Introduction

Summary of Calibration tasks, and the Outline and Philosophy of (synthesis) calibration

This chapter explains how to calibrate interferometer data within the CASA task system.  Calibration is the process of determining the net complex correction factors that must be applied to each visibility in order to make them as close as possible to what an idealized interferometer would measure, such that when the data is imaged an accurate picture of the sky is obtained.  This is not an arbitrary process, and there is a philosophy behind the CASA calibration methodology.  For the most part, calibration in CASA using the tasks is not too different than calibration in other packages such as AIPS or Miriad.

Calibration tasks

Alert: The calibration table format changed in CASA 3.4.  CASA 4.2 is the last version that will support the caltabconvert function that provides conversions from the pre-3.4 caltable format to the modern format; it will be removed for CASA 4.3.  In general, it is best to recalculate calibration using CASA 3.4 or later.

Alert: In CASA 4.2 the gaincurve and opacity parameters have been removed from all calibration tasks (as advertised in 4.1).  These calibration types are supported via the gencal task.

Alert: As part of continuing development of a more flexible and improved interface for specifying calibration for apply, a new parameter has been introduced in applycal and the solving tasks: docallib.  This parameter toggles between use of the traditional calibration apply parameters ( gaintable, gainfield, interp, spwmap, and calwt), and a new callib parameter which currently provides access to the experimental Cal Library mechanism, wherein calibration instructions are stored in a file.  The default remains docallib=False in CASA 4.5, and this reveals the traditional apply parameters which continue to work as always, and the remainder of this chapter is still written using docallib=False.  Users interested in the Cal Library mechanism's flexibility are encouraged to try it and report any problems; see here for information on how to use it, including how to convert traditional applycal to Cal Library format.  Note also that plotms and mstransform now support use of the Cal Library to enable on-the-fly calibration when plotting and generating new MSs.

The standard set of calibration solving tasks (to produce calibration tables) are:

  • bandpass --- complex bandpass (B) calibration solving, including options for channel-binned or polynomial solutions
  • gaincal --- complex gain (G,T) and delay (K) calibration solving, including options for time-binned or spline solutions
  • polcal --- polarization calibration including leakage, cross-hand phase, and position angle
  • blcal --- baseline-based complex gain or bandpass calibration

There are helper tasks to create, manipulate, and explore calibration tables:

  • applycal --- Apply calculated calibration solutions
  • clearcal --- Re-initialize the calibration for a visibility dataset
  • fluxscale --- Bootstrap the flux density scale from standard calibration sources
  • listcal --- List calibration solutions
  • plotcal --- Plot calibration solutions
  • plotbandpass --- Plot bandpass solutions
  • setjy --- Compute model visibilities with the correct flux density for a specified source
  • smoothcal --- Smooth calibration solutions derived from one or more sources
  • calstat --- Statistics of calibration solutions
  • gencal --- Create a calibration tables from metadata such as antenna position offsets, gaincurves and opacities
  • wvrgcal --- Generate a gain table based on Water Vapor Radiometer data (for ALMA)
  • uvcontsub --- Carry out uv-plane continuum fitting and subtraction

The Calibration Process

A work-flow diagram for CASA calibration of interferometry data is shown in the following figure.  This should help you chart your course through the complex set of calibration steps.  In the following sections, we will detail the steps themselves and explain how to run the necessary tasks and tools.

 

Type Figure 1
ID CASA_cal_flow
Caption Flow chart of synthesis calibration operations. Not shown are use of table manipulation and plotting tasks: plotcal and smoothcal

 

The process can be broken down into a number of discrete phases:

