VLBI Calibration

Bourke

with Baan, Beswick,Venturi

Assumptions:

1. Tom Muxlow: imaging and self-calibration.
2. Stephen Bourke: how to do bandpass calibration.
3. Rick Perley: how to do polarisation calibration.

• Prepared data: N09L2, a latest EVN Network Monitoring experiment. See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts Continuum: VLBI
• Reference document: http://www.evlbi.org/user_guide/guide/userguide.html EVN Data Analysis Guide
• Main steps:
  1. Load the data into AIPS (FITLD, MSORT, INDXR, LISTR).
  2. Calibrate amplitude (UVFLG, ANTAB, SNEDT, APCAL, CLCAL, SNPLT).
  3. Fringe-fitting (VLBAPANG, FRING, CLCAL, SNPLT).
  4. *Polarization calibrations (See the later tutorial by Perley).
  5. Bandpass calibration (BPASS/CPASS).
  6. Split the data (SPLAT, FITTP).
  7. *Imaging and self-calibration (See the earlier tutorial by Muxlow).

VLBI Tutorial

This tutorial concentrates on the analysis of an EVN (European VLBI Network) data. The observation consists of about a 2 hour phase referenced portion switching between 4C39.25 (the calibrator) and J0916+3854 (the target source). In the second half of the observation a number of bright sources are observed.

Loading

Use the task FITLD to load the data. The CLINT parameter defines the time interval between entries in the calibration tables. Setting this to a lower value (default is 1 minute) will allow us to calibrate on lower time scales later in the process (assuming we have good enough signal to noise). We'll set it to to 1/3 (20 seconds) VLBI dataset are often quite large (many GBs), setting DOUVCOMP = 1 saves disk space.

 default fitld
 datain 'PWD:n09l2_1_1.IDI
 clint 1/3
 douvcomp 1
 go
 

• Note that there is no closing quote ' after the file name. This means AIPS will keep the case in the string (normally it converts to upper case).

The dataset will have a name like MULTI .UVDATA.

The tasks LISTR and PRTAN can used to generate a scan listing, and an antenna listing respectively.

default listr
 getn 2
 optype 'scan'
 docrt -1
 outprint 'PWD:scans.txt
 go 

• getn 2 sets INNAME, INCLASS, INSEQ to the values of dataset 2 in the catalog (as seen when you 'pcat'). Use the value of the newly loaded data.
• the default value of 'docrt' is 132. This will cause the listing to be displayed in your terminal.

default prtan
 getn 2
 docrt -1
 outprint 'PWD:ants.txt
 go 

Typing 'imh' will show the data header. We can see it has 4 sub bands (AIPS calls these IFs) and 16 channels per sub band. As the data contains many sources the RA and DEC will either contain the coordinates of one of the sources in the data or may be 00.

Interlude: Plots

To get a feeling for interferometry, plotting the data, and calibration tables is important. Any plot will create a plot table (PL) by default unless you specify dotv 1. PL files are useful when you want to export a plot as a postscript file. The task LWPLA exports postscript.

• Useful plots:
  UVPLT can display your visibilities. You can specify what the axis represent. Default is amplitude vs uv-distance. Type 'explain uvplt' and look at the bparm value to see what can be on the axes. POSSM will plot your amplitudes for each baseline as a function of frequency.

VPLOT can plot each baseline, amplitude or phase, etc as a function of time, optionally averaging channels. SNPLT can plot your solution (SN) and calibration (CL) tables. It's very important to look at the calibration tables after a calibration task. Look out for large (suspicious) jumps in amplitude and phase.

When plotting uv data, if the datasets are large, plotting can take a long time. The 'xinc' parameter can be set to skip data eg, xinc 10 will plot every tenth point. timerang specifies a limited time range to plot. The format is startday, starthour, startminute, startsecond, endday, endhour, endminute, endsecond.

eg

timerang 0 13 0 0 0 13 5 0

would select five minutes of data from 13:00 to 13:05 on the first day.

   tvin
    default vplot
    getn 2
    bchan 1
    echan 16
    avgchan 1
    dotv 1
    docalib 1
    nplots 8
    go
 
    tvin
    default possm
    getn 2
    sources '4C39.25' ''
    aparm(9) 1
    dotv 1
    nplots 9
 
    tvin
    default uvplt
    getn 2
    sources '4C39.25' ''
    bparm 6 7 2 0
    xinc 100
    dotv 1
    go
 
    tvin
    default snplt
    getn 2
    ine 'sn'
    inv 3
    dotv 1
    nplots 8
    go
 

Applying External Flag Tables

Flag tables are provided with the data. These are generated by the scheduling program at the observatory and the observation stations during the observation and flag the times when each dish is not on target due to slewing, source set, etc. These can be attached to the dataset with the task UVFLG.

