Portage Lake Observatory  The Portage Lake Observatory (PLO) was an astronomical observatory owned and operated by the University of Michigan (UM). It was located near the village of Dexter, Michigan (USA), about northwest of Ann Arbor. Construction at the site began in 1948, and the facility was closed in 1975. Other observatories that UM has operated include the Detroit Observatory (Ann Arbor, Michigan, 1854), the Angell Hall Observatory (Ann Arbor, Michigan, 1927), the Lamont-Hussey Observatory (South Africa, 1928), the McMath-Hulbert Observatory (Lake Angelus, Michigan, 1930), and the Peach Mountain Observatory (Dexter, Michigan, 1955).