EVLA high frequency spectral line tutorial - IRC+10216 - calibration
This guide is under development.
Getting the data
The data for this tutorial can be obtained by anonymous FTP from ftp://ftp.aoc.nrao.edu/staff/gvanmoor/community_day/. Download all 4 files TAR in this directory.
# In UNIX wget 'ftp://ftp.aoc.nrao.edu/staff/gvanmoor/community_day/*'
Inspection and Flagging
List observational parameters
listobs provides almost all relevant observational parameters such as correlator setup (frequencies, bandwidths, channel number and widths, polarization products), sources, scans, scan intents, and antenna locations. The following command writes the listobs output to a file on disk. You can review the output at any time using less from the casapy or UNIX command line.
# In CASA vis='day2_TDEM0003_20s_full' listobs(vis=vis, verbose=True, listfile=vis+'.listobs.txt')
Here is a subset of the listobs output.
Fields: 4 ID Code Name RA Decl Epoch SrcId nVis 2 D J0954+1743 09:54:56.82363 +188.8.131.524 J2000 2 32726 3 NONE IRC+10216 09:47:57.38200 +184.108.40.20600 J2000 3 99540 5 F J1229+0203 12:29:06.69973 +02.03.08.5982 J2000 5 5436 7 E J1331+3030 13:31:08.28798 +220.127.116.1189 J2000 7 2736 (nVis = Total number of time/baseline visibilities per field) Spectral Windows: (2 unique spectral windows and 1 unique polarization setups) SpwID #Chans Frame Ch1(MHz) ChanWid(kHz) TotBW(kHz) Corrs 0 64 TOPO 36387.2295 125 8000 RR RL LR LL 1 64 TOPO 36304.542 125 8000 RR RL LR LL Sources: 10 ID Name SpwId RestFreq(MHz) SysVel(km/s) 0 J1008+0730 0 0.03639232 -0.026 0 J1008+0730 1 0.03639232 -0.026 2 J0954+1743 0 0.03639232 -0.026 2 J0954+1743 1 0.03639232 -0.026 3 IRC+10216 0 0.03639232 -0.026 3 IRC+10216 1 0.03639232 -0.026 5 J1229+0203 0 0.03639232 -0.026 5 J1229+0203 1 0.03639232 -0.026 7 J1331+3030 0 0.03639232 -0.026 7 J1331+3030 1 0.03639232 -0.026 Antennas: 19: ID Name Station Diam. Long. Lat. 0 ea01 W09 25.0 m -18.104.22.168 +22.214.171.124 1 ea02 E02 25.0 m -107.37.04.4 +33.54.01.1 2 ea03 E09 25.0 m -126.96.36.199 +188.8.131.52 3 ea04 W01 25.0 m -107.37.05.9 +33.54.00.5 4 ea05 W08 25.0 m -184.108.40.206 +220.127.116.11 5 ea07 N06 25.0 m -107.37.06.9 +18.104.22.168 6 ea08 N01 25.0 m -107.37.06.0 +33.54.01.8 7 ea09 E06 25.0 m -22.214.171.124 +126.96.36.199 8 ea12 E08 25.0 m -188.8.131.52 +184.108.40.206 9 ea15 W06 25.0 m -220.127.116.11 +18.104.22.168 10 ea19 W04 25.0 m -22.214.171.124 +126.96.36.199 11 ea20 N05 25.0 m -107.37.06.7 +33.54.08.0 12 ea21 E01 25.0 m -107.37.05.7 +188.8.131.52 13 ea22 N04 25.0 m -107.37.06.5 +33.54.06.1 14 ea23 E07 25.0 m -184.108.40.206 +220.127.116.11 15 ea24 W05 25.0 m -18.104.22.168 +22.214.171.124 16 ea25 N02 25.0 m -107.37.06.2 +33.54.03.5 17 ea27 E03 25.0 m -107.37.02.8 +33.54.00.5 18 ea28 N08 25.0 m -107.37.07.5 +126.96.36.199
Note that the Rest Frequency and Systemic Velocity are wrong in the listobs log by a factor 10^6 and 1000, respectively, given the quoted units (MHz) and (km/s). This was due to a temporary error in the EVLA Observing Tool that has subsequently been fixed. Because the sky frequencies are correct, and we set the rest frequency explicitly later in the deconvolution stage, this does not present a problem for the data reductions.
We summarize the observing strategy in this table.
|Gain calibrator||J0954+1743||field id = 2|
|Bandpass calibrator||J1229+0203||field id = 5|
|Flux calibrator||J1331+3030 (3C286)||field id = 7|
|Science target||IRC+10216||field id = 3|
Plot antenna positions
We can use task plotants to generate a plot of the antenna positions. This is useful for choosing a reference antenna.
