Time exposure image of star trails over the tower of the McMath-Pierce solar telescope at the Kitt Peak 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. The mirrors track the Sun and reflects its light down a 152-metre long diagonal optical tunnel. Deep underground is a 1. 5 metre focus mirror; this reflects the light half way back up the tunnel to a further mirror which directs the light down into an underground observing room. Star trails are caused by the apparent motion of the stars in the sky as the Earth rotates. The red appearance of the telescope is caused by sunset afterglow.