Pointing

An interferometric mode for radio pointing is available with the OBS $\backslash$POINT command. The idea is to make cross scans with some or all antennas simultaneously and to record the correlated output power as a function of position (instead of the total power for each antenna as in single-dish mode). Pointing parameters can be recovered if all the following conditions are satisfied

• At least 3 antennas are available OR at least 1 antenna is fixed.

• The pointing source MUST be a point-like object (with respect to the current baseline lengths).

• If all antennas are moved the pointing errors must be similar within 20".

The advantage over single-dish pointing is better sensitivity, due to two major reasons:

• Atmospheric total power fluctuations are eliminated, and

• Integration goes on all the time, instead of only a fraction of the time in beam-switch single-dish mode.

A signal to noise ratio of 10, corresponding to an accuracy of 1.5" on the pointing corrections, can be reached on 1 Jy sources at 3 mm with a standard drift (1 minute of time, 1 arcmin on the sky). The /FAST option is now available in INTER mode, and can be used on strong sources.

In this mode the correlator is used to measure the correlated power. Data are stored in the Date.IPB file and no real-time processing is done. Quasi real-time processing can be done within CLIC using the CLIC $\backslash$SOLVE POINTING command. The reduced data can be used by the POINT program. CLIC command CLIC $\backslash$SOLVE POINTING /PRINT produces a command procedure named pointing.obs which can be used in OBS to enter the appropriate corrections. These corrections are biased towards 0 by 1 $\sigma$ to avoid unrealistic excursions.

Note that pointing (and focus) can be done in a ``pseudo-continuum'' mode if the correlator subband selects a narrow band around a spectral line. This is particularly useful for SiO masers.