What happens when the pipeline runs? (part II)
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:
The calibration process attempts to remove these affects from the target
- 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 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.