BlackGEM is an array of optical telescopes under construction at La Silla Observatory in Chile, to follow up GW detections from Virgo and LIGO. Current plans are 15 telescopes of 65cm diameter, each with a 2-square-degree field of view, the aim being to search a significant portion of the sky quickly. Plans are to carry out surveys while awaiting alerts of GW detections.
Construction of the first phase (three telescopes) continues as of March 2019. BG4 is a term for four of the telescopes.