From the official website:

The Richmond Hill David Dunlap Observatory (RHDDO) houses the largest telescope in Canada. A number of important studies have taken place here, including providing the first direct evidence that Cygnus X-1 was a black hole, pioneering measurements of the distance to globular clusters and the discovery that Polaris was stabilizing. 

History

The RHDDO property was 76.5 hectares (189 acres) bordered by Hillsview Drive to the north, Bayview Avenue to the east, 16th Avenue to the south and the Canadian National Railway Bala Line to the west.

The property was the site of a 19th century farmstead owned by Alexander Marsh, comprised of a brick farmhouse, a lane from Yonge Street, agricultural fields with hedgerows and an orchard.

When the observatory in downtown Toronto could no longer function due to light pollution, the University of Toronto identified the Marsh farmstead as being suitable for a new astronomical facility. As a result, Jessie Donalda Dunlap purchased the property and donated it to the University as a memorial to her husband, David Alexander Dunlap, who was an avid astronomer.

The University constructed the Observatory on the site. It included a dome, housing a 74-inch (1.88m) reflector telescope, and an Administration Building, with three smaller telescope domes. When construction was complete in 1935, the main telescope was the second largest in the world and the largest in Canada.

From 1935 to 2007, the Observatory was at the forefront of Canadian astronomical research. Achievements at the site included advances in radio astronomy and the first direct evidence that Cygnus X-1 was a black hole.

In June 2008, the University sold the property to Corsica Developments Inc.

The sale and subsequent development proposal by Corsica caused concern in the community. In response, Richmond Hill undertook a number of important studies to protect the features on the property which are of cultural and natural heritage significance. Additionally, Richmond Hill passed a heritage designation by-law to ensure that the significant cultural heritage features on the property are protected.

In 2012, Richmond Hill, York Region, the Toronto and Region Conservation Authority and the David Dunlap Observatory Defenders reached an agreement with Corsica through Ontario Municipal Board mediation. Corsica agreed to transfer approximately half of the property (40 hectares, 99 acres) to Richmond Hill for public park use and cultural heritage protection as a condition of approval of the subdivision that Corsica applied for and appealed to the OMB. 

In 2017, 40 hectares of parkland, the Observatory Dome facility with the original 1.88 metres telescope and the Administration Building on the property was transferred to Richmond Hill.

In 2018, Richmond Hill opened the RHDDO to the community for astronomy programs and summer camps.

Heritage Conservation

On September 29, 2009, Richmond Hill Council designated the Richmond Hill David Dunlap Observatory Property under By-law 100-09 as a "property of cultural heritage value or interest" under Part IV, Section 29 of the Ontario Heritage Act R.S.O. 1990, Chapter 018, as amended.

On January 24, 2011, Richmond Hill Council approved a Conservation Management Plan for the RHDDO property. The Conservation Management Plan provides Council with a means to assess any applications to alter heritage features within the RHDDO property that are designated under the Ontario Heritage Act. The Conservation Management Plan also sets out short, medium and long term maintenance and conservation treatments for the identified heritage attributes.

In 2019, the RHDDO was recognized by the federal government as a national historic designation.