Getting Started

Documentation

Glish

Learn More

Programming

Contact Us

Version 1.9 Build 803

News	FAQ
Search	Home

Summary of Data Reduction Steps

Listed below are the basic steps for typical continuum and spectral line data reduction sessions which solve for and apply the standard complex gain (G), the polarization leakage (D), and the relative bandpass (B).

Since reduction of continuum, spectral-line, and polarimetry data in AIPS++ differ essentially only in the addition or omission of specific steps, and in the details of a few parameter settings, only one pass through the basic reduction path is described here. In the remainder of this document, annotated sequences of glish for both continuum polarimetry and spectral-line reduction are included in parallel. Some actions apply to only one type of reduction, but all methods are instructive for either type of observation.

The basic reduction sequence is:

1. Import the data. Data may be imported from either a VLA archive tape using the vlafiller tool, or from a UVFITS disk file (e.g., as written by AIPS) using the ms tool's ms.fitstoms tool constructor. The result is a Measurement Set table (the AIPS++ internal data format). The ms tool is used to obtain a summary of the dataset.

2. Examine and edit the data using the flagger tool for basic data editing, the autoflag tool for automatic data editing, and the msplot tool for interactive editing.

3. Set the initial calibration models. By default, all sources have unit flux density point source models. Use the Imager.setjy function to set (point-source) flux densities for the absolute flux calibrators.

4. Obtain the complex gain (G) calibration for calibrators using the calibrater tool. If planning to do instrumental polarization (D) calibration, application of parallactic angle (P) corrections is required at this point.

5. Obtain the bandpass calibration (B) (spectral-line observations only) for bandpass calibrator(s) using the calibrater tool. The G calibration obtained above (and P, if necessary) is applied on-the-fly to ensure that the B calibration is normalized.

6. Transfer the flux density scale derived from the absolute flux calibrator(s) using the calibrater tool. Only the G calibration solutions obtained for the amplitude calibrators are properly scaled in amplitude. The solutions for the other calibrators are rescaled with the calibrater.fluxscale function by demanding that the mean gain amplitude be the same for all calibrators.

7. Apply the derived calibration to the source data.

8. Obtain the instrumental polarization calibration (D) (polarization observations only). Use a full-polarization model for a calibrator (obtained by imaging^1.1, and set with the Imager.setjy function). All previous calibration factors are applied before the D-terms or ``leakage terms'' are determined.

9. Apply all calibrations to the target source data. The calibrater.correct function applies calibration to the observed data and writes the result in the CORRECTED_DATA column in the MeasurementSet.

10. Image the target source. The imager tool is used to form images from the calibrated data, deconvolve these images, and restore the residual errors to the model image.

11. As necessary, self-calibrate the target source data and reimage. New and improved calibration may be obtained by repeating the initial calibration step(s), this time using the model image obtained in the first round of imaging. Re-image.

12. Repeat the self-calibration as necessary. Self-calibration need not be limited to only the G calibration factor; any solvable calibration component (G, T, B, and/or D, in the present example) may be self-calibrated.

Keep in mind that these examples are basic ones. More sophisticated sequences are possible. For example, it may be desirable to obtain bandpass calibration from an optimum calibrator, and to apply this on-the-fly during full-out G calibration on a different calibrator. This operation can be performed in the G and B calibrations steps listed above for the bandpass calibrator only, then repeating the G calibration step for the other calibrators with the B calibration applied. In fact, any sequence of operations of this nature is possible; the fundamental requirement is that calibration components that significantly influence the one being solved for should be applied prior to the solution.