Dutch Open
Telescope. 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.