VLA Data Combination-CASA6.2.0
This tutorial was created and tested using CASA 6.2.0
Introduction
A perfect image requires measurement of all spatial scales, which is equivalent to filling in the complete uv plane. Unfortunately, this can never be achieved with an aperture synthesis interferometer, although a large number of baselines, long integration times, and multi-frequency-synthesis are all good approaches to increase the uv-coverage. One method to obtain more baselines is to observe in different array configurations and to combine the data afterwards. Deconvolution algorithms are then given good starting conditions to interpolate across the gaps in the uv-plane to achieve an image that combines the surface brightness sensitivity of the compact baselines with the spatial resolution of the extended baselines.
The VLA can be configured into four principal array configurations, A, B, C, and D; A is the most extended and D is the most compact configuration. Consequently, A configuration data exhibit the highest spatial resolution whereas D configuration delivers the best surface brightness sensitivity and also images the largest scales of the sky brightness distribution.
In this tutorial, we will combine data from three VLA configurations (A, B, and C) that were obtained in a monitoring campaign of Sgr A*, the central supermassive black hole of our Milky Way.
Typical Observing Times
When an object is being observed by the VLA in different array configurations, on-source integration times are ideally matched to reach a common surface brightness sensitivity for all scales. Adjacent VLA configurations result in synthesized beams that differ in linear size by approximately a factor of 3. The beam areas therefore change by factors of ~10 and the more extended configuration would ideally need to have 10 times more integration time than the next compact one. This, however, is frequently not very practical and it turns out that integration times that differ by factors of ~3 deliver data that can be satisfactorily combined. This combination matches the sensitivity of overlapping VLA visibilities when the data are convolved to the same scales.
Although overlapping uv-coverages are essential for the best imaging, it is possible to combine non-overlapping data if one understands that some spacings are not present and that the adjustment of the individual datasets is somewhat subjective. Weighting will be primarily achieved during imaging by the Briggs scheme that allows one to adjust imaging weights between the natural (weighting by the number of visibilities that are gridded in each cell) and uniform (weighting by the cells themselves) extremes.
Additionally, each visibility exhibits weights that should only depend on correlator integration time, bandwidth, and system temperature (Tsys). Note that the VLA does not apply Tsys in the online system. Visibility weights between different observations will therefore need to be adjusted, as described below, before they can be combined.
The Data
We will be combining three different datasets, S-band A, B, and C configuration data, from the NRAO Monitoring of the Galactic Center/G2 Cloud Encounter. The data were all calibrated and the science target split out.
The calibrated measurement sets can be downloaded here: https://casa.nrao.edu/Data/EVLA/SgrA/VLA-combination-SgrA-files.tar.gz (1.4GB)
As a first step download the file above, then untar:
# In a Terminal tar -xzvf VLA-combination-SgrA-files.tar.gz
This will unpack three MeasurementSets (MSs), one for each array configuration:
VLA-SgrA-Sband-Aconfig.ms VLA-SgrA-Sband-Bconfig.ms VLA-SgrA-Sband-Cconfig.ms
Initial Imaging
We will inspect the data and create separate images to better understand the image parameters such as on-source integration time, resolution, and rms.
