View source for VLA Source Subtraction Topical Guide-CASA6.5.4

''EDITING: This tutorial was created and tested using CASA 6.5.4''
This tutorial uses [https://cartavis.org/ Carta] to image visualization. We suggested downloading Carta before start the tutorial. You will find more information about how to use Carta in the link:
[https://carta.readthedocs.io/en/3.0/ https://carta.readthedocs.io/en/3.0/]

== Introduction ==

Observers may find on occasion that the emission of one source overlaps with that of nearby sources, or a source in the field dominates emission. This overlap can present analytical challenges if the nearby sources, which are the targets of interest, become obscured or limits the achievable dynamic range. These problems may arise when observing crowded fields or very bright objects. By modelling the emission of the obscuring source, or sources, one can then subtract it from the dataset and thus disentangle the emission.

In this tutorial we will subtract the variable and unresolved point source of Sgr A*, the central supermassive black hole of our Milky Way. The dataset used was obtained in a monitoring campaign that includes observations from three different configurations of the VLA. In the final section we will show how to insert back into the dataset a model of the subtracted source, which we do using a single combined dataset. Prior to that section the user may refer to our [[VLA_Data_Combination|CASA guide on data combination]] to understand how the data combination may be performed.

== The Data ==

We will be removing the point source from three different S-band datasets obtained in the A, B, and C configurations, from the [https://science.nrao.edu/science/service-observing NRAO Monitoring of the Galactic Center/G2 Cloud Encounter]. The datasets have all been calibrated and had their science targets [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.manipulation.split.html split] out. 

 The calibrated measurement sets can be downloaded here: 
 [https://casa.nrao.edu/Data/EVLA/SgrA/VLA-combination-SgrA-files.tar.gz 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:

<pre style="background-color: lightgrey;”>
# In a Terminal
tar -xzvf VLA-combination-SgrA-files.tar.gz
</pre>

This will unpack three MeasurementSets (MSs), one for each array configuration: 

<pre>
VLA-SgrA-Sband-Aconfig.ms

VLA-SgrA-Sband-Bconfig.ms

VLA-SgrA-Sband-Cconfig.ms
</pre>

== 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: 

<pre style="background-color: lightgrey;”>
# In a terminal, type:
casa
</pre>

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: 

<source lang="python">
# In CASA
listobs(vis='VLA-SgrA-Sband-Aconfig.ms')
</source>

<pre>
##########################################
##### 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       4701.6Difference between revisions of "VLA Data Combination-CASA6.2.0"
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(→‎Removal of the variable Sgr A* point source)
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(→‎Removal of the variable Sgr A* point source)
 413    -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              #####
##########################################
</pre> 

We see that the target source for A-configuration is J1745-2900 (Sgr A*) (the target source has been [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.manipulation.split.html?highlight=split split] out to remove all calibrators). The on-source integration time amounts to only six minutes and the 16 spectral windows span a frequency range from 1.988 to 4.012GHz. Inspection of the other array configuration files show almost identical setups. 

To better understand the data, let's check the uv-coverage of each of the three datasets:

<source lang="python">
# In CASA
# A-config: 
plotms(vis='VLA-SgrA-Sband-Aconfig.ms',xaxis='Uwave',yaxis='Vwave')
</source>

<source lang="python">
# In CASA
# B-config
plotms(vis='VLA-SgrA-Sband-Bconfig.ms',xaxis='Uwave',yaxis='Vwave')
</source>

<source lang="python">
# In CASA
# C-config:
plotms(vis='VLA-SgrA-Sband-Cconfig.ms',xaxis='Uwave',yaxis='Vwave')
</source>

The uv-coverage plots are shown in Figures 1a-c. The u:v aspect ratio of each uv-coverage is high since 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. 


