Image
World map small

EVN Data Reduction Guide

Become familiar with EVN data reduction in AIPS or CASA. Here you can find the basic commands to calibrate your EVN data using any of the two packages (AIPS instructions on the left column, CASA instructions on the right column). Hover your mouse over the different parameters to know their meaning and the explanation about the chosen values. Note that VLBI data reduction under CASA is still under heavy development, and thus several options available in AIPS may still be missing in CASA.
You can also access the EVN Data Reduction Tutorials to follow a hands on calibration.

Users analyzing EVN data

Obtaining the data

The very first step is to obtain your EVN data, that can be retrieved from the EVN Data Archive. See the detailed instructions from the EVN Data Access page.

At this point you should have several FITS IDI files containing the visibility data from your observation and a tasav.FITS file containing the calibration tables generated by the EVN Pipeline.
 

Start AIPS and load the data

AIPS can be started by typing (you may need to initialize the environment before hand): aips tv=local:0. Introduce the AIPS user number that you wish to use for the session.

The EVN data need to be loaded as:

default fitld
datain 'PWD:experiment_1_1.IDI
digicor -1
doconcat 1
ncount n
outname 'exp'
go fitld
recat

 

And the same for the tasav FITS file:

tget fitld
datain 'PWD:experiment_tasav.FITS
digicor -1
doconcat -1
ncount 1
outname 'exp'
go fitld

 

The useful calibration tables from the EVN Pipeline must be transfered from the tasav file to the data. In the case of EVN data, this means the CL table no. 2 (calibration table containing the parallactic angle and a-priori gain corrections), the a-priori flagging (FG) table, and, optionally, the bandpass (BP) calibration.

default tacop
getn 2
geton 1
inext 'CL'
invers 2
ncount 1
go tacop
tget tacop
inext 'FG'
invers 1
ncount 1
go tacop
Optionally the bandpass calibration from the EVN Pipeline can be copied. Otherwise a manual bandpass calibration may be preferred.
tget tacop
inext 'BP'
invers 1
ncount 1
go tacop

 

Start CASA and load the data

Before starting CASA it is required to download some external Python scripts located under this link (append_tsys.py, flag.py, key.py, gc.py). A new environmental variable $PYCAPATH needs to be set, pointing to the directory where these scripts are located, and $PYTHONPATH also needs to be set/updated to include the same directory.
Additionally to the previous downloaded files, another two files from the EVN Data Archive are required. Under the "Pipeline" tab, download both the Associated EVN calibration (antab file) and the UVFLG flagged data (uvflg file).

First, the a-priori flag commands will be converted into a CASA-compatible format:

python $PYCAPATH/flag.py experiment.uvflg experiment_1_1.IDI1 > experiment.flag

 

Second, the a-priori gain calibration (system temperature measurements) will be incorporated into the IDI FITS files:

casa --nogui -c $PYCAPATH/append_tsys.py experiment.antab experiment_1_1.IDI*
Note that this step can only be run once. Multiple runs will cause problems later and it cannot be undone.
Finally, the gain curve calibration table (how the system temperature values are converted to Jansky units) must be generated with:
casa --nogui -c $PYCAPATH/gc.py experiment.antab EVN.gc
In some cases a restricted range for the aceptable elevation values must be set with the options , as some extreme values may cause the gain curves to fail.

 

CASA can be started by typing casa. The IDI FITS files may be converted to a Measurement Set (MS) file, the native format for CASA:

import glob
myidifiles = sorted(glob.glob('experiment*IDI*'))
importfitsidi(vis='experiment.ms',
    fitsidifile=myidifiles,
    constobsid=True,
    scanreindexgap_s=15.0)

 

The flag table may be applied to the data:

flagdata(vis='experiment.ms',
    mode='list',
    inpfile='experiment.flag',
    reason='any',
    action='apply',
    flagbackup=True,
    savepars=False)
And an a-priori gain calibration may be generated with:
gencal(vis='experiment.ms',
    caltable='cal.tsys',
    caltype='tsys',
    uniform=False)

gencal(vis='experiment.ms',
    caltable='cal.gcal',
    caltype='gc',
    infile='EVN.gc')
Note that CASA handles differently the parallactic angle corrections. They are applied on-the-fly for each calibration task instead of being corrected at the beginning like in AIPS.

 

Inspect the data

First, information about the number of scans, times, sources, and antennas can be retrieved by

In AIPS:

default listr
getn 1
optype 'SCAN'
go listr
default prtan
getn 1
go prtan
Data should be checked carefully before calibration to identify possible issues like radio frequency interference (RFI), dropouts of stations and some particular times, or bad data in general. We recommend to do a manual flagging of the data at this point and indentify the edge channels where the signal already dropped (and should thus be flaged). AIPS has a large number of possible tasks to perform these steps, among others UVPLT/UVFLG, SPFLG, or TVFLG. Pick your preferred one.
 

