Chapter 4
Calibrating Interferometer Data

This chapter focuses on ways to do the initial calibration of interferometric fringe-visibility data in . The sections which follow concentrate primarily on calibration for connected-element interferometers, especially the Karl G. Jansky very Large Array, called the EVLA hereafter. However, the information in these sections is useful for data from the historic VLA and other interferometers and to spectral-line, solar, and VLBI observers as well. For specific advice on the historic VLA, consult Appendix O. while the original appendix on EVLA data reduction has been deleted. For the gory details of VLBI, read Chapter 9. After the initial calibration has been completed, data for sources with good signal-to-noise are often taken through a number of cycles of imaging with self-calibration. See §5.4 for information on these later stages of the reduction process. For accurate calibration, you must have accurate a priori positions and structural information for all your calibration sources and accurate flux densities for at least one of them. It is best if the calibration sources are unresolved “point” sources, but it is not required.

For the basic calibrations, visibility (“uv”) data are kept in “multi-source data sets,” each of which contains, in time order, visibility data for one or more “unknown” sources and one or more calibration sources. Associated with these data are “extension” files containing tables describing these data. When VLA archive data are first read into  a number of basic tables are created and filled with information describing the data set. A complete set of these would include

1. AN (antennas) for sub-array geometric data, date, frequency, polarization information, etc.,
2. CD (CalDevice) for noise tube values,
3. CL (calibration) for calibration and model information,
4. CT (Calc) for astrometric data used in the correlation,
5. FG (flag) for flagging (editing) information,
6. FQ (frequency) for frequency offsets of the different IFs (IF pairs in VLA nomenclature),
7. GC (gain curve) for nominal sensitivity and antenna gain functions,
8. HI (history) for history records,
9. NX (index) to assist rapid access to the data,
10. SU (source) for source specific information such as name, position, velocity,
11. SY (SysPower) for system gains and measured total power with the noise tubes on and with them off, and
12. WX (Weather) for weather data.

An initial CL table contains gains due to known antenna functions of elevation and measured atmospheric opacities. VLBI, and especially VLBA, data sets will end up with even more table files. Calibration and editing tasks then create, as needed, other tables including

1. BL (baseline) for baseline-, or correlator-, dependent corrections,
2. BP (bandpass) for bandpass calibration,
3. PD (Polarization) for spectral dependent polarization D terms,
4. SN (solution) for gain solutions from the calibration routines.

All of these tables can be written to FITS files along with the visibility data using FITTP or FITAB. They can be read back in with FITLD. These, and any other,  tables can be manipulated and examined using the general tasks PRTAB, TACOP, TABED, TAMRG, TASRT, TAFLG and TAPPE.

The visibility data within the multi-source data set are not normally altered by the calibration tasks. Instead, these tasks manipulate the tabular information to describe the calibration corrections to be applied to the data and any flagging (deletion) of the data.

The  programs discussed in this chapter are part of a package that has been developed to calibrate interferometer data from a wide range of connected-element and VLB arrays, especially the VLA and VLBA. These programs therefore support many functions (and inputs) that are not required when calibrating normal VLA data. The examples given below show only the essential parameters for the operation being described, but, to get the results described, it is essential that you check all the input parameters before running any task. Remember that  adverbs are global and will be “remembered” as you proceed. A list of calibration-related symbols is given in §13.6, but a possibly more up-to-date list can be obtained by typing ABOUT CALIBRAT in your  session. More general information on calibration can be routed to your printer by typing DOCRT FALSE ; EXPLAIN CALIBRAT  C R, while deeper information on a specific task is obtained with EXPLAIN taskname  C R.

When you are satisfied with the calibration and editing (or are simply exhausted), the task SPLIT is used to apply the calibration and editing tables and to write uv files, each containing the data for only one source. These “single-source” uv files are used by imaging and deconvolution tasks that work with only one source at a time. Many of the tasks described in this chapter will also work on single-source files. For VLA calibration, there are several useful procedures described in this chapter and contained in the RUN file called VLAPROCS. Each of these procedures has an associated HELP file and inputs. Before any of these procedures can be used, this RUN file must be invoked with:

There is a “pipeline” procedure designed to do a preliminary calibration and imaging of ordinary VLA data sets. This provides a good first look at the data. Nonetheless, the results are still not likely to be of publishable quality. To run the pipeline, enter

 4.1 Copying data into multi-source disk files
  4.1.1 Reading EVLA archive files into
  4.1.2 Reading data from FITS files with FITLD
 4.2 Record keeping and data management
  4.2.1 Calibrating data with multiple FQ entries
  4.2.2 Recommended record keeping
 4.3 Beginning the calibration
  4.3.1 Baseline corrections
  4.3.2 Correcting EVLA 3-bit data
  4.3.3 Check your data with POSSM
  4.3.4 Remove spectral ringing with SPLAT
  4.3.5 General considerations in flagging
  4.3.6 Correcting delay errors with FRING
  4.3.7 Primary flux density calibrators
  4.3.8 Calibrating the bandpass shape with BPASS
  4.3.9 Calibrating the complex antenna gains with VLACALIB
  4.3.10 Bootstrapping secondary flux-density calibrators
  4.3.11 Editing visibility data with EDITA
  4.3.12 Applying solutions to the calibration table with CLCAL
  4.3.13 Flagging RFI with RFLAG
  4.3.14 Restart the calibration
  4.3.15 Calibration with the SysPower table
  4.3.16 Calibrating polarization
  4.3.17 Additional calibrations for EVLA data
  4.3.18 Making images from multi-source data with IMAGR
  4.3.19 Back up your data with FITTP or FITAB
  4.3.20 Creating single-source data files with SPLIT
 4.4 Concluding remarks, early science
 4.5 Additional recipes