{|
|[[Image:UvAconfig_CASA5.7.png|400px|thumb|left|'''Figure 1a:''' <br />UV-coverage of the A-configuration data.]]
|[[Image:UvBconfig_CASA5.7.png|400px|thumb|left|'''Figure 1b:''' <br />UV-coverage of the B-configuration data.]]
|[[Image:UvCconfig_CASA5.7.png|400px|thumb|left|'''Figure 1c:''' <br />UV-coverage of the C-configuration data.]]
|}

The next step is to create images we will then model and subtract the point source from. 

We will use common cell sizes of 0.1 arcsec, which samples the A-configuration beam well and oversamples the more compact configurations. We create images of 1280 pixels on each side, and we will [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.imaging.tclean.html tclean] 1000 iterations (see the [http://casaguides.nrao.edu/index.php/VLA_CASA_Imaging VLA CASA imaging guide] for more sophisticated imaging parameter choices).
 
<source lang="python">
# 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)
</source>

<source lang="python">
# 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)
</source>

<source lang="python">
# 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)
</source>

The clean images, shown in Figures 2a-c, give a first impression of the data. You can see the images using Carta in a terminal outside CASA:

<source lang="python">
# In Terminal
carta SgrA-Aonly.image
</source>

You will need to paste the path into an internet browser. Once you have opened the first image, you can use Carta menu: file -> open archive to load the others.

{|
|[[File:SgrA_AonlyCASA6.4.png|400px|thumb|left|'''Figure 2a:''' <br />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, 0.05]]]
|[[File:SgrA BonlyCASA6.4.png|400px|thumb|left|'''Figure 2b:''' <br />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, 0.2]]]
|[[File:SgrA ConlyCASA6.4.png|400px|thumb|left|'''Figure 2c:''' <br />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 [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.information.imhead.html imhead] for the spatial resolution and determining the rms when checking the statistics of empty areas within the viewer. 

<source lang="python">
# In CASA
imhead('SgrA-Aonly')
imhead('SgrA-Bonly')
imhead('SgrA-Conly')
</source>

