McMath-Pierce Solar Telescope at the Kitt Peak National Observatory in Arizona, USA. This is the world's largest solar telescope. Its 2.1 metre Sun-tracking mirror (heliostat) is at the top of a 30 metre tower (right). The heliostat tracks the Sun and reflects its light down a 152 metre long diagonal optical tunnel. Deep underground is a 1.5 metre focus mirror, which reflects the light half way back up the tunnel to a further mirror that directs the light down into an underground observing room. The projected image of the Sun allows studies to be made on its composition and activity.