National Radio
Astronomy Observatory

4.2.1 The Philosophy of Calibration in CASA

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 in Appendix E. The user need not worry about the details of this mathematics as the CASA software does that for you. Anyway, its 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 Measurement Set (§ 2.1). 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 parameters, solving for new calibration tables, and then applying the calibration tables to form “calibrated” data. Iteration can occur as necessary, with the insertion of other non-calibration steps (e.g. imaging to generate improved source models for “self-calibration”).

More information about CASA may be found at the CASA web page

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