NGC3256Band3 for CASA 3.3

Overview

[To be written by Eric]

Retrieving the Data

The data were taken in six different datasets over two consecutive nights: April 16-17, 2011. There are three datasets for April 16th and three for April 17th. Here we provide you with "starter" datasets, where we have taken the raw data in ALMA Science Data Model (ASDM) format and converted them to CASA Measurement Sets (MS). We did this using the importasdm task in CASA.

[What else are we going to do to the data we provide?]

Along with the Measurement Sets, we also provide some tables that you will need for the calibration. These include the System Temperature (Tsys) tables, which contain corrections for atmospheric opacity, and Water Vapor Radiometer (WVR) tables, which contain the atmospheric phase corrections determined by the water vapor radiometers on each antenna.

You can download the data here: [Provide link to the raw .ms files in tar'd, gzip'd format]

Once the download has finished, unpack the file:

# In a terminal outside CASA
tar -xvf ngc3256band3.tgz

[Also provide links to the calibrated data (but maybe not here...maybe better at end of calibration page?)]

Initial Inspection and A priori Flagging

We will eventually concatenate the six datasets used here into one large dataset. However, we will keep them separate for now, as some of the steps to follow require individual datasets (specifically, the application of the Tsys and WVR tables). We therefore start by defining an array 'basename' that includes the names of the six files. This will simplify the following steps by allowing us to loop through the files using a simple for-loop in python.

# In CASA
basename=['uid___A002_X1d54a1_X5','uid___A002_X1d54a1_X174','uid___A002_X1d54a1_X2e3','uid___A002_X1d5a20_X5','uid___A002_X1d5a20_X174','uid___A002_X1d5a20_X330']

The usual first step is then to get some basic information about the data. We do this using the task listobs, which will output a detailed summary of each dataset supplied.

# In CASA
for name in basename:
        listobs(vis=name+'.ms')

The output will be sent to the CASA logger. You will have to scroll up to see the individual output for each of the six datasets. Here is an example of the most relevant output for the first file in the list.

Fields: 3
  ID   Code Name         RA            Decl           Epoch   SrcId nVis   
  0    none 1037-295     10:37:16.0790 -29.34.02.8130 J2000   0     38759  
  1    none Titan        00:00:00.0000 +00.00.00.0000 J2000   1     16016  
  2    none NGC3256      10:27:51.6000 -43.54.18.0000 J2000   2     151249 
  (nVis = Total number of time/baseline visibilities per field) 
Spectral Windows:  (9 unique spectral windows and 2 unique polarization setups)
  SpwID  #Chans Frame Ch1(MHz)    ChanWid(kHz)TotBW(kHz)  Ref(MHz)    Corrs   
  0           4 TOPO  184550      1500000     7500000     183300      I   
  1         128 TOPO  113211.988  15625       2000000     113204.175  XX  YY  
  2           1 TOPO  114188.55   1796875     1796875     113204.175  XX  YY  
  3         128 TOPO  111450.813  15625       2000000     111443      XX  YY  
  4           1 TOPO  112427.375  1796875     1796875     111443      XX  YY  
  5         128 TOPO  101506.187  15625       2000000     101514      XX  YY  
  6           1 TOPO  100498.375  1796875     1796875     101514      XX  YY  
  7         128 TOPO  103050.863  15625       2000000     103058.675  XX  YY  
  8           1 TOPO  102043.05   1796875     1796875     103058.675  XX  YY  
Sources: 48
  ID   Name         SpwId RestFreq(MHz)  SysVel(km/s) 
  0    1037-295     0     -              -            
  0    1037-295     9     -              -            
  0    1037-295     10    -              -            
  0    1037-295     11    -              -            
  0    1037-295     12    -              -            
  0    1037-295     13    -              -            
  0    1037-295     14    -              -            
  0    1037-295     15    -              -            
  0    1037-295     1     -              -            
  0    1037-295     2     -              -            
  0    1037-295     3     -              -            
  0    1037-295     4     -              -            
  0    1037-295     5     -              -            
  0    1037-295     6     -              -            
  0    1037-295     7     -              -            
  0    1037-295     8     -              -            
  1    Titan        0     -              -            
  1    Titan        9     -              -            
  1    Titan        10    -              -            
  1    Titan        11    -              -            
  1    Titan        12    -              -            
  1    Titan        13    -              -            
  1    Titan        14    -              -            
  1    Titan        15    -              -            
  1    Titan        1     -              -            
  1    Titan        2     -              -            
  1    Titan        3     -              -            
  1    Titan        4     -              -            
  1    Titan        5     -              -            
  1    Titan        6     -              -            
  1    Titan        7     -              -            
  1    Titan        8     -              -            
  2    NGC3256      0     -              -            
  2    NGC3256      9     -              -            
  2    NGC3256      10    -              -            
  2    NGC3256      11    -              -            
  2    NGC3256      12    -              -            
  2    NGC3256      13    -              -            
  2    NGC3256      14    -              -            
  2    NGC3256      15    -              -            
  2    NGC3256      1     -              -            
  2    NGC3256      2     -              -            
  2    NGC3256      3     -              -            
  2    NGC3256      4     -              -            
  2    NGC3256      5     -              -            
  2    NGC3256      6     -              -            
  2    NGC3256      7     -              -            
  2    NGC3256      8     -              -            
Antennas: 7:
  ID   Name  Station   Diam.    Long.         Lat.         
  0    DV04  J505      12.0 m   -067.45.18.0  -22.53.22.8  
  1    DV06  T704      12.0 m   -067.45.16.2  -22.53.22.1  
  2    DV07  J510      12.0 m   -067.45.17.8  -22.53.23.5  
  3    DV08  T703      12.0 m   -067.45.16.2  -22.53.23.9  
  4    DV09  N602      12.0 m   -067.45.17.4  -22.53.22.3  
  5    PM02  T701      12.0 m   -067.45.18.8  -22.53.22.2  
  6    PM03  J504      12.0 m   -067.45.17.0  -22.53.23.0 

