A month and a half after its launch, the performance verification phase of the Euclid space telescope began on August 12, 2023, after the unveiling on July 31, 2023 of the first images of the VIS and NISP instruments. By their excellent quality, these images attest to the proper functioning of the entire satellite, and are promising for the achievement of the scientific objectives of the mission.

After launching from Space Force Station in Cape Canaveral, Florida on July 1, 2023, the Euclid satellite traveled 1.5 million kilometers (nearly four times the Earth-Moon distance) in the opposite direction to the Sun, in order to reach its observation orbit at the Lagrange point L2 of the Sun-Earth system. The radius of this orbit varies between 400,000 and 800,000 kilometers, and the satellite coexists in this region of the solar system with the JWST and Gaia telescopes, placed in separate orbits. The L2 Lagrange point allows a telescope to look in the opposite direction to the Sun in order to study the astronomical sky without the Sun, Earth or Moon interfering with these observations, while pointing an antenna towards the Earth in order to remain in close communication with it for the management of observations and the transfer of data.

The first tests of the satellite were carried out a month after its launch, aiming in particular to achieve the development of the telescope, and to proceed with the calibration of the “fine guidance sensor” (FGS), which uses stars to point the telescope extremely stably at its targets in space. The telescope was then able to take its first images of the sky pointing to a region between the constellations of Carina and Eridanus.

The images of the Euclid telescope are obtained with the visible imager VIS (VISible instrument) and the infrared spectrometer NISP (Near Infrared Spectro Photometer). Both instruments were developed by the Euclid consortium, of which France is the first contributing country, mainly involving the CNRS, the CEA and partner universities, and whose project manager is Yannick Mellier, senior researcher at the Institut d’astrophysics de Paris (IAP), a CNRS and Sorbonne University laboratory. The Euclid consortium also benefits from the support of CNES for all its management, development and instrument production activities, as well as the processing of mission data.

The VIS instrument operates in the visible wavelength range (between 550 and 900 nanometers). Each VIS image contains 609 million 0.1 arc-second pixels, covering a total area of 0.57 square degrees (almost three times the angular diameter of the Moon). Figure 1 shows such a full image (on the left), including a detector that contains in its field a cluster of stars located in our Milky Way galaxy, as well as two distant spiral galaxies (near the center and on the right). The images are remarkably sharp. We also see in the four quadrants of this same detector very luminous stars of the Milky Way. Their center is often saturated, and they also have egrets coming from the supports of the secondary mirror which reflects the light received by the primary mirror of 1.2 meter in diameter of Euclid, towards the instruments.

Figure 1 : Image de test de la mise en service précoce, instrument VIS
Figure 1 : Images obtained by a single exposure of 566 seconds of the VIS instrument of the Euclid telescope. The one on the left is the entire image of the instrument's 36 focal plane detectors. The image on the right framed in white corresponds to a single detector, located at the top left of the entire image. Two spiral galaxies can be seen in this individual field, one of which is seen face-on in the lower right quadrant, and another tilted and therefore seen edge-on in the upper left quadrant. Also seen in the upper right quadrant is a cluster of stars located in our Milky Way galaxy. These VIS images have not undergone any processing and include effects that will have to be cleaned up by the processing chain developed by the ground segment. Credits : ESA, Euclid, Euclid Consortium, NASA

In addition to many astronomical objects, one can see in this raw image artifacts that will be cleaned when all the corrections included in the VIS data processing chain are applied. By zooming in on the image on the right of Figure 1, we see in particular the effects of cosmic rays, these very energetic particles which leave straight trails when they pass through the VIS detector, damaging the light distribution of some of the observed objects.

The quality of the images obtained with VIS and NISP demonstrates to the proper functioning of the entire satellite, and indicates that the telescope and its instruments seem to have the characteristics necessary to achieve the scientific objectives for which they were designed. For all the teams of the Euclid project, this materializes eleven years of work since the design of the satellite, and it is with great emotion that the members of the project discovered these first astronomical images.

