Amplitude self calibration
The amplitude calibration (magentaSELFCAL AMPLI) is a secondary step in the self-calibration process.
In general, it should only be attempted if the phase calibration was
already excellent, i.e., once you obtained the best solution by
adjusting the blueSELF_TIMES parameter for the
magentaSELFCAL PHASE command. If possible or needed, the amplitude
self-calibration should use a longer timescale than the phase
calibration (typically, blueSELF_TIMES=120s). magentaSELFCAL
automatically adjusts the gains so that their mean is 1, in order to
avoid changing the flux scale. In practice, it is useless if the expected noise
limited dynamic range is less than about 300.
Cases where Amplitude self calibration may be essential
If PHASE self-calibration does not sufficiently improve the image
despite ample signal-to-noise, Amplitude self calibration may do it.
This situation often occurs when the observations span different dates,
so that the relative flux calibration between the separate dates is
inconsistent. In this case, a AMPLI self-calibration may
be of great help. For compact sources, the flux consistency scale across dates can be
also checked and cured using command magentaSCALE_FLUX.
Note however that the resulting improvement does not necessarily produce a higher fidelity image. It removes the inconsistencies, but the selected (average) flux scale has no guarantee to be good. Flux calibration should be independently checked if this situation occurs.
Support restriction, flux threshold
Support restriction in the magentaCLEAN process may be needed to build a
simpler model for very complex, extended sources only. Command magentaMASK THRESHOLD can
be useful in this respect. Similarly, limiting
the flux per pixel in the model (see blueSELF_MINFLUX) may help, since
noise peaks are then ignored. However, the later may fail if the
source is too extended: low level brightness can be important for
self-calibrating short baselines.
Next: Transfering the solution to
Up: Self Calibration
Previous: Quality assessment and data
Contents
Index
Gildas manager
2023-06-01