Start CASA:
# In a Terminal casa
As a first step, let's have a look at the 'listobs' output for the different MSs. For example, the A-configuration MS has the following structure:
# In CASA
listobs(vis='VLA-SgrA-Sband-Aconfig.ms')
##########################################
##### Begin Task: listobs #####
listobs(vis="VLA-SgrA-Sband-Aconfig.ms",selectdata=True,spw="",field="",antenna="",
uvrange="",timerange="",correlation="",scan="",intent="",
feed="",array="",observation="",verbose=True,listfile="",
listunfl=False,cachesize=50,overwrite=False)
================================================================================
MeasurementSet Name: /lustre/aoc/sciops/jott/casa/topicalguide/combination/test/VLA-SgrA-Sband-Aconfig.ms MS Version 2
================================================================================
Observer: lorant sjouwerman Project: uid://evla/pdb/11434214
Observation: EVLA
Data records: 528000 Total elapsed time = 360 seconds
Observed from 31-May-2014/09:07:57.0 to 31-May-2014/09:13:57.0 (UTC)
Compute subscan properties
ObservationID = 0 ArrayID = 0
Date Timerange (UTC) Scan FldId FieldName nRows SpwIds Average Interval(s) ScanIntent
31-May-2014/09:07:57.0 - 09:13:57.0 63 0 J1745-2900 528000 [0,3,4,5,6,7,8,9,10,11,12,13,14,15] [3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3] [OBSERVE_TARGET#UNSPECIFIED]
(nRows = Total number of rows per scan)
Fields: 1
ID Code Name RA Decl Epoch SrcId nRows
0 NONE J1745-2900 17:45:40.038300 -29.00.28.06899 J2000 0 528000
Spectral Windows: (14 unique spectral windows and 1 unique polarization setups)
SpwID Name #Chans Frame Ch0(MHz) ChanWid(kHz) TotBW(kHz) CtrFreq(MHz) BBC Num Corrs
0 EVLA_S#A0C0#80 64 TOPO 1988.000 2000.000 128000.0 2051.0000 12 RR LL
3 EVLA_S#A0C0#83 64 TOPO 2372.000 2000.000 128000.0 2435.0000 12 RR LL
4 EVLA_S#A0C0#84 64 TOPO 2500.000 2000.000 128000.0 2563.0000 12 RR LL
5 EVLA_S#A0C0#85 64 TOPO 2628.000 2000.000 128000.0 2691.0000 12 RR LL
6 EVLA_S#A0C0#86 64 TOPO 2756.000 2000.000 128000.0 2819.0000 12 RR LL
7 EVLA_S#A0C0#87 64 TOPO 2884.000 2000.000 128000.0 2947.0000 12 RR LL
8 EVLA_S#B0D0#88 64 TOPO 2988.000 2000.000 128000.0 3051.0000 15 RR LL
9 EVLA_S#B0D0#89 64 TOPO 3116.000 2000.000 128000.0 3179.0000 15 RR LL
10 EVLA_S#B0D0#90 64 TOPO 3244.000 2000.000 128000.0 3307.0000 15 RR LL
11 EVLA_S#B0D0#91 64 TOPO 3372.000 2000.000 128000.0 3435.0000 15 RR LL
12 EVLA_S#B0D0#92 64 TOPO 3500.000 2000.000 128000.0 3563.0000 15 RR LL
13 EVLA_S#B0D0#93 64 TOPO 3628.000 2000.000 128000.0 3691.0000 15 RR LL
14 EVLA_S#B0D0#94 64 TOPO 3756.000 2000.000 128000.0 3819.0000 15 RR LL
15 EVLA_S#B0D0#95 64 TOPO 3884.000 2000.000 128000.0 3947.0000 15 RR LL
Sources: 16
ID Name SpwId RestFreq(MHz) SysVel(km/s)
0 J1745-2900 0 - -
0 J1745-2900 1 - -
0 J1745-2900 2 - -
0 J1745-2900 3 - -
0 J1745-2900 4 - -
0 J1745-2900 5 - -
0 J1745-2900 6 - -
0 J1745-2900 7 - -
0 J1745-2900 8 - -
0 J1745-2900 9 - -
0 J1745-2900 10 - -
0 J1745-2900 11 - -
0 J1745-2900 12 - -
0 J1745-2900 13 - -
0 J1745-2900 14 - -
0 J1745-2900 15 - -
Antennas: 26:
ID Name Station Diam. Long. Lat. Offset from array center (m) ITRF Geocentric coordinates (m)
East North Elevation x y z
0 ea01 N32 25.0 m -107.37.22.0 +33.56.33.6 -441.7442 4689.9683 -16.9356 -1600781.062100 -5039347.430600 3558761.526300
1 ea02 N64 25.0 m -107.37.58.7 +34.02.20.5 -1382.3632 15410.1417 -40.6233 -1599855.668100 -5033332.388100 3567636.626500
2 ea03 E64 25.0 m -107.27.00.1 +33.50.06.7 15507.5911 -7263.7210 67.2006 -1587600.203200 -5050575.869700 3548885.404900