Invested since 2010 in Euclid's ground segment, in charge of planning and implementing observations, as well as processing, archiving data and delivering them to scientific teams, the team of Euclid researchers and engineers at IAP is specifically invested in the production of algorithms of the VIS instrument image processing. The images shown in Figure 1 have not received any processing and it was all the work of this team to develop the algorithms for processing these images in order to make them scientifically exploitable, which requires understanding the optical and electronic effects that affect them. These effects must be studied in detail to obtain the measurements of the positions, shapes and fluxes of the sources as accurately as possible, in order to achieve the scientific objectives of the mission. This work is also based on the simulation of VIS images, i.e. the production of synthetic images resembling real images as closely as possible, to which the IAP team also contributes, in order to assess the impact of instrumental effects and of the entire image processing chain on the measurements which will be deduced therefrom.

After the commissioning phase of Euclid and its instruments, the calibration of the instruments began on August 12 as part of the “performance verification” phase for a period of about two months, and while tests of the reliability of the telescope guidance system are still underway. It will be at the end of this phase that the survey observations can begin. at the end of which the survey observations can begin. These observations make it possible to measure Euclid's performance in real conditions in space, and to determine with what precision Euclid will be able to achieve its scientific objectives. The very first steps of this verification phase show that the entire telescope and VIS instrument offer a remarkable image quality allowing the identification and understanding of very subtle effects in the data at a level of precision never achieved previously in space, and therefore to correct them. The IAP team is working closely with the other project members, some of whom have gathered, for the intensive work of this new phase of the project, at European Space Astronomy Center (ESAC) in Madrid (Spain).

The Euclid telescope and its instruments were designed to increase our knowledge of two still mysterious components of our Universe, dark matter and dark energy, which together contribute 95% of the energy content of the Universe. Thanks to the NISP instrument, Euclid will draw up a three-dimensional infrared map of the Universe, observing billions of galaxies up to 10 billion light-years away, and spread over a third of the sky. As for the VIS instrument, it will measure the shapes of distant galaxies in the visible with extreme precision, inaccessible with ground-based telescopes. At the end of the performance verification phase, images of the sky exposed for longer will then be fully processed by the Euclid Consortium in order to get rid of observational artefacts and prepare them for scientific analysis. The first availability of scientific data is expected by the end of 2023.


puce CNES press release, July 31, 2023 (in French) : Un mois après son envol, la mission Euclid dévoile ses premières observations” (“A month after its flight, the Euclid mission unveils its first observations”)

puce ESA press release, July 31, 2023 : “Euclid test images tease of riches to come”

puce ESA press release, August 25, 2023 : “Follow Euclid’s first months in space”

puce Post on the Euclid Consortium blog, July 31, 2023 : “Euclid sees first light”

puce Post on the Euclid Consortium blog, August 1, 2023 : “What’s in Euclid’s first light images?”

puce Video on the Euclid France channel: “La première lumière du satellite Euclid enfin dévoilée !” (“The first light from the Euclid satellite finally unveiled!”)

puce Video on the Euclid France channel: “Le segment sol Euclid, Traiter les images de l’instrument VIS #3” (“The Euclid ground segment, Processing images from the VIS instrument #3”)

puce Video on the Euclid France channel: “Le segment sol Euclid, Simuler les images #2” (“The Euclid ground segment, simulating images #2”)

puce On Euclid France: “Le segment sol d’Euclid, sa particularité, une série vidéo” (“The Euclid ground segment, its particularities, a video series”)

puce Euclid, a Science and Exploration mission: « Exploring the dark Universe, latest news and Euclid overview »


  • Yannick Mellier
    Institut d’astrophysique de Paris, CNRS, Sorbonne Université
    yannick [dot] mellier [at] iap [dot] fr
  • Audrey Le Reun
    Institut d’astrophysique de Paris, CNRS, Sorbonne Université
    audrey [dot] lereun [at] iap [dot] fr

Writing and layout: Valérie de Lapparent

Layout and inconography: Jean Mouette

August 2023

Institut d’Astrophysique de Paris - 98 bis boulevard Arago - 75014 Paris