VLBI Data Reduction and Imaging Tutorial
Advanced Tutorial 3: Introduction to VLBI data reduction and imaging
Originally developed for the 10th European Radio Interferometry School in Granada (IAA-CSIC, September 30 – October 4, 2024)
Overview
This is a hands-on introduction to VLBI data reduction using CASA. The tutorial dataset will be updated with data from the e-EVN observation conducted during the school.
Note: This is a simplified introduction. For more detailed guides, please refer to:
Data Download
Download the tutorial dataset (320 MB):
- n23c2-eris-vlbi.tar.gz
- Alternative:
wget -r ftp.astron.nl:outgoing/jive/marcote/n23c2*
Dataset Information
Observation: N23C2 continuum dataset
Type: EVN network monitoring test with phase-referencing
Duration: ~2 hours
Method: Telescopes nodding between calibrator (C) and target (T) in C-T-C-T pattern
| Source Type | Source Name |
|---|---|
| Calibrator (C) | J0854+2006 |
| Target (T) | J0905+2052 |
Data Reduction in CASA
Setup
Navigate to your working directory and start CASA:
cd ~/ERIS/n23c2-eris-vlbi/
casaStep 1: Loading and Inspecting Data
Load the data:
default(importfitsidi)
fitsidifile='n23c2_1_1.IDI'
vis='n23c2.ms'
constobsid=True
scanreindexgap_s=15
importfitsidi()
Load flag table: Download the flag file: n23c2.flag
default(flagdata)
vis='n23c2.ms'
mode='list'
inpfile='n23c2.flag'
flagdata()
Understand your data:
default(listobs)
vis='n23c2.ms'
listfile='n23c2.ms.listobs'
overwrite=True
listobs()
Inspect uncalibrated data:
default(plotms)
vis='n23c2.ms'
xaxis='frequency'
yaxis='amp'
field='J0854+2006'
avgtime='3600'
antenna='EF&*'
gridrows=3
gridcols=3
iteraxis='antenna'
coloraxis='scan'
plotms()Step 2: A Priori Amplitude Calibration
Generate system temperature information:
default(gencal)
vis='n23c2.ms'
caltable='n23c2.tsys'
caltype='tsys'
uniform=False
gencal()
Generate gain curves:
default(gencal)
vis="n23c2.ms"
caltable= "n23c2.gcal"
caltype="gc"
gencal()
Inspect Tsys corrections:
default(plotms)
vis='n23c2.tsys'
xaxis='frequency'
yaxis='tsys'
gridrows=3
gridcols=3
coloraxis='corr'
iteraxis='antenna'
plotms()
xaxis='time'
plotms()Step 3: Data Inspection and Flagging
Plot amplitude vs frequency on sensitive baseline:
default(plotms)
vis='n23c2.ms'
xaxis='channel'
yaxis='amp'
field='J0854+2006'
avgtime='3600'
antenna='EF&JB'
iteraxis='spw'
gridrows=1
gridcols=4
plotms()
Flag band edges:
default(flagdata)
vis='n23c2.ms'
mode='manual'
spw='*:0~3;60~63'
flagdata()
Flag autocorrelations:
default(flagdata)
vis='n23c2.ms'
mode='manual'
autocorr=True
flagdata()Step 4: Fringe Fitting
Identify good timerange and check data:
default(plotms)
vis='n23c2.ms'
xaxis='frequency'
yaxis='phase'
ydatacolumn='data'
antenna='EF&*'
coloraxis='corr'
timerange='14:32:00~14:33:00'
averagedata=True
avgtime='120'
iteraxis='antenna'
gridrows=3
gridcols=3
plotms()
Calculate delays (SBD - Single Band Delays):
default(fringefit)
vis='n23c2.ms'
caltable='n23c2.sbd'
timerange='14:32:00~14:33:00'
solint='inf'
zerorates=True
refant='EF'
minsnr=10
gaintable=['n23c2.gcal','n23c2.tsys']
interp=['nearest','nearest,nearest']
parang=True
fringefit()
Apply SBD solutions:
default(applycal)
vis='n23c2.ms'
gaintable=['n23c2.gcal','n23c2.tsys','n23c2.sbd']
interp=['nearest','nearest,nearest','nearest']
parang=True
applycal()
Check corrected data:
tget(plotms)
ydatacolumn='corrected'
plotms()
Multi-band delays and rates (MBD):
default(fringefit)
vis='n23c2.ms'
caltable='n23c2.mbd'
field='J0854+2006'
solint='120s'
zerorates=False
refant='EF'
combine='spw'
minsnr=7
gaintable=['n23c2.gcal','n23c2.tsys','n23c2.sbd']
interp=['nearest','nearest,nearest','nearest']
parang=True
fringefit()
Inspect MBD solutions:
default(plotms)
vis='n23c2.mbd'
xaxis='time'
yaxis='delay'
gridrows=4
gridcols=4
coloraxis='corr'
iteraxis='antenna'
plotms()
Apply phase referencing:
default(applycal)
vis='n23c2.ms'
field='J0854+2006,J0905+2052'
gaintable=['n23c2.gcal','n23c2.tsys','n23c2.sbd','n23c2.mbd']
interp=['nearest', 'nearest,nearest','nearest','linear']
spwmap=[[],[],[],[0,0,0,0]]
parang=True
applycal()Step 5: Bandpass Calibration
Calculate bandpass:
default(bandpass)
vis='n23c2.ms'
caltable='n23c2.bpass'
field='J0854+2006'
gaintable=['n23c2.gcal','n23c2.tsys','n23c2.sbd','n23c2.mbd']
interp=['nearest','nearest,nearest','nearest','linear']
solnorm=True
solint='inf'
refant='EF'
bandtype='B'
spwmap=[[],[],[],[0,0,0,0]]
parang=True
bandpass()
Inspect bandpass solutions:
default(plotms)
vis='n23c2.bpass'
xaxis='frequency'
yaxis='amp'
coloraxis='corr'
iteraxis='antenna'
plotms()
Apply all calibrations:
default(applycal)
vis='n23c2.ms'
field='J0854+2006,J0905+2052'
gaintable=['n23c2.gcal','n23c2.tsys','n23c2.sbd','n23c2.mbd','n23c2.bpass']
interp=['nearest','nearest,nearest','nearest','linear','nearest,nearest']
spwmap=[[],[],[],[0,0,0,0],[]]
parang=True
applycal()Step 6: Export Data
Create calibrated datasets:
# Calibrator
default(mstransform)
vis='n23c2.ms'
outputvis='n23c2_J0854+2006.ms'
field='J0854+2006'
datacolumn='corrected'
keepflags=True
chanaverage=True
chanbin=64
mstransform()
# Target
default(mstransform)
vis='n23c2.ms'
outputvis='n23c2_J0905+2052.ms'
field='J0905+2052'
datacolumn='corrected'
keepflags=True
chanaverage=True
chanbin=64
mstransform()
Export to UV FITS:
# Calibrator
default(exportuvfits)
vis='n23c2_J0854+2006.ms'
fitsfile='n23c2_J0854+2006.uvfits'
combinespw=True
padwithflags=True
datacolumn='corrected'
exportuvfits()
# Target
default(exportuvfits)
vis='n23c2_J0905+2052.ms'
fitsfile='n23c2_J0905+2052.uvfits'
combinespw=True
padwithflags=True
datacolumn='corrected'
exportuvfits()