This output shows that three fields were observed: 1037-295, Titan, and NGC3256. Field 0 (1037-295) will serve as the gain calibrator and bandpass calibrator; field 1 (Titan) will serve as the flux calibrator; and field 2 (NGC3256) is, of course, the science target.

Note that there are more than four SpwIDs even though the observations were set up to have four spectral windows. The spectral line data themselves are found in spectral windows 1,3,5,7, which have 128 channels each. The first one (spw 1) is centered on the CO(1-0) emission line in the galaxy NGC 3256 and is our highest frequency spectral window. There is one additional spectral window (spw 3) in the Upper Side Band (USB), and there are two spectral windows (spw 5 and 7) in the Lower Side Band (LSB). These additional spectral windows are used to measure the continuum emission in the galaxy, and may contain other emission lines as well.

Spectral windows 2,4,6,8 contain channel averages of the data in spectral windows 1,3,5,7, respectively. These are not useful for the offline data reduction. Spectral window 0 contains the WVR data. You may notice that there are additional SpwIDs listed in the "Sources" section which are not listed in the "Spectral Windows" section. These spectral windows are reserved for the WVRs of each antenna (seven in our case). At the moment, all WVRs point to spw 0, which contains nominal frequencies. The additional spectral windows (spw 9-15) are therefore not used and can be ignored.

Another important thing to note is that the position of Titan is listed as 00:00:00.0000 +00.00.00.0000. This is due to the fact that for ephemeris objects, the positions are currently not stored in the asdm. This will be handled correctly in the near future, but at present, we have to fix this offline. We will correct the coordinates below by running the procedure fixplanet.

There were seven antennas in the array for these observations. Note that numbering in python always begins with "0", so the antennas have IDs 0-6. To see what the antenna configuration looked like at the time of the observations, we will use the task plotants. Since the configuration did not change during the course of the observations, we will simply look at the first dataset from the list.

# In CASA
plotants(vis=basename[0]+'.ms', figfile='ngc3256band3_plotants.png')

This will plot the antenna configuration on your screen as well as save it under the specified filename for future reference. This will be important later on when we need to choose a reference antenna, since the reference antenna should be close to the center of the array (as well as stable and present for the entire observation).

