are: 5 GHz - 0.013, 8 GHz - 0.013
15 GHz - 0.016, 23 GHz - 0.051, 43 GHz - 0.07
async -- Run asynchronously
default = False; do not run asychronously
