Project Calibrations

We first give calibration modes that may have to be done for a given project. Some will be done once when the project is started or ended; some will be done at regular intervals during a session. This interval will appear as a parameter in the description of observing.

Amplitude calibration Needed for all astronomical observations: Short auto-correlations on blank sky, hot and cold loads. Used to measure system temperatures at the observing frequency. Blank sky to be done as often as needed to follow variations in atmospheric transparency; loads need to be done only if receiver performance varies. One may need to store the results for the source and the phase calibrator separately (for fast switching phase calibration).

Phase calibration Independent phase calibration is needed for most interferometric and phased array observations. Can be done frequently if needed, to correct for atmospheric phase fluctuations. One observes a point source (phase calibrator), near the project source. Some projects may need several different calibrators. The observation may be done at the observing frequency or at a different (low) frequency.

Bandpass calibration Needed for most interferometric observations. One observes a strong point source (calibrator), in the observable sky. The integration time will vary according to the project needs. The same calibrator must be observed both at the observing frequency and at the frequency used for phase calibrations.

Pointing/Focus calibration Needed for most astronomical observations. Done by cross-correlation scans, either on-the-fly or five-point scans on pointing calibrator, at low frequency, every 10-30 min. Extrapolated pointing parameters from last few pointing calibrations need to be fed back into antenna control. One may also be able to self-calibrate the pointing/focus from the observed source itself, if it is strong enough. At the low frequencies, pointing/focus calibrations may not be necessary.

Flux calibration Used to set up the absolute flux scale, by observing a source of predictable continuum flux (could be in total power or interferometric mode).

Polarization calibration This can probably be derived from phase calibration observations.