O'Brien Observatory in Marine on St. Croix, Minnesota, about thirty-five kilometers northeast of Minneapolis, is an astronomical observatory operated by the University of Minnesota since opening in 1968. The facility is built around a single 0.76-meter Cassegrain reflector, a thirty-inch telescope capable of observing in both optical and infrared wavelengths. Its chief role is instruction, while the telescope also serves for testing instruments and is sometimes used for research.