<pre>
INFO imhead	##### Begin Task: imhead             #####
INFO imhead	imhead( imagename='SgrA-Aonly.image', mode='summary', hdkey='', hdvalue='', verbose=False )
INFO ImageMetaData	   
INFO ImageMetaData	Image name       : SgrA-Aonly.image
INFO ImageMetaData	Object name      : J1745-2900
INFO ImageMetaData	Image type       : PagedImage
INFO ImageMetaData	Image quantity   : Intensity
INFO ImageMetaData	Pixel mask(s)    : mask0
INFO ImageMetaData	Region(s)        : None
INFO ImageMetaData	Image units      : Jy/beam
INFO ImageMetaData	Restoring Beam   : 1.40376 arcsec, 0.400036 arcsec, 8.32141 deg
INFO ImageMetaData	   
INFO ImageMetaData	Direction reference : J2000
INFO ImageMetaData	Spectral  reference : LSRK
INFO ImageMetaData	Velocity  type      : RADIO
INFO ImageMetaData	Rest frequency      : 2.99881e+09 Hz
INFO ImageMetaData	Pointing center     :  17:45:40.038300  -29.00.28.068994
INFO ImageMetaData	Telescope           : EVLA
INFO ImageMetaData	Observer            : lorant sjouwerman
INFO ImageMetaData	Date observation    : 2014/05/31/09:07:58.500000
INFO ImageMetaData	Telescope position: [-1.60116e+06m, -5.04199e+06m, 3.55488e+06m] (ITRF)
INFO ImageMetaData	   
INFO ImageMetaData	Axis Coord Type      Name             Proj Shape Tile   Coord value at pixel         Coord incr Units
INFO ImageMetaData	----------------------------------------------------------------------------------------------------- 
INFO ImageMetaData	0    0     Direction Right Ascension   SIN  1280  160  17:45:40.038   640.00      -1.000000e-01 arcsec
INFO ImageMetaData	1    0     Direction Declination       SIN  1280  160 -29.00.28.069   640.00       1.000000e-01 arcsec
INFO ImageMetaData	2    1     Stokes    Stokes                    1    1             I
INFO ImageMetaData	3    2     Spectral  Frequency                 1    1 2998809251.46     0.00 2.023871358925e+09 Hz
INFO ImageMetaData	                     Velocity                                     0     0.00      -2.023274e+05 km/s
INFO imhead	Task imhead complete. Start time: 2023-01-19 17:52:14.304617 End time: 2023-01-19 17:52:14.307656
INFO imhead	##### End Task: imhead               #####
INFO imhead	##########################################
INFO imhead	##########################################
INFO imhead	##### Begin Task: imhead             #####
INFO imhead	imhead( imagename='SgrA-Bonly.image', mode='summary', hdkey='', hdvalue='', verbose=False )
INFO ImageMetaData	   
INFO ImageMetaData	Image name       : SgrA-Bonly.image
INFO ImageMetaData	Object name      : J1745-2900
INFO ImageMetaData	Image type       : PagedImage
INFO ImageMetaData	Image quantity   : Intensity
INFO ImageMetaData	Pixel mask(s)    : mask0
INFO ImageMetaData	Region(s)        : None
INFO ImageMetaData	Image units      : Jy/beam
INFO ImageMetaData	Restoring Beam   : 4.12117 arcsec, 1.30587 arcsec, 20.9395 deg
INFO ImageMetaData	   
INFO ImageMetaData	Direction reference : J2000
INFO ImageMetaData	Spectral  reference : LSRK
INFO ImageMetaData	Velocity  type      : RADIO
INFO ImageMetaData	Rest frequency      : 2.99914e+09 Hz
INFO ImageMetaData	Pointing center     :  17:45:40.038300  -29.00.28.068994
INFO ImageMetaData	Telescope           : EVLA
INFO ImageMetaData	Observer            : lorant sjouwerman
INFO ImageMetaData	Date observation    : 2013/10/27/00:11:04.500000
INFO ImageMetaData	Telescope position: [-1.60116e+06m, -5.04199e+06m, 3.55488e+06m] (ITRF)
INFO ImageMetaData	   
INFO ImageMetaData	Axis Coord Type      Name             Proj Shape Tile   Coord value at pixel         Coord incr Units
INFO ImageMetaData	----------------------------------------------------------------------------------------------------- 
INFO ImageMetaData	0    0     Direction Right Ascension   SIN  1280  160  17:45:40.038   640.00      -1.000000e-01 arcsec
INFO ImageMetaData	1    0     Direction Declination       SIN  1280  160 -29.00.28.069   640.00       1.000000e-01 arcsec
INFO ImageMetaData	2    1     Stokes    Stokes                    1    1             I
INFO ImageMetaData	3    2     Spectral  Frequency                 1    1 2999135410.54     0.00 2.024091446898e+09 Hz
INFO ImageMetaData	                     Velocity                                     0     0.00      -2.023274e+05 km/s
INFO imhead	Task imhead complete. Start time: 2023-01-19 17:55:34.641808 End time: 2023-01-19 17:55:34.644805
INFO imhead	##### End Task: imhead               #####
INFO imhead	##########################################
INFO imhead	##########################################
INFO imhead	##### Begin Task: imhead             #####
INFO imhead	imhead( imagename='SgrA-Conly.image', mode='summary', hdkey='', hdvalue='', verbose=False )
INFO ImageMetaData	   
INFO ImageMetaData	Image name       : SgrA-Conly.image
INFO ImageMetaData	Object name      : J1745-2900
INFO ImageMetaData	Image type       : PagedImage
INFO ImageMetaData	Image quantity   : Intensity
INFO ImageMetaData	Pixel mask(s)    : mask0
INFO ImageMetaData	Region(s)        : None
INFO ImageMetaData	Image units      : Jy/beam
INFO ImageMetaData	Restoring Beam   : 11.1818 arcsec, 3.98467 arcsec, 9.05307 deg
INFO ImageMetaData	   
INFO ImageMetaData	Direction reference : J2000
INFO ImageMetaData	Spectral  reference : LSRK
INFO ImageMetaData	Velocity  type      : RADIO
INFO ImageMetaData	Rest frequency      : 2.99912e+09 Hz
INFO ImageMetaData	Pointing center     :  17:45:40.038300  -29.00.28.068994
INFO ImageMetaData	Telescope           : EVLA
INFO ImageMetaData	Observer            : lorant sjouwerman
INFO ImageMetaData	Date observation    : 2013/08/09/04:26:49.500000
INFO ImageMetaData	Telescope position: [-1.60116e+06m, -5.04199e+06m, 3.55488e+06m] (ITRF)
INFO ImageMetaData	   
INFO ImageMetaData	Axis Coord Type      Name             Proj Shape Tile   Coord value at pixel         Coord incr Units
INFO ImageMetaData	----------------------------------------------------------------------------------------------------- 
INFO ImageMetaData	0    0     Direction Right Ascension   SIN  1280  160  17:45:40.038   640.00      -1.000000e-01 arcsec
INFO ImageMetaData	1    0     Direction Declination       SIN  1280  160 -29.00.28.069   640.00       1.000000e-01 arcsec
INFO ImageMetaData	2    1     Stokes    Stokes                    1    1             I
INFO ImageMetaData	3    2     Spectral  Frequency                 1    1 2999123164.42     0.00 2.024083211560e+09 Hz
INFO ImageMetaData	                     Velocity                                     0     0.00      -2.023274e+05 km/s
INFO imhead	Task imhead complete. Start time: 2023-01-19 17:55:39.040544 End time: 2023-01-19 17:55:39.043544
INFO imhead	##### End Task: imhead               #####
INFO imhead	##########################################
</pre> 