The first editing we will do is some a priori flagging. We will start by flagging the shadowed data and the autocorrelation data:

# In CASA
for name in basename:
	flagdata(vis=name+'.ms', flagbackup = F, mode = 'shadow')
	flagautocorr(vis=name+'.ms')

There are a number of scans in the data that were used by the online system for pointing calibration. These scans are no longer needed, and we can flag them easily by selecting on 'intent':

# In CASA
for name in basename:
        flagdata(vis=name+'.ms', mode='manualflag', flagbackup = F, intent='*POINTING*')

Similarly, we can flag the scans corresponding to atmospheric calibration:

# In CASA
for name in basename:
        flagdata(vis=name+'.ms', mode='manualflag', flagbackup = F, intent='*ATMOSPHERE*')

We will then store the current flagging state for each dataset using the flagmanager:

# In CASA
for name in basename:
        flagmanager(vis = name+'.ms', mode = 'save', versionname = 'Apriori')

We will continue with some initial flagging/corrections specific to these datasets. For uid___A002_X1d54a1_X174.ms there is a outlying feature in spw=7, antenna DV04. This corresponds to scans 5 and 9, so we flag those data:

# In CASA
flagdata(vis='uid___A002_X1d54a1_X174.ms', mode='manualflag',
	antenna='DV04', flagbackup = F, scan='5,9', spw='7')

Antenna DV07 shows large delays for the first three datasets. We correct this by calculating a K-type delay calibration table with gencal. The parameters are the delays measured in nanoseconds, first cycling over polarization product, and then over spectral window (thus giving eight numbers in total). Before creating these tables, make sure to delete any existing versions.

# In CASA
for i in range(3): # loop over the first three ms's
	name=basename[i]
	os.system('rm -rf '+name+'_del.K')
	gencal(vis=name+'.ms', caltable=name+'_del.K',
	caltype='sbd', antenna='DV07', pol='X,Y', spw='1,3,5,7',
        parameter=[0.99, 1.10, -3.0, -3.0, -3.05, -3.05, -3.05, -3.05])

[MARTIN, HOW DID YOU GET THESE PARAMETERS? I ASSUME YOU FIT THE WRAPPING SOMEHOW AND PERHAPS WE SHOULD MENTION IT]

[ALSO, WHEN I RUN THIS, CASA SAYS "No scr col generation!!!" DO YOU KNOW WHAT THIS MEANS? I SEE YOU NOTE IT LATER IN YOUR SCRIPT AS WELL]

We will apply these K tables to the data in the next section.

WVR Correction and Tsys Calibration

First, we apply the delay correction table and the WVR calibration tables to the data. We do this in two steps, first cycling over the three datasets from the first day of observations because we have to correct the delay error for DV07 for those data. For the last three datasets, taken during the second day, we do not need to correct the delays, so we just apply the WVR tables. For both types of tables, we will use interpolation "nearest"... [BECAUSE WHY?]

# In CASA
for i in range(3): # loop over the first three data sets
	name=basename[i]
	applycal(vis=name+'.ms', flagbackup=F, spw='1,3,5,7',
		 interp='nearest', gaintable=[name+'_del.K',name+'.W'])

for i in range(3,6): # loop over the last three data sets
	name=basename[i]
	applycal(vis=name+'.ms', flagbackup=F, spw='1,3,5,7',
		 interp='nearest', gaintable=name+'.W')

Now we split out the datasets with delays and WVR tables applied. These datasets are given the extention "_K_WVR" to indicate that the delay tables and WVR tables have been applied. Again, we are careful to remove any previous versions of the split ms's before running the split command.

# In CASA
for name in basename:
	os.system('rm -rf '+name+'_K_WVR.ms*')
	split(vis=name+'.ms', outputvis=name+'_K_WVR.ms',
		datacolumn='corrected')

[WHEN I RUN THIS, I GET A MESSAGE ABOUT SPW 9-15 MISSING, AND IT'S GOING TO IGNORE THEM. THIS IS FINE, OF COURSE, BUT DO WE NEED TO SAY ANYTHING ABOUT IT?]

Next we do the Tsys calibration. Tsys measurements correct for the atmospheric opacity (to first-order) and allow the calibration sources to be measured at elevations that differ from the science target. The Tsys tables for these datasets were provided with the downloadable data. We will start by inspecting them:

# In CASA
for name in basename:
	plotcal(caltable='tsys_'+name+'.cal', xaxis='freq', yaxis='amp', 
                spw='1,3,5,7', timerange='<2020', subplot=221, overplot=False, 
                iteration='spw', plotrange=[0, 0, 40, 180], plotsymbol='.', 
                figfile='tsys_per_spw'+name+'.png')

Note that we only plot the spectral windows with the spectral line data, and we set timerange="<2020" because the SYSCAL table currently includes values from March 2081. [IS THERE ANY WAY TO GET AROUND EXPLAINING THIS EMBARRASSING ERROR HERE?] In addition to plotting on your screen, the above command will also produce a plot file (png) for each of the six datasets.

