The sentinel

Here we see the now-obsolete Swedish–ESO Submillimetre Telescope (SEST), watching vigilantly over the horizon at ESO’s La Silla Observatory like a sentinel. Below, stratus clouds blanket the valley.

SEST was installed at La Silla in 1987, and at the time of its first light was the only large submillimetre telescope operating in the southern hemisphere. Although it was decommissioned in 2003, the 15-metre diameter dish remains at La Silla Observatory and serves as a tribute to the groundbreaking work it accomplished — both as a standalone telescope, and also as a testbed for later projects such as the Atacama Pathfinder Experiment telescope (APEX) and the Atacama Large Millimeter/submillimeter Array (ALMA).

During its tenure, SEST made several big discoveries, including, in 1995, finding the Boomerang Nebula to be the coldest place in the Universe at just one degree above absolute zero, and discovering the first extragalactic silicon monoxide maser.

This beautiful panorama was taken by ESO photo ambassador Alexandre Santerne. Alexandre is a professional astronomer who also participates regularly in public outreach events and conferences.