3 ea04 E24 25.0 m -107.35.13.4 +33.53.18.1 2858.1804 -1349.1324 13.7306 -1598663.094300 -5043581.396100 3553767.023200
4 ea05 W08 25.0 m -107.37.21.6 +33.53.53.0 -432.1181 -272.1470 -1.5057 -1601614.092200 -5042001.651900 3554652.509800
5 ea06 N56 25.0 m -107.37.47.9 +34.00.38.4 -1105.2076 12254.3155 -34.2423 -1600128.382500 -5035104.142000 3565024.679400
6 ea07 N48 25.0 m -107.37.38.1 +33.59.06.2 -855.2644 9405.9610 -25.9303 -1600374.875000 -5036704.207500 3562667.885900
7 ea08 N16 25.0 m -107.37.10.9 +33.54.48.0 -155.8517 1426.6442 -9.3792 -1601061.957400 -5041175.883000 3556058.040000
8 ea09 W64 25.0 m -107.46.20.1 +33.48.50.9 -14240.7638 -9606.2696 17.1066 -1616361.587500 -5042770.516600 3546911.446900
9 ea10 E40 25.0 m -107.32.35.4 +33.52.16.9 6908.8305 -3240.7192 39.0202 -1595124.923100 -5045829.467200 3552210.703600
10 ea11 W24 25.0 m -107.38.49.0 +33.53.04.0 -2673.3552 -1784.5888 10.4757 -1604008.749300 -5042135.808900 3553403.716000
11 ea12 N09 25.0 m -107.37.07.8 +33.54.19.0 -77.4204 530.6453 -5.5755 -1601139.471200 -5041679.039700 3555316.553900
12 ea13 W56 25.0 m -107.44.26.7 +33.49.54.6 -11333.2004 -7637.6771 15.3707 -1613255.393400 -5042613.099800 3548545.915000
13 ea14 E08 25.0 m -107.36.48.9 +33.53.55.1 407.8298 -206.0320 -3.2196 -1600801.931000 -5042219.386500 3554706.431200
14 ea15 E56 25.0 m -107.29.04.1 +33.50.54.9 12327.6313 -5774.7469 42.6153 -1590380.611000 -5048810.243300 3550108.432300
15 ea17 E32 25.0 m -107.34.01.5 +33.52.50.3 4701.6413 -2209.7152 25.2066 -1597053.135800 -5044604.681200 3553058.995000
16 ea18 E72 25.0 m -107.24.42.3 +33.49.18.0 19041.8717 -8769.2047 4.7262 -1584460.871200 -5052385.599800 3547600.000100
17 ea19 W16 25.0 m -107.37.57.4 +33.53.33.0 -1348.7109 -890.6216 1.3005 -1602592.853600 -5042054.996800 3554140.704800
18 ea20 N40 25.0 m -107.37.29.5 +33.57.44.4 -633.6074 6878.6018 -20.7693 -1600592.756000 -5038121.357300 3560574.853200
19 ea21 E48 25.0 m -107.30.56.1 +33.51.38.4 9456.6097 -4431.6564 37.9341 -1592894.077600 -5047229.138200 3551221.206000
20 ea22 N24 25.0 m -107.37.16.1 +33.55.37.7 -290.3752 2961.8594 -12.2389 -1600930.087800 -5040316.396400 3557330.387200
21 ea23 W72 25.0 m -107.48.24.0 +33.47.41.2 -17419.4740 -11760.2758 14.9538 -1619757.312900 -5042937.664400 3545120.392300
22 ea24 W48 25.0 m -107.42.44.3 +33.50.52.1 -8707.9403 -5861.7877 15.5282 -1610451.925800 -5042471.125800 3550021.055800
23 ea25 W32 25.0 m -107.39.54.8 +33.52.27.2 -4359.4392 -2923.1244 11.7721 -1605808.634900 -5042230.089000 3552459.209500
24 ea26 W40 25.0 m -107.41.13.5 +33.51.43.1 -6377.9880 -4286.7769 8.2038 -1607962.463800 -5042338.190100 3551324.947500
25 ea28 E16 25.0 m -107.36.09.8 +33.53.40.0 1410.0378 -673.4764 -0.7821 -1599926.107500 -5042772.979700 3554319.790400
##### End Task: listobs #####
##########################################
We see that A-configuration contains only the central source, J1745-2900, which is just a different name for Sgr A* (the target source has been split out to remove all calibrators). The on-source integration time amounts to only six minutes and the 14 spectral windows span a frequency range from 1.988 to 4.012GHz. Inspection of the other array configuration files show almost identical setups. Although the integration times between the different array configurations do not follow the 1:3:9 ratios that we discussed in the previous section, we can still combine the data without any problem. In the end, having better signal to noise on the shorter baselines can only improve the overall combined image.
