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Press release | Focus | News | Matter ＆ the Universe | Astrophysics | Galaxies | Content of the Universe | Planets | Stars
‘First light’ for the Dark Energy Spectroscopic Instrument (DESI): as the installation phase nears completion, this new instrument is due to undergo final tests before starting to create a giant map of the sky in early 2020, a mission that is scheduled to run for five years. CEA, the French National Centre for Scientific Research (CNRS), Aix-Marseille University and French company Winlight System have played a significant role in the project to develop this international instrument which aims to scan the sky in an attempt to understand the effects of dark energy.
Artist's impression showing DESI fitted to the Mayall 4-metre Telescope at the Kitt Peak National Observatory in Arizona. ©2010 The Regents of the University of California, through the Lawrence Berkeley National Laboratory).
DESI is designed to measure the spectrum of 5,000 celestial objects every two minutes. It can scan five times as many objects twice as fast as the best existing instruments.
Over the next five years, DESI is intended to map the position and distance of 35 million galaxies and 2.4 million quasars covering one third of the sky. With DESI scientists will have the most detailed three-dimensional maps of the universe.
DESI will enable scientists to investigate how the universe evolved. The distance measurements obtained for the scanned objects will be compared with predictions based on the Standard Model of Cosmology. This model assumes the existence of an unknown component, which scientists refer to as 'dark energy'. This energy is deemed to be responsible for the accelerating expansion of the universe over the past five billion years.
"DESI is a very significant advance compared to previous instruments in terms of the number of objects measured. By looking at very different objects from different eras, we can actually map the history of the universe and see what the universe is made of," explains Nathalie Palanque-Delabrouille (CEA-Irfu (Institute of Research into the Fundamental Laws of the Universe), Paris Saclay), co-spokesperson for the DESI collaboration project.
Given that the light from each star takes a certain length of time to reach us, the maps created by DESI will provide access to how the universe evolved through the ages.
The most distant galaxies and quasars that are visible with current instruments take us back 11 billion years. By multiplying the number of objects analysed by 15 compared with existing data, DESI will make it possible to create the densest map over the largest area ever recorded.
The culmination of an international collaboration to track dark energy, DESI is installed on the Nicholas U. Mayall Telescope at the Kitt Peak National Observatory near Tucson, Arizona. ©Marilyn Chung/Lawrence Berkeley National Laboratory © Marilyn Chung/Lawrence Berkeley National Laboratory
Equipped with 5,000 little robots, each with a fibre-optic cable, DESI is capable of pointing automatically towards a precise list of pre-selected galaxies and quasars to detect any light and thus measure their distance from Earth. ©LBL
The light collected from a small region of the Triangle Galaxy (lower part of the image) by a single optical fibre is broken down into a spectrum (lower part of the image) that reveals the traces of chemical elements present in the galaxy and helps to measure how far it is from Earth. DESI collected the spectrum shown here on 22 October. ©DESI Collaboration
Inspecting a lens before installing in one of the spectrographs ©Winlight
A spectrograph equipped with three CCD light sensors ©Winlight
pulse tube designed by the Interdisciplinary Research Institute of Grenoble (CEA-Irig) and marketed by Thales.
"The CCD light sensors are the core of DESI. We were responsible for designing the cryomechanical system which holds these detectors and making sure they operate to an optimum level. Our teams assembled the 30 cryostats and positioned each of the CCDs with the required degree of accuracy," explains Pierre-Henri Carton (CEA-Irfu), head of the DESI cryostat project.
CEA-Irfu teams were also in charge of final assembly of the cryostats, each with their CCD, in the spectrographs on the Winlight System site in Pertuis, Vaucluse.
Teams from the CNRS and partner universities (1) carried out optical adjustments on these assemblies. "Our role is to optimise the focusing system of each spectrograph and check that optical performance complies with the specifications. It takes four weeks to test each spectrograph and we have just started adjusting the last of the ten spectrographs," explains Pierre-Eric Blanc, CNRS researcher at the Pytheas Research Institute (CNRS/Aix-Marseille University/IRD/Irstea). "To carry out these tests, we designed a calibrated light system for the spectrographs. We also designed and manufactured the calibration system for the instrument as a whole, which has just been fitted to the telescope," adds Laurent Le Guillou, lecturer and researcher at Sorbonne University in the Laboratory of Nuclear and High-Energy Physics (LPNHE) (Sorbonne University/CNRS).
When this qualification stage comes to an end, the spectrographs are sent off to the Kitt Peak Observatory, where technical teams from CEA-Irfu carry out the final installation phase and chill down the cryostats ready for operation. A team from the Laboratory of Astrophysics of Marseille (AMU/CNRS/CNES) (2) then carries out final adjustments on the spectrographs. Eight of these devices are already installed, with the last two due to arrive by the end of the year.
The 'first light' will be followed by an instrument commissioning period which is expected to last until January 2020. This will be followed in February 2020 by a programme to validate the strategy for selecting the astrophysical targets to be observed and scheduling observations, after which the scientific study programme will commence in the summer of 2020 and run for five years.
"Now that this instrument is ready, we are about to enter the important phase of validating the algorithms used to select the targets to be observed by DESI. This is a key stage in the process, and one that is critical for the success of our ambitious research programme," explains Christophe Yeche (CEA-Irfu), head of the team defining the process used to select the various types of targets to be observed by DESI, such as very bright galaxies, red light galaxies, emission-line galaxies and quasars.
"The first light is our reward for 10 years of hard work. This will allow us to further our understanding of dark energy and I am keen to see what surprises DESI has in store for us," adds Christophe Magneville (CEA-Irfu), scientific team leader for the DESI cryostat project.
The DESI installation project started back in February 2018 on the Nicholas U. Mayall Telescope at the Kitt Peak National Observatory near Tucson, Arizona. Over the past 18 months, the various components making up DESI have arrived on site from institutions throughout the world to be installed on the telescope.
The first component to arrive was a lens assembly, where each lens has a diameter of one metre. Once installed on the telescope, each lens will provide a large field of view over 8 square degrees (or, in other words, 32 times the apparent surface of the full moon). Successful tests were carried out on this assembly in April.
DESI's focal plane, which has 5,000 robotic positioners that pivot in a choreographed 'dance' to focus individually on galaxies and quasars, is located at the top of the telescope.
These small robots, each holding a fibre-optic cable about the same width as an average human hair, are DESI's eyes. It takes no more than two minutes to move from one configuration of 5,000 galaxies to the next.
In each pointing position, the light collected by the 5,000 optical fibres is directed to a series of ten spectrographs, which are in turn designed to break down the light from each object observed into wavelengths and record the spectrum in order to measure the precise distances involved.
(1) These include the Laboratory of Nuclear and High-Energy Physics (CNRS/Sorbonne University), the Centre for Particle Physics of Marseille (CNRS/Aix-Marseille University) and the Laboratory of Astrophysics of Marseille (LAM, AMU/CNRS/CNES).
(2) It is part of the Pytheas Research Institute.
CEA is a French government-funded technological research organisation in four main areas: low-carbon energies, defense and security, information technologies and health technologies. A prominent player in the European Research Area, it is involved in setting up collaborative projects with many partners around the world.