The plots look acceptable upon examination, so we will apply the Tsys tables with applycal: We do this for each field separately so that the appropriate calibration data are applied to the right fields. The "field" parameter specifies the field to which we will apply the calibration, and the "gainfield" parameter specifies the field from which you wish to take the calibration solutions from the gaintable.

# In CASA
for name in basename:
	for field in ['Titan','1037*','NGC*']:
		applycal(vis=name+'_K_WVR.ms', spw='1,3,5,7', flagbackup=F, field=field, gainfield=field,
			interp='nearest', gaintable=['tsys_'+name+'.cal'])

[**I COULDNT GET THIS TO WORK: KEPT GETTING SEVERE ERROR: SEVERE applycal::Calibrater::setapply(type, applypar) Error loading 'tsys_uid___A002_X1d5a20_X330.cal' with parsed selection: (FIELD_ID IN [2]). 2011-05-20 01:11:47 SEVERE applycal::Calibrater::setapply(type, applypar)+ (/var/rpmbuild/BUILD/casapy-stable/casapy-stable-32.0.15107/code/include/calibration/CalTables/CalSet.tcc : 320) Failed AlwaysAssert numberAnt==nElem_ Check inputs and try again. 2011-05-20 01:11:47 SEVERE applycal:::: Exception Reported: Error in Calibrater::setapply.

IT SEEMS TO BE COMPLAINING ABOUT THE FIELD SPECIFICATION IN THE TSYS SCRIPT. I GENERATED THE TSYS TABLES THE WAY MARTIN DID IN THE SCRIPT...IS THAT NOT CORRECT?]

We then split out spectral windows 1,3,5,7. This will get rid of the extraneous spectral windows, including the channel averaged spectral windows and spw 0, which is the one for the WVR data.

# In CASA
for name in basename:
	os.system('rm -rf '+name+'_line.ms*')
	split(vis=name+'_K_WVR.ms', outputvis=name+'_line.ms',
		datacolumn='corrected', spw='1,3,5,7')

The WVR and Tsys tables are now applied in the DATA column of the resultant measurement sets. The new data sets have the extension "_line" to indicate that they only contain the line data, and no longer the "channel average" spectral windows. These measurement sets therefore have four spectral windows.

Now that we have applied the Tsys calibration and WVR corrections, we can concatenate the six individual data sets into one big measurement set. We define an array "comvis" that contains the names of the measurement sets we wish to concatenate, and then we run the task concat.

# In CASA
comvis=[]
for name in basename:
	comvis.append(name+'_line.ms')

os.system('rm -rf ngc3256_line.ms*')
concat(vis=comvis, concatvis='ngc3256_line.ms')

---More to come here--- [IS THIS STILL TRUE?]

Additional Data Inspection

Now that the data are concatenated into one dataset, we will do some additional inspection with plotms. First we will plot amplitude versus channel, averaging over time and baselines in order to speed up the plotting process.

# In CASA
plotms(vis='ngc3256_line.ms', xaxis='channel', yaxis='amp',
       averagedata=T, avgbaseline=T, avgtime='1e8', avgscan=T)

From this plot we see that the edge channels have abnormally high amplitudes. We will use flagdata to remove some channels from both sides of the bandpass:

# In CASA
flagdata(vis = 'ngc3256_line.ms', flagbackup = F, spw = ['*:0~10','*:125~127'])

Next, we will look at amplitude versus time, averaging over all channels and colorizing by field. Since the observations take place over two days, you will need to zoom in to examine the data in more detail. In particular, note the difference in Titan's amplitude between the two days and the change in amplitude during the second day.

# In CASA
plotms(vis='ngc3256_line.ms', xaxis='time', yaxis='amp',
       averagedata=T, avgchannel='128', coloraxis='field')

Titan is our primary flux calibrator. However, for the second day of observations, Titan had moved to close to Saturn, and Saturn's rings moved into the primary beam. Another way to see this is to plot amplitude versus uv-distance and colorize by scan: [MARTIN, IS THIS WHAT YOU HAD IN MIND FOR THIS PLOT?]