To better understand the data, let's check the uv-coverage of each of the three datasets:
# In CASA
# A-config:
plotms(vis='VLA-SgrA-Sband-Aconfig.ms',xaxis='Uwave',yaxis='Vwave')
# In CASA
# B-config
plotms(vis='VLA-SgrA-Sband-Bconfig.ms',xaxis='Uwave',yaxis='Vwave')
# In CASA
# C-config:
plotms(vis='VLA-SgrA-Sband-Cconfig.ms',xaxis='Uwave',yaxis='Vwave')
The uv-coverage plots are shown in Fig. 1a-c. The u:v aspect ratio of each uv-coverage is high as Sgr A* is a very southern source. We also see that the longest baseline in each array differs by about a factor of 3, as expected, between the VLA A, B, and C configurations.
 UV-coverage of the A-configuration data. |
 UV-coverage of the B-configuration data. |
 UV-coverage of the C-configuration data. |
The next step is to determine the image quality, the synthesized beam, and the rms of each image. The images do not have to be perfectly deconvolved as we only would like to see how the combination will improve over the individual arrays.
To keep things simple, we will use common cell sizes of 0.1 arcsec, which samples well the A-configuration beam and oversamples the more compact configurations. We create images of 1280 pixels on each side, and we will tclean 1000 iterations (see the VLA CASA imaging guide for more sophisticated imaging parameter choices). :
# In CASA
# A-config:
tclean(vis='VLA-SgrA-Sband-Aconfig.ms', imagename='SgrA-Aonly',field='J1745-2900',
specmode='mfs',cell='0.1arcsec',imsize=[1280,1280],niter=1000,weighting='briggs',robust=0)
# In CASA
# B-config
tclean(vis='VLA-SgrA-Sband-Bconfig.ms', imagename='SgrA-Bonly',field='J1745-2900',
specmode='mfs',cell='0.1arcsec',imsize=[1280,1280],niter=1000,weighting='briggs',robust=0)
# In CASA
# C-config:
tclean(vis='VLA-SgrA-Sband-Cconfig.ms', imagename='SgrA-Conly',field='J1745-2900',
specmode='mfs',cell='0.1arcsec',imsize=[1280,1280],niter=1000,weighting='briggs',robust=0)
The clean images, shown in Fig. 2a-c, give a first impression of the data. Note that when combining the images, we will improve on the uv-coverage and will therefore not only combine high resolution with high surface brightness sensitivity on large scales, but also expect a higher image fidelity, i.e. fewer artifacts due to better deconvolution.
 Simple image of the A-configuration data. To get a similar image, in the Viewer task, click on the Wrench icon and in the Basic setting change the data range to [-0.01, 0.03] |
 Simple image of the B-configuration data. To get a similar image, in the Viewer task, click on the Wrench icon and in the Basic setting change the data range to [-0.01, 0.115] |
 Simple image of the C-configuration data. To get a similar image, in the Viewer task, click on the Wrench icon and in the Basic setting change the data range to [-0.1, 1] |
The basic parameters of the individual images can be checked through imhead for the spatial resolution and determining the rms when checking the statistics of empty areas within the viewer. The central point source, Sgr A*, is variable and we see this reflected in the flux density values from the images taken in the three different configurations, which were separated in observing time by several months (get the peak flux values from within the viewer or use the imstat task):
A-configuration: synthesized beam 1.40" x 0.40"; rms ~0.8mJy; peak flux density of Sgr A* 0.713 Jy/beam B-configuration: synthesized beam 4.12" x 1.31"; rms ~0.7mJy; peak flux density of Sgr A* 0.691 Jy/beam C-configuration: synthesized beam 11.18" x 3.98"; rms ~0.6mJy; peak flux density of Sgr A* 1.314 Jy/beam
Removal of the variable Sgr A* point source
This step is only necessary for our case which includes a variable source. In most cases, one can skip this step.
We have seen in the initial images that Sgr A* is a variable source. This introduces some problems for data combination as the visibilities of the different sessions will reflect these variations. Cleaning will be difficult in such a situation as flux levels at different uv points (times and baselines) are not consistent in their portion for the central point source. Therefore, we will remove the unresolved Sgr A* in each dataset. For those who are interested in having Sgr A* back in the image, we will later show how to re-introduce a nominal point source with a flux of 1Jy. It is perfectly fine, however, to image without the artificial source.
