The Great Refractor of Meudon rediscovers the light of the stars

The Great Refractor of Meudon, the third largest night-time telescope in the world, built at the end of the 19th century at the initiative of Jules Janssen, has several notable achievements to its name : highlighting the retrograde rotation of Uranus in 1902, disproving the existence of artificial Martian canals in 1909, locating Mars’ clouds above dust storms in 1924, and observing various binaries, novae, and comets, including comet 1P/Halley in 1986.

However, due to a lack of maintenance, the rotation of the dome stopped working in 1991, leading to the end of observations. Then, in December 1999, two storms tore off plates from the dome. A project to refurbish the roof of the coupola began in January 2005. In 2011, new hatches were installed, and attempts to open and close the dome were then undertaken, but were generally unsuccessful.

Work on the mount’s mechanics and attempts to restart operations were undertaken by Jean-Pierre Michel, Daniel Crussaire, engineers from LESIA at the time, and members of the recently established MCO (Maintenance of Operational Domes) structure. However, due to ongoing issues with opening and closing the hatches and concerns about water infiltration, it was decided in 2019 to seal the summit with a reversible blocking of the hatch opening.

In October 2021, following a brief condition report written by Miguel Montargès, Vincent Lapeyrère, and Daniel Rouan (then members of LESIA), the President of the Observatory, Fabienne Casoli, and the Vice-President of the Scientific Council, Mathieu Puech, decided to create a working group with the aim of allowing Observatory staff to visit, and to assess the condition of the dome and the instrument for a future use to be defined. The working group, set up in April 2022, is led by Yann Clénet, responsible for the Meudon domes, and included staff from LIRA as well as from the MCO dome maintenance structure within UNIDIA, as well as staff from the Real Estate and Logistics Service and the Library of the Observatory.

The first mandate quickly materialised with the event organised by the Communication Department of the Observatory for the families of staff members in April 2022. The development of visitor guidelines and a major cleaning effort, funded by PSL University, has since enabled numerous visits by staff, external collaborators, and, exceptionally, schoolchildren and the public during the 2023 and 2024 Science Festival.

It was with great emotion that staff members of the Observatory are discovering this Great Refractor, next to which they had passed for years without being able to see it. The members of the working group were also moved by the wonder of children and the public during their visits.

Emotion was also present for the working group when it came to responding to the second mandate. It was necessary to use a crane to lift the heavy objectives weighing 120 and 200 kg, which had been stored for 20 years in a room in the Château. Then, they had to tackle the opening of the hatches with the help of rope access technicians, with results achieved more easily than the difficulties encountered previously had suggested.

After these operations, carried out thanks to funding from LESIA and MCO, and a few interventions to restart the telescope’s tracking, observations became possible once again. And so, on 25th July 2024, the telescope experienced a "new light," discovering again the light of the stars, with observations of the Moon and Saturn. The successful experience was repeated on 30th January 2025 with observations of Jupiter, Mars, and M42.

There is still work to be done, and funding to be secured, in order to ensure safe observations and public visits, but the demonstration of the possibility to visit and observe has been made, opening a new future for the Grande Coupole and the Great Refractor of Meudon.