. It is located on the island of La Palma
at an altitude
of about 2.5 km
and used for looking at the sun
The telescope is called open because of it's unusual construction.
The earth's atmosphere causes a lot of distortion in ground based telescopes.
Hot and cold air have a slightly different index of refraction. Light travelling through hot air bubbles is distorted and the telescope's image becomes less clear.
During day time, when the sun is out, this problem is even worse. The DOT is constructed in such a way as to reduce distortion from hot air. It also has some devices which correct for remaining distortions.