To start, we create a component list for each dataset that includes only a point source at the position of Sgr A*. To do so, we first need to find out the position of the central point source. This is performed by fitting a point source (2d-Gaussian function) to the A-configuration data, which has the largest angular resolution. We use imfit:
# In CASA
imfit(imagename='SgrA-Aonly.image',box='630,620,650,660')
The output of imfit into the logger is as follows:
Fit on SgrA-Aonly.image component 0
Position ---
--- ra: 17:45:40.038545 +/- 0.000057 s (0.000748 arcsec along great circle)
--- dec: -029.00.28.051094 +/- 0.004412 arcsec
--- ra: 639.9679 +/- 0.0075 pixels
--- dec: 640.1790 +/- 0.0441 pixels
We will be using RA (J2000) =17h45m40.039s and DEC (J2000) = -29d00'28.05" for the central position of Sgr A*. We will use these coordinates for all three configurations, although it might be advisable to fit each dataset separately in case the peak may slightly shift. In this case, a single position works for all of our three data sets.
The component list will be inserted as a MODEL column into the respective MS via ft. Task uvsub will then be used to subtract the point source model from the CORRECTED data column. Since there's no CORRECTED column to start with, uvsub will initially copy the DATA column to the CORRECTED column, then subtract. Note that running uvsub twice will oversubtract; we recommend to create backup copies of the previous datasets before subtracting Sgr A*:
# In a Terminal cp -r VLA-SgrA-Sband-Aconfig.ms VLA-SgrA-Sband-Aconfig-sub.ms cp -r VLA-SgrA-Sband-Bconfig.ms VLA-SgrA-Sband-Bconfig-sub.ms cp -r VLA-SgrA-Sband-Cconfig.ms VLA-SgrA-Sband-Cconfig-sub.ms
Let's start with the A-configuration data and create a component list of a point source with a flux of 0.712 Jy at a position of RA (J2000) =17h45m40.038547s and DEC (J2000) = -29d00'28.051226" (more examples of component lists are provided in the CASA guide Create a Component List for Selfcal):
# In CASA
cl.addcomponent(flux=0.713, fluxunit='Jy',shape='point', dir='J2000 17:45:40.039 -29.00.28.05')
cl.rename('component-SgrA-A.cl')
cl.close()
(We have used the CASA coordinate convention with colons for h:m:s and dots for d.m.s.)
The component list is now stored in the file 'component-SgrA-A.cl' and we will use it to populate the MODEL column via ft:
# In CASA
ft(vis='VLA-SgrA-Sband-Aconfig-sub.ms', complist='component-SgrA-A.cl', usescratch=True)
At this step the MODEL column is created.
Finally we will subtract the MODEL from the DATA/CORRECTED_DATA with uvsub. This task will also create CORRECTED as it does not exist yet:
# In CASA
uvsub(vis='VLA-SgrA-Sband-Aconfig-sub.ms')
Note that to revert back to the original state, you can use task clearcal to reset the MODEL column and to set CORRECTED to be identical with DATA (thus undoing the source subtraction).
