9.1 VLBI data calibration recipe
See Appendix C for simpler and shorter recipes suitable for straightforward observations.
- LOAD THE DATA For data from the VLBA correlator, run FITLD (§18.104.22.168); if needed, follow
up with MSORT, USUBA, INDXR, VBGLU, VBMRG, and MERGECAL (§22.214.171.124–§126.96.36.199). For data from a MkIII
correlator, see §O.2. Data from the Penticton correlator should be loaded using FITLD, sorted (MSORT,
§188.8.131.52), and indexed (INDXR, §184.108.40.206).
- POLARIZATION: The combination of the VLBA correlator and FITLD incorrectly labels
polarizations for dual parallel-hand correlation (RR and LL only), even if RR and LL are in different
frequency bands (e.g., LL at 5 GHz and RR at 8.4 GHz). For these types of data, you must run FXPOL
- EXAMINE THE DATA It is important to familiarize yourself with the data set before proceeding
further, especially if you have little experience with VLBI data. There are many tasks for the
examination of your data (see §9.4 for a fuller discussion). Minimally, you should at first run LISTR,
IMHEAD, EDITR, POSSM, VPLOT, and PRTAN. At later stages you will probably find SNPLT, PRTAB, DTSUM,
and SHOUV useful for examining data and calibration tables.
- SVLBI: Task OBPLT allows you to examine different aspects of the spacecraft orbit.
- PROCESS THE CALIBRATION FILES You will have either received calibration files, or instructions
on where to obtain them. Some calibration files can be automatically processed into a form suitable for
use within using VLOG (§9.5.2). ANTAB is now the primary task for loading calibration
information from log files.
- VLBA CORRELATOR: The VLBA correlator will usually attach calibration information directly to
your data for all VLBA and some other antennas. This obviates the need to run VLOG, ANTAB, PCLOD,
and UVFLG to process your a priori calibration information for VLBA antennas. Some information for
non-VLBA antennas must usually still be loaded from text files.
- POLARIZATION: Be careful to make sure that the polarization labeling of the IFs in the calibration
text files is the same as the labeling in the data.
- CORRECT FOR THE IONOSPHERE For low frequency experiments TECOR should be run to
remove at least part of the ionospheric contribution to the phase offsets. This should also be
considered for higher frequencies (e.g., 8 GHz) depending on the amount of phase wrapping caused
by the ionosphere.
- CORRECT FOR THE EARTH ORIENTATION PARAMETERS For phase referencing experiments
correlated at the VLBA correlator, particularly between 5-May-2003 and 2-August-2005, CLCOR
(OPCODE=’EOPS’) should be run. This will correct the possibly inaccurate Earth Orientation
Parameters used by the VLBA correlator. This is particularly important for astrometry experiments
but can effect any phase referencing experiment including those correlated outside the above range
- EDIT THE DATA Identifying and editing bad data now can save you time later. Data should first
be edited using UVFLG to apply editing information supplied with your calibration files (§9.5.3). Some
useful tasks for examining and editing data are EDITR, UVFLG, TVFLG, SPFLG, EDITA, BPEDT, FLAGR,
FINDR, VPLOT, and QUACK.
- POLARIZATION: You may want to edit the data consistently in all polarizations (select STOKES =
’IQUV’ within EDITR, TVFLG or SPFLG) — this can greatly simplify the imaging stage (see step 15).
- POLARIZATION: ADD PARALLACTIC ANGLE CORRECTION For alt-az mounted antennas,
a parallactic angle correction for the rotating orientation of the antenna feeds with respect to the
observed source must be performed as the first step in the phase calibration using CLCOR (§220.127.116.11).
This step should be performed no later than immediately after the Tsys calibration.
- PHASE REFERENCING: You will want to perform the parallactic angle correction described above
for phase referencing observations even if you only correlated the parallel hands (RR, LL).
- APPLY SAMPLER CORRECTION We now advocate a new amplitude calibration strategy based
on VLBA Scientific Memo #37 (Walker 2015). This strategy interleaves the classic a priori calibration
with instrumental delay and bandpass calibration to improve the calibration of data from the
new Roach Digital Backend (RDBE) on the VLBA (see VLBA Observational Status Summary for
a description of the RDBE and VLBA Scientific Memo #37 for a discussion of amplitude problems
when using the RDBE). With data from before the RDBE you can use either the old or new strategy.
First corrections for sampler biases should be applied using ACCOR (§18.104.22.168) for data from the VLBA
and some other correlators. Some correlators apply this correction to the data before writing them out
— notably the EVN JIVE correlator and correlators used for the Australian LBA. The VLBA hardware
and software (DiFX) correlators do not apply this correction to the data. Therefore, ACCOR is required
for the VLBA correlators and any others that do not apply the correction. ACCOR should be benign (do
nothing) for those correlators that do apply the correction prior to reading the data into . Note
that you can always run ACCOR and look at the SN table produced with PRTAB or SNPLT to see if it was
benign or not.
- CALIBRATE THE INSTRUMENTAL DELAYS Phase-cals, or measured single-band and multi-band
instrumental phase errors, should be applied using PCCOR (§22.214.171.124). You can manually perform
a phase-cal by running FRING on a limited subset of your data to account for missing phase-cal
information or to refine the reported phase-cal measurements (§126.96.36.199).
