Baseline measurements

To measure the baselines after a reconfiguration, it will be necessary to observe some (strong) continuum sources. A sideband average of the visibilities (weighted by the image-to-signal gain ratio, provided we know it) is used to remove instrumental phase offsets and get only geometric and atmospheric phases in the solution. No specific OBS command is necessary for this; the OBS $\backslash$START command is used to make cross-correlation. Data are written in the Date.IPB file. Typically 4 minutes per source are required. Reduction is done within the CLIC program using the CLIC $\backslash$SOLVE BASELINE command. The CLIC $\backslash$PRINT BASELINE command produces a procedure named dd-mmm-yyyy-base.obs, which can be used to enter the appropriate values.

It is important to have good hour angle and declination coverage for a complete baseline determination. The observer must also avoid systematic scanning in azimuth, in order to disentangle atmospheric variations (time dependent) from baseline effects (hour angle and declination dependent). Low elevation sources must be avoided; high declination sources are necessary, but not low declination ones. finally, a baseline determination should not last longer than 2 hours; otherwise, time variations of the atmospheric phases begin to dominate. Even 2 hours may be too long for long baselines. A procedure @ PR:BASELINE is available to select automatically appropriate sources.