On the night of April 28 (Chilean time), interference fringes were successfully detected in an interferometer test using two Japanese antennas at a distance of 1966 meters at the ALMA Array Operations Site (AOS) at 5000 m asl.

ALMA is an interferometer consisting of multiple antennas. In an interferometer observation, the longer the distance between the antennas, the higher resolution will be obtained. The antenna distance of 1966 meters is the longest ever achieved with the ALMA antennas.

The observed object is SiO maser emission around VX Sagittarii. The figure below shows the obtained spectral graph (left) and phase diagram. These two graphs demonstrate that the two antennas worked properly as an interferometer.

ALMA has already started its early scientific observation with 16 antennas; the number of antennas will be increased to 66, which will form an array spread over a maximum distance of 18.5 km when completed. With this successful interferometer test, we are steadily making strides forward to the start of full operation.

The picture below shows a Japanese antenna used in this interferometer test, 1966 m away from the ALMA antenna array. [Credit: ALMA (ESO/NAOJ/NRAO), R. Hills (ALMA)]