default uvflg
 getn 2
 intext 'PWD:n09l2.uvflg
 go
 

Amplitude Calibration

To use an astronomical source for amplitude calibration it must meet certain criteria; it must be relatively near to the target in the sky, have quite high flux, be reasonably compact and have a well known source model at the resolution of the instrument. Such sources are rare at VLBI resolutions and so another means of amplitude calibration is required. To accomplish this the antennas have an electronic noise source incorporated into them which is briefly triggered many times throughout the observation to allow the measurement of the system temperature of the antenna. This is done at the observatories and the results are provided to the astronomer as an 'antab' file. This is attached to the data with the task ANTAB. Also present in the file are the Gain Curves for each antenna. These relate the gain of the antenna as a function of elevation angle. AIPS can then use this information to determine amplitude calibration values for the observation.

default antab
 getn 2
 calin 'PWD:n09l2.antab
 go
 

The task APCAL is used to generate a Solution (SN) table of amplitude corrections from the Tsys (TY) and Gain Curve (GC) tables.

default apcal
 getn 2
 go
 

We can then create a new calibration with these amplitude solutions using the task CLCAL.

default clcal
 getn 2
 interpol 'self'
 go 

This creates a new calibration table CL 2, which contains the the values from CL 1 (which had constant values, amp 1, phase 0) and the solutions from SN 1. By default CLCAL will use the highest CL table, and all SN tables. This is ok now, as we have only 1 SN table, however we should be careful in future to specify which SN tables to use as we don't want to re-apply a SN table that has already been applied. SNVER = 1 would mean only SN 1 would be applied.

Parallactic Angle correction

We can use the the utility vlbapang to correct for parallactic angle effects. It is provided with aips in the vlbautil script to use it do

run vlbautil
 default vlbapang
 getn 2
 go vlbapang
 

Phase calibration

Phase calibration is slightly more complicated at VLBI scales as the positions of the antennas can be slightly different than what the correlator thought they were. Also, each antenna uses it's own clock, so the times can be slightly different at each stations and the times can drift relative to each other. These errors are very small but cause the phases to vary with frequency.

The task FRING is similar to CALIB but will solve for the delays (time offset) and rates (drifts) as well as the phases. A strong point like calibrator is needed to solve these parameters. It can be useful to first solve for delays only with a short piece time range on a bright calibrator and then solve for all parameters with all calibrators using the delay-corrected data.

default fring
 getn 2
 timerang 0 13 5 0 0 13 7 0
 docalib 1
 refant 2
 solint 0
 dparm(1) 1
 dparm(8) 1
 go
 

We will apply these delay corrections with CLCAL

default clcal
 getn 2
 interpol '2pt'
 refant 2
 snver 2
 go
 

Now we will solve for phase, delay and rate using all calibrators:

default fring
 getn 2
 calsour '4C39.25' '3C84' '3C273B' '3C286' 'OQ208' '3C345' ''
 docalib 1
 refant 2
 solint 2
 go
 

and apply these calibrations to create a new CL table

default clcal
 getn 2
 calsour '4C39.25' '3C84' '3C273B' '3C286' 'OQ208' '3C345' ''
 sources 'J0916+3854' '4C39.25' '3C84' '3C273B' '3C286' 'OQ208' '3C345' ''
 interpol 'ambg'
 snver 3
 refant 2
 go
 

CLCAL interpolates the solutions which were calculated by CALIB. It will interpolate the calibration solutions for the calibrator source to the target source. When we last ran CALIB we requested solutions every 2 minutes (solint 2) now these will be interpolated to the CL interval (20 seconds - set when we did FITLD or INDXR). Here we have requested that all sources be calibrated. The interpolation mode 'ambg' is useful with FRING calibration.

Bandpass Calibration

We will calibrated the gain response across our bands using the task BPASS with the source 4C39.25

default   bpass
 getn 2
 docalib 1
 sources '4C39.25' ''
 timerang 0 13 4 0 0 13 8 0
 refant 2
 go
 

This creates a bandpass table BP 1.

We will now create separate datasets for each source with all the flags, and calibrations applied.

default split
 getn 2
 docalib 1
 doband 1
 bpver 1
 aparm(1) 1
 nchav 0
 go
 

Setting aparm(1) and nchav causes the channels to be averaged together - this is ok only after you've fring fitted the data. We now have many single source datasets (type pcat). We can image these as in the other tutorials. eg:

default imagr
 getn 3
 cellsize 0.001
 imsize 512
 niter 1000
 uvwtfn 'na'
 dotv 1
 go 

Still room for improvement... data editing, self cal, re-fring, etc. but this is the general idea.