 

In CASA:

listobs(vis='experiment.ms')
And you can visualize the data interactively with

plotms(vis='experiment.ms')
Among other options, you can set the parameters xaxis, yaxis, coloraxis (either when calling plotms or inside the interactive window) to display different information like amplitudes, phases, times, uv distances, or u versus v. This tool also allows you to flag the bad data.

 

Atmospheric corrections

Depending on the data to be analyzed, some atmospheric corrections may be recommended. If the observation was conducted at frequencies of around 5 GHz or below, the data may suffer from ionospheric dispersive delays (different at each antenna and proportional to the Total Electron Content, TEC, along the line of sight and inversely proportional to the square of the frequency) that can be compensated.

In AIPS vlbatecr downloads the required ionospheric data at the time of the observation and produces a calibration.

run vlbautil
default vlbatecr
getn 1
go vlbatecr
A new CL table should have been created (number 3 if no additional steps have been done up to now).

 

In CASA tec_maps downloads the required data:

from recipes import tec_maps
tec_image, tec_rms_image, plotname = tec_maps.create(vis='experiment.ms', doplot=True)

gencal(vis='experiment.ms', caltable='cal.tecim',
    caltype='tecim', infile='tec_image')

viewer('experiment.ms.IGS_TEC.im')
viewer('experiment.ms.IGS_RMS_TEC.im')

 

 

If the observation was conducted at frequencies above 20 GHz, you may want to correct for tropospheric effects.
 

Removing instrumental delay

Phase jumps between subbands (or IFs) are related to instrumental delays at each telescope and thus can be corrected by using the data from one of the strong calibrators and apply the solutions for the whole observation. Note that if the data only have one subband (or if you conduct a subband-independent global fringe calibration), this step is not necessary.

In AIPS:

default fring
getn 1
timerang d1 h1 m1 s1 d2 h2 m2 s2
docal 1
gainuse n
doband 1
bpver 1
weightit 1
refant a
solint 0
dparm(9) 1
snver 1
go fring

In CASA:

fringefit(vis="experiment.ms",
    caltable="cal.sbd",
    timerange='h1:m1:s1~h2:m2:s2',
    solint='inf',
    zerorates=True,
    refant='a'
    minsnr=10,
    gaintable=['cal.tsys', 'cal.gcal', ...]
    parang=True)

 

In AIPS look at the solutions with

default snplt
getn 1
inext 'SN'
invers 1
opty 'dela'
dotv 1
nplots 8
tvini
go snplt

 

In CASA look at the solutions with:

plotms(vis='cal.sbd',
    xaxis='frequency',
    yaxis='phase',
    antenna='a'
    correlation='rr,ll',
    timerange='h1:m1:s1~h2:m2:s2',
    averagedata=True,
    avgtime='120')
Note that they can also be plotted by using the plotcal function.

 

In AIPS apply the solutions to the data and create a new calibration (CL) table.

default clcal
getn 1
gainver n
gainuse n+1
snver 1
interpol ''
opcode 'cali'
refant a
go clcal

 

In CASA apply the calibration to the data.

applycal(vis='experiment.ms',
    gaintable=['cal.tsys', 'cal.gcal', 'cal.sbd', ...],
    parang=True)

 

NOTE: In some cases it is not possible to obtain a time range at which all stations were observing. In those cases multiple FRING instances must be run. In AIPS, run the first FRING/CLCAL in the same way but with optype 'CALP' in the later (so it keeps data from antennas with no solutions).
Then FRING can be run again for a different time range and with dofit a1, a2,... where a1, a2, ... are the antennas that were not available before. Finally, run CLCAL as mentioned above to create a new CL table.
 

Bandpass calibration

In AIPS the bandpass table can be imported (recommended) from the EVN Pipeline results with TACOP (previous steps). In some cases, a manual bandpass calibration must be performed to recover a flat response of the instruments across all the band:

default bpass
getn 1
calsour 'bandpass-calibrator',''
docal 1
gainuse n
refant a
solint -1
weightit 1
soltype 'l1r'
bpassprm(1) 0
bpassprm(2) 1
bpassprm(10) 1
go bpass

NOTE: in this case the Bandpass correction may introduce again some offsets between subbands/IFs. Therefore, it is recommended to run again the instrumental delay correction.