# In CASA
plotms(vis='ngc3256_line.ms', xaxis='uvdist', yaxis='amp', field='1',
       averagedata=T, avgchannel='128', avgtime='1e8', coloraxis='scan')

If you use the "Mark a region" and "Locate" tools in plotms, you will see the signature of a bright, resolved source during the latter three (last day's) scans. We will therefore need to flag the Titan scans for the second day:

# In CASA
flagdata(vis = 'ngc3256_line.ms', flagbackup = F,
	timerange='>2011/04/16/12:00:00', field='Titan')

Next, we will fix the position of Titan in the combined dataset. Recall that the position of the Titan field is currently set to 00:00:00.0000 +00.00.00.0000. The following procedure will replace this with the actual position observed by the telescopes and, at the same time, it will recalculate the uvw coordinates:

# In CASA
execfile(os.getenv("CASAPATH").split(' ')[0]+"/lib/python2.6/recipes/fixplanets.py")

fixplanets('ngc3256_line.ms', 'Titan', True)

Continue to inspect the data with plotms, plotting different axes and colorizing by the different parameters. Don't forget to average the data if possible to speed the plotting process. You will find the following:

• Baselines with DV07 have very high amplitudes in spw 3, correlation YY
• Baselines with DV08 have very low amplitudes in spw 3, correlation YY, but only for the last observation
• Baselines with PM03 have low amplitudes at 2011/04/17/02:15:00 for spw 0
• Baselines with PM03 have low amplitudes at 2011/04/16/04:15:15 for spw 2 and 3

We flag the bad data with the following commands:

# In CASA
flagdata(vis='ngc3256_line.ms', flagbackup=F, spw='3',
	 correlation='YY', mode='manualflag', selectdata=T,
	 antenna='DV07', timerange='')

flagdata(vis='ngc3256_line.ms', flagbackup=F, spw='3',
	 correlation='YY', mode='manualflag', selectdata=T,
	 antenna='DV08', timerange='>2011/04/17/03:00:00')

flagdata(vis='ngc3256_line.ms', flagbackup=F, spw='0',
	 mode='manualflag', selectdata=T, antenna='PM03',
	 timerange='2011/04/17/02:15:00~02:15:50')

flagdata(vis='ngc3256_line.ms', flagbackup=F, spw='2,3',
	 mode='manualflag', selectdata=T, antenna='PM03',
	 timerange='2011/04/16/04:13:50~04:18:00')

Bandpass Calibration

We are now ready to begin the bandpass calibration. The first thing we will do is run the task gaincal on the bandpass calibrator to determine phase-only gain solutions. We will use solint='int' for the solution interval, which means that one gain solution will be determined for every integration time. This will correct for any phase variations in the bandpass calibrator as a function of time, a step which will prevent decorrelation of the vector-averaged bandpass solutions. We will then apply these solutions on-the-fly when we run bandpass.

We will use the average of channels 40 to 80 to increase our signal-to-noise in the determination of the antenna-based phase solutions. Averaging over a subset of channels near the center of the bandpass is acceptable when the phase variation as a function of channel is small, which it is here. For our reference antenna, we choose PM03. We call the output calibration table "ngc3256.G1".

# In CASA
gaincal(vis = 'ngc3256_line.ms', caltable = 'ngc3256.G1', spw = '*:40~80', field = '1037*',
        selectdata=T, solint= 'int', refant = 'PM03', calmode = 'p')

We check the time variations of the phase solutions with plotcal. We will plot the XX and YY polarization products separately and make different subplots for each of the spectral windows. This is done by setting the 'iteration' parameter to 'spw' and specifying subplot=221. By setting the parameter "figfile" to a non-blank value, it will also generate png files of the plots.

# In CASA
plotcal(caltable = 'ngc3256.G1', xaxis = 'time', yaxis = 'phase',
	poln='X', plotsymbol='o', plotrange = [0,0,-180,180], iteration = 'spw',
	figfile='phase_vs_time_XX.G1.png', subplot = 221)

plotcal(caltable = 'ngc3256.G1', xaxis = 'time', yaxis = 'phase',
	poln='Y', plotsymbol='o', plotrange = [0,0,-180,180], iteration = 'spw',
	figfile='phase_vs_time_YY.G1.png', subplot = 221)

Now that we have a first measurement of the phase variations as function of time, we can determine the bandpass solutions with bandpass, using the phase calibration table 'on-the-fly'.