  • Calibrator Model Visibility Specification --- set model visibilities for calibrators, either unit point source visibilities for calibrators with unknown flux density or structure (generally, sources used for calibrators are approximately point-like), or visibilities derived from a priori images and/or known or standard flux density values.  Use the setjy task for calibrator flux densities and models.
  • Prior Calibration --- set up previously known calibration quantities that need to be pre-applied, such antenna gain-elevation curves, atmospheric models, delays, and antenna position offsets.  Use the gencal task for antenna position offsets, gaincurves, antenna efficiencies, opacity, and other prior calibrations
  • Bandpass Calibration --- solve for the relative gain of the system over the frequency channels in the dataset (if needed), having pre-applied the prior calibration. Use the bandpass task
  • Gain Calibration --- solve for the gain variations of the system as a function of time, having pre-applied the bandpass (if needed) and prior calibration. Use the gaincal task
  • Polarization Calibration --- solve for polarization leakage terms and linear polarization position angle. Use the polcal task.
  • Establish Flux Density Scale --- if only some of the calibrators have known flux densities, then rescale gain solutions and derive flux densities of secondary calibrators.  Use the fluxscale task
  • Smooth --- if necessary smooth calibration using the smoothcal task.
  • Examine Calibration --- at any point, you can (and should) use plotcal and/or listcal to look at the calibration tables that you have created
  • Apply Calibration to the Data --- Corrected data is formed using the applycal task, and can be undone using clearcal
  • Post-Calibration Activities --- this includes the determination and subtraction of continuum signal from line data (uvcontsub), the splitting of data-sets into subsets (split, mstransform), and other operations (such as simple model-fitting: uvmodelfit).

The flow chart and the above list are in a suggested order.  However, the actual order in which you will carry out these operations is somewhat fluid, and will be determined by the specific data-reduction use cases you are following.  For example, you may need to obtain an initial gain calibration on your bandpass calibrator before moving to the bandpass calibration stage.  Or perhaps the polarization leakage calibration will be known from prior service observations, and can be applied as a constituent of prior calibration.

Calibration Philosophy

Calibration is not an arbitrary process, and there is a methodology that has been developed to carry out synthesis calibration and an algebra to describe the various corruptions that data might be subject to: the Hamaker-Bregman-Sault Measurement Equation (ME), described here.   The user need not worry about the details of this mathematics as the CASA software does that for you.  Anyway, it's just matrix algebra, and your familiar scalar methods of calibration (such as in AIPS) are encompassed in this more general approach.

There are a number of ``physical'' components to calibration in CASA:

  • data --- in the form of the MeasurementSet (MS).  The MS includes a number of columns that can hold calibrated data, model information, and weights
  • calibration tables --- these are in the form of standard CASA tables, and hold the calibration solutions (or parameterizations thereof)
  • task parameters --- sometimes the calibration information is in the form of CASA task parameters that tell the calibration tasks to turn on or off various features, contain important values (such as flux densities), or list what should be done to the data.

At its most basic level, Calibration in CASA is the process of taking "uncalibrated" data, setting up the operation of calibration tasks using task parameters, solving for new calibration tables, and then applying the calibration tables to form "calibrated" data.  Iteration can occur as necessary, e.g., to re-solve for an eariler calibration table using a better set of prior calibration, often with the aid of other non-calibration steps (e.g. imaging to generate improved source models for "self-calibration").

The calibration tables are the currency that is exchanged between the calibration tasks.  The "solver" tasks ( gaincal, bandpass, blcal, polcal) take in the MS (which may have a calibration model attached) and previous calibration tables, and will output an "incremental" calibration table (it is incremental to the previous calibration, if any).  This table can then be smoothed using smoothcal if desired.

The final set of calibration tables represents the cumulative calibration and is what is applied to correct the data using applycal. It is important to keep track of each calibration table and its role relative to others.  E.g., a provisional gain calibration solution will usually be obtained to optimize a bandpass calibration solve, but then be discarded in favor of a new gain calibration solution that will itself be optimized by use of the bandpass solution as a prior; the original gain calibration table should be discarded in this case.   On the other hand, it is also permitted to generate a sequence of gain calibration tables, each relative to the last (and any other prior calibration used); in this case all relative tables should be carried forward through the process and included in the final applycal.  It is the user's responsibility to keep track of the role of all calibration tables.  Depending on the complexity of the observation, this can be a confusing business, and it will help if you adopt a consistent table naming scheme.