Mount Laguna Observatory (MLO) is an astronomical observatory owned by San Diego State University (SDSU) and operated in partnership with the University of Illinois at Urbana-Champaign (UIUC). MLO is located approximately 75 kilometers (47 mi) east of downtown San Diego, California (USA) on the eastern edge of the Cleveland National Forest in the Laguna Mountains and near the hamlet of Mount Laguna. MLO was dedicated on June 19, 1968, seven years after SDSU's Department of Astronomy became an independent academic department of SDSU's College of Sciences. The dedication took place during the 1968 summer meeting of the Astronomical Society of the Pacific.
- The 1.06 m (42 in) MLO 40-inch Telescope is a Cassegrain reflector built by Astro Mechanics in 1966. It was originally fitted with a 1.02 m (40 in) primary mirror which has since been replaced. It is known informally as the Illinois telescope because it was located at UIUC's Prairie Observatory until 1981.
- The 0.6 m (24 in) Clifford Smith 24-inch Telescope is a Cassegrain reflector. It was built by the SDSU Department of Astronomy and used at the main campus from 1961 to 1966. In 1971 it was installed at MLO after modifications.
- The 0.52 m (20 in) Reginald Buller 21-inch Visitors' Telescope was built in 1950 by J.W. Fecker, Inc. It was donated to SDSU by Reginald Buller and dedicated at MLO in 1988. It is used primarily by students for direct viewing, and for outreach.
- The 1.26 m (50 in) 50-inch Phillips Claud Telescope was under construction as of June 2010University of Kansas (KU), and is a follow-on to the ULTRA project.
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- A 0.4 m (16 in) telescope built by Boller and Chivens was the first telescope at the observatory, and was replaced by the ULTRA telescope.
- A 0.4 m (16 in) telescope built by Nishimura and used by visitors was moved to the UCSD campus in 1988. It was replaced by the Buller telescope.
- The 1.0 m (39 in) Ultra-Light Technology for Research in Astronomy (ULTRA) telescope was a test bed for lightweight carbon-fiber mirrors. It was located in the 0.4 m Boller and Chivens building from 2006 until 2008. Before it was installed, the original dome was replaced with a larger one, and the mount upgraded. The project was collaboration of SDSU, KU, Dartmouth College, and Composite Mirror Applications, Inc. (CMA) of Tucson, Arizona.