- SPECTRAL-LINE: Delay calibration should be carried out only on the continuum sources at this
stage. Since there should be no pulse-cals, the “manual” phase-cal method should be used.
- POLARIZATION: If running FRING to determine the instrumental delays, be certain to solve for
independent left- and right-polarization delay solutions APARM(3) = 0. Run RLDLY after calibrating
the instrumental delays, to determine a single delay offset between left and right polarization
- CALIBRATE THE COMPLEX BANDPASS RESPONSE FUNCTION Run BPASS or CPASS to
determine the bandpass response function using the cross-power spectra (§188.8.131.52). The normalization
should be over the full bandwidth, be careful because the default channel selection is the inner 75%
of the band. If you use the inner 75%, it can lead to amplitude errors of up to 15%.
- SPECTRAL-LINE: The bandpass response function should be determined using only the continuum
- APPLY ONE MORE AUTOCORRELATION CORRECTION AND THEN FINISH A PRIORI
CALIBRATION After determining the bandpass calibration the autocorrelations are probably
different from unity by a few percent. To correct this run the task ACSCL, applying the previous
calibration and bandpass correction. Then finally use APCAL to complete the a priori amplitude
calibration (§184.108.40.206) — this is called the Tsys method of amplitude calibration. APCAL can also be
used to perform opacity corrections.
- SPECTRAL-LINE: Unless the line emission is very weak, you may wish to defer amplitude
calibration of your line sources only until step 16 below. The template method described there is much
more accurate than the Tsys method.
- FRINGE FIT THE DATA Estimate and remove residual delays, rates and phases using FRING or
BLING and CLCAL (§220.127.116.11–§18.104.22.168).
- SPECTRAL-LINE: Only fringe-fit the calibrator source at this stage. Check the coherence of the target
source using the resulting solutions to decide whether or not to zero the rate solutions using the
’ZRAT’ option in SNCOR (§22.214.171.124).
- PHASE REFERENCING: You should not fringe-fit on the target, or phase-referenced source. Rather,
you should fringe-fit on the cal, or phase-reference calibrator. When you apply the solution, be sure
to set the CALSOUR and SOURCES adverbs in CLCAL appropriately to interpolate the solutions for the
cal source onto the target source (see §126.96.36.199). If you are not interested in astrometric calibration and
your target source is strong enough, you may wish to consider fringe-fitting on it to further refine the
phase calibration (§188.8.131.52).
- POLARIZATION: ESTIMATE THE INSTRUMENTAL POLARIZATION . Correct for the
instrumental polarization terms, commonly known as ‘D-terms’ using PCAL, LPCAL, or SPCAL on the
polarization calibrator (§184.108.40.206). This polarization calibrator should first be fully calibrated and
imaged before this step can be performed.
- POLARIZATION: CALIBRATE THE POLARIZATION POSITION ANGLE If a calibration source
with known polarization orientation is available, use CLCOR to make a final correction to adjust the
polarization angles of the target source data (§220.127.116.11).
- SPECTRAL-LINE or POLARIZATION: The bandpass response function should be determined using
only the calibrator source. Unlike step 7, this step cannot be skipped.
- APPLY THE DOPPLER CORRECTION (spectral-line data only). Run CVEL to compensate for
the changing Doppler shifts of the antennas with respect to the source during the observation and
between the different observations (§9.5.5).
- SPECTRAL-LINE: REFINE THE AMPLITUDE CALIBRATION Run ACFIT to amplitude calibrate
the program source using the template spectra method (§9.5.6). Note that the traditional Tsys method
(§18.104.22.168) can also be used if the line emission is too weak for the template method to work
- SPECTRAL-LINE:DETERMINE RESIDUAL RATES Now estimate the residual rates ONLY by
running FRING or BLING on one or a few spectral points on the target source (§22.214.171.124).
- APPLY CALIBRATION, AVERAGE, AND INSPECT THE FINAL DATA Run SPLIT or SPLAT to
apply the calibration solutions and to average the data in frequency if appropriate (§9.6.1), and UVAVG
to average the data in time (§9.6.2). You can also run SPLAT to combine these three operations into
a single step. It is recommended that you take the time to inspect the calibrated data to see if more
editing is needed, and to check that no gross calibration errors remain in the data (§9.6.3).
- SELF-CALIBRATE/IMAGE OR SELF-CALIBRATE/MODEL-FIT THE DATA The final complex
gain corrections are determined by iterating self-calibration with imaging of the resultant data set.
This is called hybrid-mapping. Alternatively, self-calibration can be iterated while fitting models
directly to the data — the goal is to self-calibrate using the best model possible. The options are
outlined in §9.7.
- SPECTRAL-LINE: One final distinction remains between continuum and spectral-line data. Only
one or a few spectral points are used to determine final complex gain corrections which are then
applied to all spectral points in the line data. After applying these gains, the line source data can be
imaged to form an image cube.
- POLARIZATION: While the Stokes I and Stokes V images formed using the RR and LL visibilities
will be real-valued, the Stokes Q and Stokes U images formed using LR and RL visibilities can, in
principle, be complex-valued. You must use a fully complex imaging and deconvolution technique
(see the HELP files for CXPOL and CXCLN) or you can simply edit the LR and RL visibilities to enforce the
condition that the whenever you have a RL visibility on a baseline, you also have the LR visibility on
the same baseline; this ensures that the Stokes Q and U images are real-valued and allows you to use
the standard imaging tasks.