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. We can get the peak flux values from within the [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.visualization.viewer.html viewer] or use the [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.information.imstat.html 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 ==
We have seen in the initial images that Sgr A* is a variable source. The variability would introduce problems if a user wished to combine the datasets because the visibilities of the different sessions will reflect those variations. Cleaning would be difficult in such a situation as flux levels at different uv points (times and baselines) would not be 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 [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.analysis.imfit.html imfit]:

 <source lang="python">
# In CASA
imfit(imagename='SgrA-Aonly.image',box='630,620,650,660')
</source>

The output of [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.analysis.imfit.html imfit] into the logger is as follows: 

<pre>

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
</pre> 

We will use RA (J2000) =17h45m40.039s and DEC (J2000) = -29d00'28.05" as the central position of Sgr A* for all three configurations; 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 [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.imaging.ft.html ft]. Task [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.manipulation.uvsub.html 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, [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.manipulation.uvsub.html uvsub] will initially copy the DATA column to the CORRECTED column, then subtract. Note that running [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.manipulation.uvsub.html uvsub] twice will ''oversubtract''; we recommend to create backup copies of the previous datasets before subtracting Sgr A*:

<pre style="background-color: lightgrey;”>
# 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
</pre> 


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 [https://casaguides.nrao.edu/index.php?title=Create_a_Component_List_for_Selfcal Create a Component List for Selfcal]):

<source lang="python">
# 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()
</source>
(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 [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.imaging.ft.html ft]:

<source lang="python">
# In CASA
ft(vis='VLA-SgrA-Sband-Aconfig-sub.ms', complist='component-SgrA-A.cl', usescratch=True)
</source>
At this step the MODEL column is created. 

Finally, we subtract the MODEL from the DATA/CORRECTED_DATA with [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.manipulation.uvsub.html uvsub]. This task will also create CORRECTED as it does not exist yet:

<source lang="python">
# In CASA
uvsub(vis='VLA-SgrA-Sband-Aconfig-sub.ms')
</source>

Note that to revert back to the original state you can use task [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.calibration.clearcal.html clearcal] to reset the MODEL column and to set CORRECTED to be identical with DATA (thus undoing the source subtraction).

