The full plans for the £3.5m redevelopment of the birthplace of astronomy in the nation's capital, designed as a "temple to the Enlightenment", have been revealed.

The Collective Gallery, already based in the City Observatory complex on Calton Hill, have revealed the extent of their plans for the redevelopment of the historic site.

The observatory was designed by William Henry Playfair in 1818 but is in need of major conservation, is in a state of disrepair and is officially a building "at risk".

Collective is working with the City of Edinburgh Council to conserve and reinstate the original 19th century Playfair designs, display and interpret the telescopes and astronomical instruments, restore the grounds and enable public access to the site for the first time.

A new "subterranean" gallery space and restaurant will be also be created with Edinburgh architectural practice Malcolm Fraser Architects as the project architects.

The first phase of work was completed in 2014 with the restoration and opening of the City Dome, where Collective shows the work of established and international artists.

The majority of funding for this £3.5m project has been raised.

Collective is continuing to raise funds in order to start on site at the end of 2015 and aim to open up the entire City Observatory walled complex at the end of 2016.

Two open days are being held at Collective in the City Dome on April 17 and 18 for the public to talk to staff and view the plans.

Kate Gray, Collective's artistic director, said: "Historically observatories are places to observe the stars and make scientific discoveries but Collective's vision is to be a new kind of City Observatory, a place to look at art and view the city that art was made to be seen in.

"We will bring people together to engage with art, science, and Edinburgh's heritage.

"This unique and iconic place will be fully opened for the first time to the public providing a space for reflection on the past, present and importantly the future through creating an exciting environment for both artists and visitors."

Richard Lewis, culture and sport convenor for the City of Edinburgh Council said: "Under Collective's redevelopment plans, the Observatory's original 19th Century William Playfair designs would be reinstated to create a hub of art, science, history and leisure right in the heart of the Athens of the North for residents and visitors to enjoy."

Malcolm Fraser, the architect, said the site is one of a creative interaction between art and science.

He said: "Art might just deliver something forward-looking for astronomy that is thrillingly-complementary to the site's history.

"Our concern has been with the evolution of the site.

"Robert Adam urged James Craig - architect for the New Town and the site's first designer - to consider it as a fortified enclosure with towers at its corners.

"Playfair's temple precinct magnificently advanced that vision, except at the north-west corner, where the site breaks down into minor buildings.

"We propose a Pavilion there, engaging the wall and with inspirational views out over Edinburgh to the Forth Bridges and back to Playfair's temple."

Funding is being provided by City of Edinburgh Council, Collective, Creative Scotland, Heritage Lottery Fund, Edinburgh World Heritage, Wolfson Foundation and the Architectural Heritage Fund.