The night sky, the setting moon, and passing clouds over La Silla telescope domes. Sirius, the brightest star in the night sky, is on the upper left. Constellation Orion is setting to the left of the moon and the Beehive star cluster (M44) is on the upper right. The European Southern Observatory's (ESO) site at La Silla, Chile, has telescopes which observe at optical and infrared. The largest optical telescope has a mirror with a diameter of 3.6 meters. The high altitude of La Silla (2400 meters), the dark sky, and the clear air above it (reducing atmospheric distortions of incoming light), make the site an ideal location for astronomical observations.