 

In CASA the manual bandpass calibration must be performed by running:

bandpass(vis='experiment.ms',
    caltable='cal.bpass'
    field='bandpass-calibrator',
    gaintable=['cal.tsys', 'cal.gcal', ...],
    solnorm=True,
    solint='inf',
    refant='a',
    bandtype='B',
    parang=True)

 

Global fringe (or frequency and time-dependent phase calibration)

Phases must be corrected for the whole observation. For VLBI data that means that delays and rates must be corrected by using fringe fit. If the target source is bright and compact enough, it can directly be used in the fringe-fit. However, in most of the cases a nearby calibrator (the so-called phase calibrator) should be used for that and then solutions must be extrapolated to the target source position.

In AIPS:

default fring
getn 1
calsour 'calsource1', 'calsource2', ''
docal 1
gainuse n
doband 1
bpver 1
timerang 0
weightit 1
refant a
solint t
aparm(5) 1
aparm(6) 3
aparm(9) 1
dparm 1 200 50 0
search a2, a3, a4
snver 2
go fring

 

In CASA:

fringefit(vis='experiment.ms',
    caltable='cal.mbd',
    solint='t',
    combine='spw',
    field='calsource1, calsource2,...',
    refant='a',
    minsnr=7,
    gaintable=['cal.tsys', 'cal.gcal', 'cal.bpass', 'cal.sbd', ...],
    parang=True)

 

Note that the solution interval (solint) should be long enough to produce solutions with a good signal-to-noise ratio, but it must be smaller than the time interval at which you see a significant phase evolution (which would de-correlate the signal). Typically it is of the order of a few minutes for standard arrays and EVN frequencies. Also note that it is not possible to combine IFs (aparm(5) 1 in AIPS or combine='spw' in CASA) if the instrumental delay correction was not performed beforehand.

In AIPS apply the solutions to the data and create a new calibration (CL) table. It is recommended to apply the solutions to one source at the time:

default clcal
getn 1
calsour 'sourceN' ''
source calsour
gainver n
gainuse n+1
snver 2
interpol 'self'
opcode 'cali'
refant a
go clcal
Repeate to all sources used in fring. Then if solutions must be transfer to the target (like in phase-referencing experiments) then:
tget clcal
calsour 'phaseCalibrator' ''
source 'target'
interpol 'ambg'
opcode 'cali'
refant a
go clcal

 

In CASA apply the calibration to the data:

applycal(vis='experiment.ms',
    gaintable=['cal.tsys', 'cal.gcal', 'cal.bpass', 'cal.sbd', 'cal.mbd', ...],
    spwmap=[[], [], [], N*[0], ...],
    parang=True)

 

NOTE: Fringe fit assumes that your source(s) are point-like (unresolved for all baselines) and located at the phase center position. However, in some cases this may not be true and a source model can be provided to for a more accurate fit. In FRING (in AIPS), add the parameters get2n i (where i refers to the image file of the source) and cmethod 'dft' (for a better model interpolation).
In the case of spectral line observations, CVEL may be executed to provide the conversion between frequencies and velocities.

Split and store the data

The final calibration may be applied to the data and different datasets can be produced for the different sources.

In AIPS:

default split
getn 1
source ''
docal 1
gainuse n
doband 1
bpver 1
flagv 0
aparm 2 0
go split
To store the UVDATA outside AIPS:
default fittp
getn i
dataout 'PWD:filename.uvfits'
go fittp

 

 

In CASA:

split(vis='experiment.ms',
    field='source_name',
    outputvis='source_name.ms',
    datacolumn='corrected')
To store the data as UVFITS:
exportuvfits(vis='filename.ms',
    field='sources',
    fitsfile='filename.uvfits',
    datacolumn='corrected')

 

Initial imaging

Now images for each source can be produced. Depending on the type of observations, the required parameters may differ significantly. In the case of continuum data, an image can be obtained with:

In AIPS:

default imagr
getn n
imsize 512
cellsize c
niter 10000
gain 0.05
cmethod 'dft'
robust r
dotv 1
go imagr
In the case of spectral line data, a cube may be wanted. Select multiple channels with the desired averaging to obtain it.

 

 

In CASA:

tclean(vis='filename.ms',
    field='source',
    imagename='source.image',
    specmode='mfs',
    nterms=1,
    deconvolver='hogbom',
    gridder='standard',
    imsize=1024,
    cell='XXmas',
    weighting='natural',
    niter=10000,
    interactive=True,
    savemodel='modelcolumn')

 

Image fitting and statistics

Different measurements can be performed in the obtained images. The most basic ones are:

In AIPS:

  • tvall: show the image.
  • imstat: provides the statistics of the image (like rms, maximum and minumum brightnesses).
  • tvstat: similar to imstat but a region in the image can be specified.
  • jmfit: fit a Gaussian to the image.
To store the FITS images outside AIPS:
default fittp
getn i
dataout 'PWD:filename.fits'
go fittp

 

In CASA:

  • imview or viewer: show the image and allows interactive measurements.
  • imstat: provides the statistics of the image (like rms, maximum and minumum brightnesses).
  • imfit: fit a Gaussian to the image.
     