First, plot the phase as a function of frequency for 1037. We use avgscan=T and avgtime='1E6' to average in time over all scans, and coloraxis='baseline' is used to colorize by baseline.

# In CASA
plotms(vis='ngc3256_line.ms', xaxis='freq', yaxis='phase', selectdata=True,
	field='1037*', avgtime='1E6', avgscan=T, coloraxis='baseline', iteraxis='antenna')

and the amplitudes

# In CASA
plotms(vis='ngc3256_line.ms', xaxis='freq', yaxis='amp', selectdata=True, spw='*:10~120',
	field='1037*', avgtime='1E6', avgscan=T, coloraxis='baseline', iteraxis='antenna')

Bandpass calibration, using the first gaincal on-the-fly

# In CASA
bandpass(vis = 'ngc3256_line.ms', caltable = 'ngc3256.B1', gaintable = 'ngc3256.G1',
	field = '1037*', minblperant=3, minsnr=1, solint='inf',
	bandtype='B', fillgaps=1, refant = 'PM03', solnorm = F)

# In CASA
plotcal(caltable = 'ngc3256.B1', xaxis='freq',yaxis='phase', spw='',
	subplot=212, overplot=False, plotrange = [0,0,-70,70],
	plotsymbol='.', timerange='')

plotcal(caltable = 'ngc3256.B1', xaxis='freq',yaxis='amp', spw='',
	subplot=211, overplot=False, 
	figfile='bandpass.B1.png', plotsymbol='.', timerange='')

[MOVE THIS SOMEWHERE ELSE?] Get flux density for Titan using the Butler-JPL-Horizons 2010 model. The flux density of Titan is 296 mJy

# In CASA
setjy(vis='ngc3256_line.ms', field='Titan', spw='', modimage='',
      scalebychan=False, fluxdensity=-1,
      standard='Butler-JPL-Horizons 2010')

Plot amplitude as function of uv distance for Titan for the remaining data. It looks unresolved

# In CASA
plotms(vis='ngc3256_line.ms', xaxis='uvdist', yaxis='amp',
	ydatacolumn='corrected', selectdata=True, field='Titan',
	spw='', averagedata=True, avgchannel='128', avgtime='',
	avgscan=True, avgbaseline=F)

Gain Calibration

Now do a new gaincal, using the bandpass on-the-fly

# In CASA
gaincal(vis = 'ngc3256_line.ms', caltable = 'ngc3256.G2', spw =
	'0:30~90,1:30~90,2:30~90,3:30~90', field = '1037*,Titan',
	solint= 'inf', selectdata=T, solnorm=False, refant = 'PM03',
	gaintable = ['ngc3256.B1'], calmode = 'ap')

Generate plots:

# In CASA
plotcal(caltable = 'ngc3256.G2', xaxis = 'time', yaxis = 'phase',
	poln='X', plotsymbol='o', plotrange = [0,0,-180,180], iteration
	= 'spw', figfile='phase_vs_time_XX.G2.png', subplot = 221)

plotcal(caltable = 'ngc3256.G2', xaxis = 'time', yaxis = 'phase',
	poln='Y', plotsymbol='o', plotrange = [0,0,-180,180], iteration
	= 'spw', figfile='phase_vs_time_YY.G2.png', subplot = 221)

plotcal(caltable = 'ngc3256.G2', xaxis = 'time', yaxis = 'amp',
	poln='X', plotsymbol='o', plotrange = [], iteration = 'spw',
	figfile='amp_vs_time_XX.G2.png', subplot = 221)

plotcal(caltable = 'ngc3256.G2', xaxis = 'time', yaxis = 'amp',
	poln='Y', plotsymbol='o', plotrange = [], iteration = 'spw',
	figfile='amp_vs_time_YY.G2.png', subplot = 221)

# In CASA
fluxscale( vis='ngc3256_line.ms', caltable='ngc3256.G2',
        fluxtable='ngc3256.G2.flux', reference='Titan',
        transfer='1037*', append=False)