'''uvsub in CASA 6.4.1 has a bug and will show an error message like 'name 'vis' is not defined'. The task still appears to work correctly though, just the history entry seems to be compromised. '''


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:

<source lang="python">
# 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')
</source>

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*):

<source lang="python">
# 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')
</source>


Image the three data sets with Sgr A* removed. We run [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.imaging.tclean.html 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: 

<source lang="python">
# 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)
</source>

<source lang="python">
# 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)
</source>

<source lang="python">
# 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)
</source>

{|
|[[File:SgrA Aconfig_noPNTCASA6.4.png|400px|thumb|left|'''Figure 3a:''' <br />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, 0.02]]]
|[[File:SgrA Bconfig_noPNTCASA6.4.png|400px|thumb|left|'''Figure 3b:''' <br />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, 0.1]]]
|[[File:SgrA Cconfig_noPNTCASA6.4.png|400px|thumb|left|'''Figure 3c:''' <br />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 [-0.01, 0.5]]]
|}

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.

== Inserting an artificial Point Source for Sgr A* ==
<!-- '''Prior to this step, please refer to the CASA guide on multi configuration data combination on how to combine the datasets as we will now be using a combined set of the three configurations''' -->

We can add a point source with a canonical flux density of 1 Jy to bring back Sgr A*. Again create a component list and use [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.imaging.ft.html ft] to attach the source model. Using [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.manipulation.uvsub.html 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 recommend to not introduce an artificial source during the calibration steps as it has no phase errors imprinted. For this example, we re-introduce Sgr A* to the A-configuration dataset. First copy the dataset to the dataset the point source will be added to.

<pre style="background-color: lightgrey;”>
# In a Terminal
cp -r VLA-SgrA-Sband-Aconfig-sub.ms VLA-SgrA-Sband-Aconfig-addPNT.ms 
</pre>

Create a component list with a 1 Jy point source at the previously derived position of Sgr A*. 

<source lang="python">
# In CASA
cl.addcomponent(flux=1, fluxunit='Jy',shape='point', dir='J2000 17:45:40.039 -29.00.28.05')
cl.rename('component-SgrA.cl')
cl.close()
</source>

Insert the point source component into the model column with [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.imaging.ft.html ft] and add it to the data with [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.manipulation.uvsub.html uvsub] to the MS:
<source lang="python">
# In CASA
ft(vis='VLA-SgrA-Sband-Aconfig-addPNT.ms', complist='component-SgrA.cl', usescratch=True)
#
uvsub(vis='VLA-SgrA-Sband-Aconfig-addPNT.ms',reverse=True)

</source>

After inserting Sgr A* back into the dataset we image the measurement set once again.

<source lang="python">
# In CASA
tclean(vis=['VLA-SgrA-Sband-Aconfig-addPNT.ms'], 
      imagename='GC-addPNT',field='J1745-2900',specmode='mfs',cell='0.1arcsec',imsize=[1280,1280],niter=5000,
      threshold='5mJy',weighting='briggs',robust=0)
</source>

In Fig. 4 we show the resulting image on the same scales as in Fig. 2a. As expected the main difference is the added point source but some differences due to the [https://casadocs.readthedocs.io/en/v6.2.0/api/tt/casatasks.imaging.tclean.html tclean] algorithm are also visible. Again, one can improve the images as described above.

{|
|[[File:GC-addPNT CASA6.4.png|400px|thumb|left|'''Figure 4:''' <br />Combined image with a steady point source re-added. 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, 0.1]]]
|}



<!--
created: Juergen Ott August 2016
Edited: Tony Perreault October 2017
Edited: Updated to CASA 5.4.0 version, Angelica Vargas October 2018
Edited: Updated to CASA 5.5.0, Tony Perreault May 2019
Edited: Split off from VLA Data Combination guide to form new guide, Trent Seelig 2021-10-14
Edited: Documentation edits, Tony Perreault October 2021
Edited: Version CASA 6.4.1+CARTA figures, Pedro P.B. Beaklini on January,20th 2023 
-->

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