Real Time Calibration

Under this denomination we group a certain number of calibration operations that need to be done on the visibilities, and on autocorrelation data. These calibration steps are performed by the data acquisition processes (CORREL and RDI) running on the control computer, BURE01::. It is nevertheless useful to briefly describe these operations here. They are:

• Clipping correction of line correlator data (the clipping correction for the continuum correlator is performed in real time on the control computer). For this we use the approximation of Kulkarni and Heiles (Astron.J.85, 10, 1413) for correlation data, and a numeric integration using cubic splines for auto correlation data.
• Fourier transform of autocorrelation data
• Fourier transform and sideband separation of correlation data
• Correction of phases for delay rounding errors
• Reduction of atmospheric calibrations, using an atmospheric model and recorded atmospheric parameters. A calibration constant ($T_{CAL}$) is determined, as well as the gains in each channel for each antenna.
• Reduction of the IF Passband calibration scans.
• Amplitude calibration of correlation data using the last available autocorrelation data, and system temperatures from atmospheric calibrations
• Calculation of velocity scales for each sub-band and side band.

The atmospheric calibration may be re-computed off-line by using the command ATMOSPHERE. The atmospheric calibration is normally performed according to parameters given in real time in OBS. Each antenna may be set in one of three calibration modes: FILE, AUTO, or TREC. FILE mode (the default) just means that the mode used will be the one given in OBS for the corresponding antenna. Any antenna in TREC mode is first used to determine a value of the water content. If more than one is available, an average is made. Then the resulting water content is used for the antennas in AUTO, for which a value of Trec is then determined.

Note that the command ATMOSPHERE may be run several times on the same data. The mode for some antennas may be forced in CLIC with commands SET ATMOSPHERE TREC temp /ANTENNA i, or SET ATMOSPHERE AUTO water /ANTENNA i. This may be useful if, by chance, the amplitude calibration parameters were not set to the correct values in OBS by the observer. Incorrect calibration parameters may be updated by command MODIFY HEADER.