Let's repeat the steps for B-configuration, now using a flux of 0.691 Jy. Although a separate fit would be even better, the subtraction also works well when using the A-configuration position:
# In CASA
cl.addcomponent(flux=0.691, fluxunit='Jy',shape='point', dir='J2000 17:45:40.039 -29.00.28.05')
cl.rename('component-SgrA-B.cl')
cl.close()
#
ft(vis='VLA-SgrA-Sband-Bconfig-sub.ms', complist='component-SgrA-B.cl', usescratch=True)
#
uvsub(vis='VLA-SgrA-Sband-Bconfig-sub.ms')
And for C-configuration we subtract a point source of 1.314 Jy (note that the central flux includes some extended emission, but is still dominated by Sgr A*):
# In CASA
cl.addcomponent(flux=1.314, fluxunit='Jy',shape='point', dir='J2000 17:45:40.039 -29.00.28.05')
cl.rename('component-SgrA-C.cl')
cl.close()
#
ft(vis='VLA-SgrA-Sband-Cconfig-sub.ms', complist='component-SgrA-C.cl', usescratch=True)
#
uvsub(vis='VLA-SgrA-Sband-Cconfig-sub.ms')
Let's image the three data sets with Sgr A* removed. We will run tclean, but since the point source is removed, we only do 500 iterations for A and B configurations, and 200 iterations for the C configuration data to avoid cleaning into the noise:
# In CASA
# A-config:
tclean(vis='VLA-SgrA-Sband-Aconfig-sub.ms', imagename='SgrA-Aonly-noPNT',field='J1745-2900',
specmode='mfs',cell='0.1arcsec',imsize=[1280,1280],niter=500,weighting='briggs',robust=0)
# In CASA
# B-config:
tclean(vis='VLA-SgrA-Sband-Bconfig-sub.ms', imagename='SgrA-Bonly-noPNT',field='J1745-2900',
specmode='mfs',cell='0.1arcsec',imsize=[1280,1280],niter=500,weighting='briggs',robust=0)
# In CASA
# C-config:
tclean(vis='VLA-SgrA-Sband-Cconfig-sub.ms', imagename='SgrA-Conly-noPNT',field='J1745-2900',
specmode='mfs',cell='0.1arcsec',imsize=[1280,1280],niter=200,weighting='briggs',robust=0)
 Simple image of the A-configuration data with the central point source removed. To get a similar image, in the Viewer task, click on the Wrench icon and in the Basic setting change the data range to [-0.01, 0.03] |
 Simple image of the B-configuration data with the central point source removed. To get a similar image, in the Viewer task, click on the Wrench icon and in the Basic setting change the data range to [-0.01, 0.115] |
 Simple image of the C-configuration data with the central point source removed. To get a similar image, in the Viewer task, click on the Wrench icon and in the Basic setting change the data range to [-0.091, 0.618] |
As expected, the images without the point sources as shown in Fig. 3a-c look similar to those in Fig. 2a-c. but with the central point source removed. Since we cleaned a given number of iterations, some differences are visible, particularly for C-configuration. This is expected.
Check and Adjust the Visibility Weights
VLA weights are currently based only on channel width and correlator integration time. In the future, the VLA may use the switched power measurements to derive absolute calibrated weights. At the moment, however, the VLA weights need to be taken as being relative to each other. The relative sensitivity within an observation is measured by the gain, so weights of single, continuous observations are self-consistent. It is important to adjust the weights between separate observations as they will be on different scales.
Let's have a look at the weights of the different datasets. We will plot the weights as a function of uv-distance for a single spectral window (spw='3'), average over all channels in one spectral window, and will colorize by antenna:
# In CASA
# A-config:
plotms(vis='VLA-SgrA-Sband-Aconfig-sub.ms',spw='3',averagedata=True,
avgchannel='64',xaxis='uvwave',yaxis='wt',coloraxis='antenna1')
# In CASA
# B-config
plotms(vis='VLA-SgrA-Sband-Bconfig-sub.ms',spw='3',averagedata=True,
avgchannel='64',xaxis='uvwave',yaxis='wt',coloraxis='antenna1')
# In CASA
# C-config:
plotms(vis='VLA-SgrA-Sband-Cconfig-sub.ms',spw='3',averagedata=True,
avgchannel='64',xaxis='uvwave',yaxis='wt',coloraxis='antenna1')
Fig. 4a-c shows that there are some differences. Data from configurations A and B have weights that are reasonably comparable. C-configuration data, however, seems to have consistently lower values.
 A-config original weights. |
 B-config original weights. |
 C-config original weights. |
The next step is to bring the weights on the same relative scale.
There are currently a number of options to do so.
1) Calculate the weights based on the rms of the visibilities themselves, using the statwt task.
2) Reset the weights with initweights to reflect the channel width and correlator integration time ([math]\displaystyle{ WEIGHT=2 \Delta \nu \Delta t }[/math], see the document on Data Weights in CASAdocs).
3) Calculate the weights based on the rms of the cross-polarization products (currently not supported in statwt).
We recommend to reset the weights when there are strong sources present in the data as they will change the rms of the visibilities and the rms will not be representative of the noise anymore. Without strong sources, statwt should deliver better results. The third option may work both cases, but requires full polarization observations and calibrations. For this guide we will perform the statwt path as an example. But the user should be aware of the different options for optimized imaging.
