Irenee du Pont telescope

The Irenee du Pont telescope, at the Las Campanas Observatory, in Chile. The telescope has a primary mirror of 100 inches (2.5 meters) in diameter, relatively large for the time of construction (the telescope was inaugurated in 1977). I took the photo from a nearby peak, home of the Magellan telescopes, each with a primary mirror of 6.5 meters in diameter. Despite having a much smaller mirror, the du Pont telescope is housed in a much larger building than each of the Magellan telescopes. This dramatic reduction in the telescope dome size is due to the introduction of computers in the control of the telescope movements. At the time in which the du Pont telescope was built, telescopes needed to have an “equatorial” mount: the telescope was hinged on an axis oriented parallel as the Earth rotation axis, so tracking the stars was a simple matter of rotation along that axis. Equatorial mounts are very large and occupies a lot of space. Modern telescopes like the Magellans have instead “alt-azimuthal” mounts: the telescope rotates on an horizontal platform and a second orthogonal axis is used to point at different altitudes on the horizon. These kind of mounts allows for a much more compact design (and thus a smaller housing for the telescope) but require a more complex control of the telescope movement on both axis simultaneously to track the stars, which was difficult to obtain with pure mechanical means.