Plateau de Bure Online Calibration

Some of the mentioned terms are partially estimated on real time so that the stored visibilities are already corrected from their contribution. Specific OBS commands, a dedicated GILDAS software package (RDI) and some CLIC procedures are used to produce and reduce the following scans:

• CALI: Each scan consists of two or three subscans, obtained by the OBS command CALIBRATE. They are used to measure the atmospheric transmission and receiver temperature, converting backend counts in antenna temperature outside atmosphere (which is a factor in $AA_i$, excluding pointing and focus terms).
• IFPB: Two subscans are obtained by the OBS command BANDPASS, which switches the correlator entries to a common noise source. Thus all the $AC_{ijk}$ and $PC_{ijk}$ are measured, except for a residual delay (visible as a constant phase slope with respect to frequency) due to path differences between the noise source and the antenna connections to the correlator.
• GAIN: This procedure allows measuring the sideband gain ratio of the receivers, by looking at a strong cosmic source (in practice, only $AI_i$ is computed, although the phase term $PI_i$ could also be derived). Since this ratio only depends on the receiver tuning, in particular on the backshort tuning, a CALIBRATE scan is made just after tuning modifications.

Results from CALI and IFPB observations are applied to the data by the automatic data compression job. Corrections from tuning modifications are introduced in OBS by the operator just after GAIN, at the beginning of the observations. Therefore, in principle, amplitude errors should only be due to phase noise, pointing and focus errors.