Calculating the weights based on the rms
statwt will bring all the weights of all observations on the same scale. The task recalculates the visibility weights based on the inverse of their rms. Task statwt is part of the VLA pipeline, which was used to calibrate the original data. This explains the similarity of the A and B configuration weights in Fig. 4a and b. We will nevertheless run statwt once more on all files in order to ensure proper compatibility of the data (older versions of CASA may have been used for the pipeline, and CASA underwent major changes in its definition of data weights).
Task statwt will be executed on each MS. The default setting calculates the weight based on the rms of each scan and spectral window. This setting works quite well for continuum observations. We would like to note though that for spectral line data the fitspw parameter should be set to exclude the line from the calculations. Otherwise, strong lines will be down-weighted.
As statwt will overwrite the WEIGHT column, we once more create copies of our MSs in case we will have to revert to the original data:
# In a Terminal cp -r VLA-SgrA-Sband-Aconfig-sub.ms VLA-SgrA-Sband-Aconfig-sub-statwt.ms cp -r VLA-SgrA-Sband-Bconfig-sub.ms VLA-SgrA-Sband-Bconfig-sub-statwt.ms cp -r VLA-SgrA-Sband-Cconfig-sub.ms VLA-SgrA-Sband-Cconfig-sub-statwt.ms
Now, we execute statwt on the new datasets:
# In CASA
# A-config:
statwt(vis='VLA-SgrA-Sband-Aconfig-sub-statwt.ms')
# In CASA
# B-config
statwt(vis='VLA-SgrA-Sband-Bconfig-sub-statwt.ms')
# In CASA
# C-config:
statwt(vis='VLA-SgrA-Sband-Cconfig-sub-statwt.ms')
Let's have a look at the weight values again to see if statwt has adjusted the weights properly:
# In CASA
# A-config:
plotms(vis='VLA-SgrA-Sband-Aconfig-sub-statwt.ms',spw='3',averagedata=True,
avgchannel='64',xaxis='uvwave',yaxis='wt',coloraxis='antenna1')
# In CASA
# B-config
plotms(vis='VLA-SgrA-Sband-Bconfig-sub-statwt.ms',spw='3',averagedata=True,
avgchannel='64',xaxis='uvwave',yaxis='wt',coloraxis='antenna1')
# In CASA
# C-config:
plotms(vis='VLA-SgrA-Sband-Cconfig-sub-statwt.ms',spw='3',averagedata=True,
avgchannel='64',xaxis='uvwave',yaxis='wt',coloraxis='antenna1')
In Fig. 6a-c we plot the new, recalculated weights for the three configurations. The absolute scaling has indeed changed, likely due to a different weight definition that was used in earlier versions of CASA when the data were calibrated. The relative weights between A and B configurations still appear to be similar to the original, relative numbers. The C-configuration weights, however, appear to be slightly lower relative to A and B, likely due to more flux in the MS.
 A-config recalculated weights. |
 B-config recalculated weights. |
 C-config recalculated weights. |
Combining and Imaging All Data
We will now create a combined image of all three re-weighted datasets.
First, let's check the new uv-coverage. We need to concatenate the data with concat (alternatively, one can use virtualconcat, a task that can also concatenate multi-MSs):
# In CASA
concat(vis=['VLA-SgrA-Sband-Aconfig-sub-statwt.ms','VLA-SgrA-Sband-Bconfig-sub-statwt.ms','VLA-SgrA-Sband-Cconfig-sub-statwt.ms'],concatvis='VLA-SgrA-Sband-statwt-combined.ms')
Now let's plot the uv-coverage of the combined MSs, e.g.:
# In CASA
plotms(vis='VLA-SgrA-Sband-statwt-combined.ms',xaxis='Uwave',yaxis='Vwave')
Fig. 7 shows the combined uv-coverage, which extends to A-configuration baselines but with a much higher density at the intermediate and shorter baselines contributed from B and C configurations.
 Combined uv-coverage. |
Although concat merges all three MSs into a single one, it is actually not a required step before imaging. Task tclean will take care of the combination when all datasets are specified as a list. By default, tclean will image the data in the CORRECTED columns, i.e. it will use the portion of the MS which exhibits the data with the subtracted Sgr A* point source (if CORRECTED is not present, it will image the visibilities stored in the DATA column). Tclean will also perform the spectral regridding of all datasets on the fly, in particular in mode="velocity" or mode="frequency". There's no need to run cvel/cvel2 (or mstransform) to Doppler correct the MSs beforehand.
