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Prior to 1860, there were only two 15-inch telescopes in the world. They were at Harvard Observatory and the Pulkovo Observatory in Russia. Dr. F.A.P. Barnard, president of the University of Mississippi, comissioned Alvan Clark, a lens maker of Cambridge, MA, to make an 18.5" lens. When the Civil War broke out, the agreement with the University of Mississippi failed, and the lens went idle. In 1863, The Chicago Astronomical Society and the University of Chicago purchased the lens for $18,187, and the construction on the observatory commenced, paid for by J. Y. Scammon, who named it for his deceased wife, Mary Ann Haven Dearborn.

In 1864, the lens arrived at the observatory, located at 3400 South Cottage Grove in Chicago. The building was completed in 1865, and the first observations were made in 1866. The Chicago Astronomical Society was active over the next 30 years, but in 1886, the original University of Chicago became bankrupt. The telescope was determined to be the property of the Chicago Astronomical Society, which was instructed to remove it.

In 1887, the telescope was moved to Northwestern University. J. B. Hobbs donated the new home at a cost of $25,000. Observatory contributions have included the discover of 102 binary stars, study of faint red stars, and photographing the asteriod Eros, which contributed to improved accuracy of distances in the solar system.

In 1939, the observatory moved locations again to its present site next to the Technological Institute in order to make room for the new Tech building. It took 26 men three months to move the building, with their highest speed being 20 inches per minute.

In 1997, the rotating dome was replaced with a new Observa-Dome, and the entire dome was beautifully refurbished in 1998. DFM Engineering, Inc. reinstalled the telesceope and retrofitted it with a precise computer control system. Regular observing sessions began for NU astronomy classes and for the public. Today, the telescope is used for teaching and outreach purposes.

For a timeline of Dearborn's history, see here.