Projects are now routinely carried out with 4 antennas.
We use a total of 5 configurations (instead of 9 with 3 antennas): 2 ``C'' (intermediate length), 1 ``D'' (very compact array), 2 ``B'' (long baselines). The ``BC'' set has 24 baselines and provides a significantly better sensitivity than the previous 3-antenna choice. The ``CD'' set has 18 baselines. Projects can request the ``BCD'' combination (30 baselines) if needed; this choice must be justified.
As an indication, the sensitivity for a full synthesis with 4 antennas, 1 MHz effective resolution (0.625 MHz channel separation), 300 K system temperature is 0.4 K using the BC configuration set (0.15 K using CD).
The configurations are given in the following table (the beam sizes are for 100 GHz):
Beam sizes (FWHM) and orientations: