The observatory is home to the Dearborn 18 1/2 inch refractor, which was the largest telescope in the United States in the late 1860s.

[2] Due to the complicated history, it was operated from a different site at that time, and the original tube and mounting is at the Adler Planetarium since 1929.

[2] In 1911 the original tube and mounting were replaced, and eventually donated to Adler Planetarium in 1929 by the Chicago Astronomical Society.

[2] The telescope was surpassed by the 26-inch (66 cm) Great Refactor installed at the United States Naval Observatory in 1873, then located at Foggy Bottom.

He commissioned the renowned firm of Alvan Clark & Sons of Cambridge, Massachusetts, to construct the lens.

The lens was initially intended for Harvard University, but was bought instead by Chicagoan Thomas Hoyne, a founding member of the Chicago Astronomical Society, for the sum of $11,187.

The new observatory was designed by architectural firm Cobb and Frost and constructed of limestone in the Richardsonian Romanesque style.

[2] In the summer of 1939, the Dearborn Observatory building had to be moved 200 metres (660 ft) southeast to its present location to make way for the construction of the Technological Institute.

On January 31, 1862, American telescope-maker and astronomer Alvan Graham Clark first observed the faint companion, which is now called Sirius B, or affectionately "the Pup".