To store the images as FITS files:
exportfits(imagename='imagefilename',
    fitsname='imagefilename.fits')

 

Self-calibration

Typically this calibration can still be improved by self-calibrating the target data (or the phase calibrator data and then transfer the solutions to the target if this is faint). If a good source model has already been created, then this one can be used to improve the calibration:

In AIPS:

default calib
getn n
get2n m
nmaps 1
refant a
weightit 1
soltype 'l1r'
cmethod 'dft'
solint t
aparm 3 0
solmode 'p'
go calib
It will create a CALIB file with the calibrated data which can be used to image again the source (now with a better calibration). Repeat this process iteractively until reaching the best possible calibration. After some rounds of phase-only selfcalibration, and only if the model is good enough, a phase-and-amplitude selfcalibration may be performed (setting solmode 'a&p').

 

If the selfcalibration has been performed in a calibrator source, then the solutions may be transfered to the original UVDATA file and applied to the target source, to be later on imaged again.
First the SN table created in the SPLIT file used for the selfcalibration must be copied to the original UVDATA:

default tacop
getn n
geton 1
inext 'SN'
invers s
ncount 1
outvers 0
go tacop
Now a new CL (calibration) table may be created with CLCAL in the same way as before by using this SN table. Split the target source data again applying this CL table and the fully calibrated dataset will be ready to be imaged.

 

In CASA: (NOTE: THIS SECTION IS UNDER TESTING)

gaincal(vis='filename.ms',
    caltable='cal.pcal'
    field='calibrator',
    gaintype='T',
    solint='inf',
    refant='a',
    calmode='p',
    combine='spw',
    minsnr=10,
    minblperant=4)
Apply the calibration:
applycal(vis='filename.ms',
    caltable='cal.pcal'
    field='sources',
    gainfield='calibrator',
    gaintable=['cal.pcal'],
    calwt=False,
    flagbackup=False,
    interp='linearperobs')
Repeat this process iteractively until reaching the best possible calibration. After some rounds of phase-only selfcalibration, and only if the model is good enough, a phase-and-amplitude selfcalibration may be performed (setting calmode='ap').

 

Multi phase-center observations

The EVN supports multi phase-center correlations, where different correlations at different positions (within the primary beam) can be performed to observe multiple source with only one pointing. In these cases, the calibration may be performed in the same way for only one phase center. Once it is done, you can apply the same calibration to the other phase centers.

Primary beam corrections

When you image a source that is significantly offset from the phase center (the position in the sky where all antennas were pointing to; like in the case of multi phase-cetner observations), you may need to correct your data for the primary beam of each individual antenna (their response pattern outside the pointing position). JIVE is working in an automatic process to provide you these corrections within the EVN Pipeline results. Contact the JIVE Support Scientists to get information about how to apply this correction in your case.

Frequency Asked Questions (FAQ)

Uncertainties in total flux densities in VLBI data
Amplitudes in VLBI data are based on the measurement of antenna sensitivities and system temperatures during the observation. Given that no reference source can be used to set the true amplitude level (like it is done in connected interferometers with the known amplitude calibrators), the final amplitudes fully rely on these measurements and estimations. There is thus an uncertainty that propagates to the final absolute flux density scale in your data. Although it cannot be easily measured, in VLBI observations this method is typically accurate up to 10-15%. This should thus be the uncertainty quoted in the absolute flux density measurements that you perform on your data.
How to measure flux densities from the data
There are typically two different approaches to measure the flux density of a source in your VLBI data: doing it in the image plane or in the uv plane.
DO NOT USE CLCOR (AIPS) on EVN data
The use of CLCOR in AIPS to move the phase center of your data to a different position should be avoided.
Gain calibration issues
In some data it may happen that one (or some) antennas exhibit a significant offset in the gain calibration, producing amplitudes that are significantly off from what it was expected (e.g. when compared to the other baselines). This issue can be corrected via self-calibration (producing an accurate model with the other baselines), or with tasks like gscale in packages like Difmap.
When do self-calibration?
Self-calibration can only be reliably used when certain conditions are met. You can read the guide from Sabrian Stierwalt (NRAO) about when, why, and how to do self-calibration on your data.