# In CASA plotants(vis=vis,figfile=vis+'.plotants.png')
We will use ea02 as the reference antenna.
Next, let's look at the elevation as a function of time for all sources. We will use plotms to plot the data. It's not the case for these data, but if the elevation is very low (usually at start or end of track) you may want to flag. Also, how near in elevation your flux calibrator is to your target will impact your ultimate absolute flux calibration accuracy. Unfortunately, the target and flux calibrator are not particularly well-matched for this observation, as you can show by plotting the elevation for each source (each sources has a different colors). Thus we are strongly dependent on the opacity and gaincurve corrections to get the flux scale right for these data. (This is something to keep in mind when planning observations!)
# In CASA plotms(vis=vis, xaxis='time',yaxis='elevation',correlation='RR,LL', avgchannel='64',spw='0:4~60', coloraxis='field')
Next, let's look at all the source amplitudes as a function of time.
# In CASA plotms(vis=vis, xaxis='time',yaxis='amp',correlation='RR,LL', avgchannel='64',spw='0:4~60', coloraxis='field')
Now zoom in on the region very near zero amplitude for sources J0954+1743 and IRC+10216. To zoom, select the Zoom tool in lower left corner of the plotms GUI, then you can left click to draw a box. Look for the low values (you may want to zoom a few times to really see the suspect points clearly). Now use the Mark Region and Locate buttons (located along the bottom of the GUI) to see which antenna is causing problems. The output is be shown in the logger. Since all the "located" baselines include ea12, this is the responsible antenna.
Now click the clear region button, and then go back to the zoom button to zoom in further to note exactly what the time range is: 03:41:00~04:10:00.
Check the other sideband by changing spw to 1:4~60. You will have to rezoom. If you have trouble, click on the Mark icon and then back to zoom. In spw=1, ea07 is bad from the beginning until after next pointing run: 03:21:40~04:10:00. To see this, compare the amplitudes when antenna is set to 'ea07' and when it is set to one of the other antennas, such as 'ea08'.
If you set antenna to 'ea12' and zoom in on this intial timerange, you can also see that ea12 is bad during the same time range as for spw 0. You can also see this by entering '!ea07' for antenna, which removes ea07 from the plot (in CASA selection, ! deselects).
We can set up a flagging command to get both bad antennas for the appropriate time and spw:
# In CASA flagdata(vis=vis, field=['2,3','2,3'], spw=['','1'], antenna=['ea12','ea07'], timerange=['03:41:00~04:10:00','03:21:40~04:10:00'])
flagdata works by spanning up a matrix. The first entries in each list must be taken as one flagging command, as well as the second entries etc. Lists within lists are fine. In the above example, the first flagging command is issued for fields 2 and 3 for all spws and within the 03:41:00~04:10:00 timerange. A second command is again for the fields 2 and 3 but for spw 1 only and for the second timerange in the list '03:21:40~04:10:00'.
Note that because the chosen timerange is limited to fields 2 and 3, the field parameter is not really needed; however, flagdata will run fastest if you put as many constraints as possible.
Now remove the !ea07 from antenna and replot both spw, zooming in to be sure that all obviously low points are gone. Also zoom in and check 3C286 (J1229+0203 is already obvious because it is so bright!).
Lets look more closely at IRC+10216:
# In CASA plotms(vis=vis,field='3', xaxis='time',yaxis='amp',correlation='RR,LL', avgchannel='64',spw='0~1:4~60', coloraxis='spw')
You can see a that there are some noisy high points. But now try
# In CASA plotms(vis=vis, field='3', xaxis='uvdist', yaxis='amp', correlation='RR,LL', avgchannel='64', spw='0~1:4~60', coloraxis='spw')
Most of the high points on IRC+10216 are due to large scale emission on short baselines, but there is still some noisy stuff -- for a target like this with extended emission it's best to wait until later to decide what to do about it. We will not be able to get adequate calibration for antennas that are truly bad (even if they don't stand out here) so these will be obvious later.
Set Up the Model for the Flux Calibrator
Next, we set the model for the flux calibrator. Depending on your observing frequency and angular resolution you can do this several ways. In the past, one typically used a point source (constant flux) model for the flux calibrator, possibly with a uvrange cutoff if necessary. More recently for the VLA/EVLA, model images for the most common flux calibrators have been made available for use in cases where the sources are somewhat resolved. This is most likely to be true at higher frequencies and at higher resolutions (more extended arrays).
Currently, CASA contains models for the most common calibrators at the most common frequencies but not yet the new EVLA frequency bands S, Ku, and Ka. These will be added as son as they become available. One may check the availability of calibration models in setjy:
# In CASA setjy(vis='day2_TDEM0003_10s_norx',listmodimages=T)