We will now create a combined image in tclean. The threshold parameter was derived by a previous run of tclean on the combined MS for which we determined the rough rms noise. For our threshold, we will use the rms noise multiplied by a factor of ~2.5.
Image the data with recalculated weights:
# In CASA
tclean(vis=['VLA-SgrA-Sband-statwt-combined.ms'],
imagename='GC-statwt-all',field='J1745-2900',specmode='mfs',cell='0.1arcsec',imsize=[1280,1280],niter=3000,
threshold='5mJy',weighting='briggs',robust=0)
 Combined image with statwt. To get a similar image, in the Viewer task, click on the Wrench icon and in the Basic setting change the data range to [0, 0.05] |
The image can still be improved upon. For simplicity, we did not use any interactive cleaning in the above, but we highly recommend it for producing the final images. Improvements can also be obtained by adjusting the image weights via the Briggs robust parameter, adding a taper, or weighting the different datasets against each other using visweightscale in concat. Wide-band imaging and multi-scale imaging will also lead to better results. We refer to the VLA CASA Imaging Guide for more details and examples.
Inserting an artificial Point Source for Sgr A*
Finally, we will add in a point source with a canonical flux density of 1 Jy to bring back Sgr A*. We again create a component list and use ft to attach the source model. Using uvsub with reverse=True will add instead of subtract the MODEL to DATA/CORRECTED. This step can be skipped when Sgr A* is not required. When the data are going to be self-calibrated, we actually recommend to not introduce an artificial source during the calibration steps as it has no phase errors imprinted. For this example, we now re-introduce Sgr A*. Once more, we start by copying the combined datasets:
# In a Terminal cp -r VLA-SgrA-Sband-statwt-combined.ms VLA-SgrA-Sband-statwt-combined-addPNT.ms
Now we create a component list with a 1 Jy point source at the previously derived position of Sgr A*.
# In CASA
cl.addcomponent(flux=1, fluxunit='Jy',shape='point', dir='J2000 17:45:40.038547 -29.00.28.051226')
cl.rename('component-SgrA.cl')
cl.close()
Finally, we insert it in the model column with ft and add it to the data with uvsub to both MSs, the one with initialized weights and the one with recomputed weights:
# In CASA
ft(vis='VLA-SgrA-Sband-statwt-combined-addPNT.ms', complist='component-SgrA.cl', usescratch=True)
#
uvsub(vis='VLA-SgrA-Sband-statwt-combined-addPNT.ms',reverse=True)
Now it is time to image both combined MSs once more:
# In CASA
tclean(vis=['VLA-SgrA-Sband-statwt-combined-addPNT.ms'],
imagename='GC-statwt-all-addPNT',field='J1745-2900',specmode='mfs',cell='0.1arcsec',imsize=[1280,1280],niter=5000,
threshold='5mJy',weighting='briggs',robust=0)
 Combined image with statwt and point source. To get a similar image, in the Viewer task, click on the Wrench icon and in the Basic setting change the data range to [0, 0.05] |
In Fig. 9 we show the resulting images on the same scales as in Fig. 8a-b. As expected the main difference is the added point source but some differences due to the tclean algorithm are also visible. Again, one can improve the images as described above.
Tips for Selfcal
If the source has a bright nucleus or, more generally, a bright unresolved emission, start with the A array data, selfcal, then add B array, selfcal again, and so on. This procedure starts with a high model flux that is increased further. If the source is mostly diffuse, then there is not much signal in the A array data, so start with the D array, selfcal, then add C array, selfcal, and so on. Each selfcal step should be phase only first with maybe two or more iterations. At the end of each selfcal step, a phase+amplitude selfcal can be attempted, before merging in the next array configuration data. After each merge the selfcal steps can be repeated.
In our case the steps would be done in the images which have Sgr A* removed; otherwise the variability of Sgr A* prevents such procedures. Also, when we replaced Sgr A* by an artificial 1 Jy source, that source will not have phase fluctuations imprinted and cannot be used for selfcal. It can be reintroduced as described above after all selfcal steps are completed.
Last checked on CASA Version 6.2.0.
