A First Look at the Data

Before processing to actually calibrate the data, it is useful to evaluate the data quality. Actually this should be done on the site, as soon as possible after the data has been taken, to take the appropriate action (such as re-observing the source in the current interferometer configuration). We list here the simplest checks to be done on the data (these checks do not require knowledge of the astronomic aims of the observing program):

Plot the system temperature and water vapor content versus time

(or versus elevation). Use SET ANTENNA 1 2 3 4, SET Y TSYS WATER. TSYS should have a reasonable dependence on elevation, while WATER (the zenith opacity) will reflect any strong variation of atmospheric opacity with time.

Plot the total power output versus time.

Use Use SET ANTENNA 1 2 3 4, SET Y TOTAL. It should display a reasonable elevation dependence.

Plot the calibrator amplitude and phase versus time

The phase calibrators are selected from the data set by the command SET TYPE PHASE. Both amplitude and phase should ideally be constants.

The amplitude (in kelvins) should be in principle the same as observed in single-dish observations. Check for pointing or focussing problems if the amplitude suddenly drops in baselines connected to one antenna. Check the calibration parameters if the amplitude as a clear dependence on elevation.

If the phase has a sinusoidal dependence on hour-angle, the baseline parameters may be incorrect.

Plot the calibrator amplitude and phase versus IF-frequency.

These curves should be relatively flat, since the IF pass-band has been removed during real time calibration. No systematic slope should appear in the phase of the calibrator, if the delay offsets have been measured with sufficient accuracy. If some strange effect is seen in the correlator data (very low amplitude, very strong slope in the phases), you should check the IF passband calibrations.

Plot the IF-passband calibrations

(if anything seems wrong with the correlator). These calibrations consist of two observations, an autocorrelation and a correlation.

With SET Y AMPLITUDE PHASE, SET X I_F and SET BASELINE 12 13 23 14 24 34, you will get the correlation (normalised by the autocorrelation). Amplitudes should be very close to 1., except for possibly the edge channels of each subband. Phases show a systematic slope, varying from one correlator unit to another.

With SET Y AUTOCORR, SET X I_F and SET ANTENNA 1 2 3 4, you will get the autocorrelation spectrum for each correlator entry. The passbands should be rather smooth.

Bad channels should show up as strong oscillations in either plot.