The telescopes in the array arehoused at ESO La Silla, at the site of the former GPO/Marly telescope. Each telescope is mounted on top of a 7 meter high double-walled steel cylinder. The inner cylinder will hold the telescope and the outer cylinder the clam-shell dome. This way the telescope-bearing structure is shielded from sun and wind.
ESO La Silla is an ideal site because of its excellent weather and seeing (median value 0.9"). Since the gravitational wave events are expected to originate from within ~200 Mpc distance, many might be located on or near resolved galaxies, and be faint. Spatial resolution is therefore doubly essential: to resolve out the night-sky background (sensitivity to points sources), and to accurately locate the event on top of its host galaxy.
La Silla is the original observatory of the European Southern Observatory and currently houses the 3.6m telescope (with the immensely successful planet hunting spectrograph HARPS), the 3.5m NTT telescope, the 2.2m ESO/MPG telescope, and a number of national and robotic instruments. ESO is committed to keep La Silla running to beyond 2022, which is the current horizon for BlackGEM.