What happens when the pipeline runs? (part II)
Calibration
In a typical synthesis observation, a number of calibrators with known properties (usually point sources, and at least one of known flux density) are observed in addition to the scientific targets in order to obtain a measure of the systematic properties of the instrument and the current environmental conditions at the time of the observation.
The observed data is a product of the intrinsic properties of the source and a variety of other effects (this list taken from the AIPS++ calibrator documentation:
- Complex gain effects which are polarization-independent, e.g. tropospheric effects, high-frequency opacity corrections, antenna gain as a function of elevation, baseline corrections.
- Parallactic angle.
- Instrumental polarization response. "D-terms" describe the polarization leakage between feeds (e.g. how much the R-polarized feed picked up L-polarized emission, and vice versa).
- Electronic gain response due to components in the signal path between the feed and the correlator. This complex gain term includes the scale factor for absolute flux density calibration, and may include phase and amplitude corrections due to changes in the atmosphere.
- Bandpass response.
The calibration process attempts to remove these affects from the target source data.
The recommended strategy for calibrating ATCA data is to observe a primary flux density calibrator (normally 1934-638, and sometimes 0823-500), as well as a secondary phase calibrator close to the source (normally using a calibrator selected from the ATCA calibrator database).
The pipeline uses metadata about the observation to guess which sources were intended to be the calibrators. It then matches each target source with an appropriate secondary calibrator. These guesses will be displayed in tables as the pipeline runs, allowing you to see if they are correct. Our testing has found these guesses are reasonably accurate in many cases, but in some observations (particularly non-standard ones) it is not possible to determine the calibrator relationships. This means this data will not be able to be processed correctly.
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