WIYN telescope. Panoramic view of the dome interior of the WIYN (Wisconsin, Indiana, Yale, NOAO) telescope, with the Kitt Peak observatory, USA, in the background. The WIYN's 3.5-metre prim- ary mirror focuses light onto a 1.2 metre second- ary mirror which in turn reflects the light down onto a tertiary mirror which reflects the light sideways to instruments. The flexible primary mirror has 66 actuators which adjust its shape to compensate for distortions. The openings in the side of the dome allow the mirrors to be cooled to ambient air temperatures. The weight of the tele- scope's moving parts, including its alt-azimuth mount, is only 46 tons. It was completed in 1994.