"Ce sont des CCD très spéciaux, ils ont été développés spécifiquement pour Euclid depuis de nombreuses années, " dit Alex Short, Ingénieur charge utile VIS de l'ESA.

L'autre instrument, NISP, est dédié à la réalisation de mesures spectroscopiques de galaxies, ce qui implique de diviser leur lumière en longueurs d'onde individuelles. Cela permet de déduire les redshifts. Cette propriété permet aux cosmologistes d'estimer la distance à la galaxie en question, et permettra aux données d'Euclide d'être transformées en les plus grandes, l'enquête 3-D la plus précise jamais réalisée sur l'Univers.

"L'équipe internationale du NISP et les industries de soutien ont fait un travail incroyable pour concevoir, développer et tester cet instrument stimulant, " dit Thierry Maciaszek, Chef de projet instrumentation NISP, du CNES et du Laboratoire d'Astrophysique de Marseille, La France.

"C'est, cependant, pas la fin de l'histoire pour nous car de nombreuses activités majeures doivent être réalisées avec NISP au niveau du satellite. Nous attendons avec impatience les premières lumières en vol démontrant les excellentes performances globales."

Le détecteur NISP offrira le plus grand champ de vision jamais parcouru dans l'espace pour un instrument infrarouge.

"La qualité de l'optique est tout simplement incroyable, " dit Tobias Boenke, Ingénieur système de mission et instrument NISP à l'ESA.

Un facteur clé dans l'obtention de la précision optique exceptionnelle d'Euclid a été la décision prise au début de l'histoire du projet de construire l'intégralité du module de charge utile en carbure de silicium. L'utilisation de ce matériau à l'ESA a été pionnière dans la fabrication du télescope de la mission spatiale Herschel. Dans le cadre de la mission Gaia de l'ESA, la structure de support des sous-systèmes de l'engin spatial était montée sur un châssis en carbure de silicium. Sur Euclide, le matériau a été utilisé pour les instruments ainsi que le télescope.

Alors que le métal se dilate et se contracte lorsque sa température change, dégradant ainsi la capacité d'un système optique à focaliser la lumière, le carbure de silicium est extrêmement stable à de telles variations de température. But using the compound brings its own challenges. Silicon carbide is a ceramic and so much more brittle than metal.

"It was a big challenge to be able to manufacture the instruments from this material and make sure they can remain undamaged during the launch, " adds Tobias.

Like VIS, NISP also uses specially designed state-of-the-art detectors to record the faint light coming from distant stars and galaxies. Unlike VIS, NISP can also operate in spectrographic mode. The detectors, which are operated at –180°C to provide ultra-low noise and high sensitivity, register these 'spectra' and convert them into tiny electronic signals. These signals can then be amplified and accurately measured to provide the photometric and spectroscopic redshifts.

The instruments will receive light from Euclid's telescope, which has already been assembled at Airbus, Toulouse. Like the instruments, it too is made from silicon carbide and is a state-of-the art construction in all senses.

"We are pushing all the manufacturing levels to the limit, " says Luis Miguel Gaspar Venancio, ESA's Mission Performance &Optical Systems Engineer.

A special component behind the telescope, called the dichroic, separates the collected light and diverts the visible wavelengths to VIS and the infrared wavelengths to NISP.

When the information from VIS and NISP is combined, scientists will be able to deduce the way that the Universe's large-scale distribution of galactic structures has built up throughout cosmic history. This will help them determine the speed at which such structures grow, providing strong constraints on the nature and amount of dark matter and dark energy in the Universe.

The instruments were prepared for delivery to Airbus just before the COVID-19 pandemic imposed restrictions and lockdowns in many ESA member states. Heureusement, VIS was already at Airbus, and NISP had to wait for a few weeks for shipment from Marseille to Toulouse, but was not on the critical path.

"I am extremely grateful to all project parties:institutes, industry and colleagues at ESA for their dedication and commitment during these difficult times, " says Giuseppe Racca, ESA's Euclid project manager.

"We were hit by the pandemic in a particularly critical moment when both VIS and NISP were to be transferred to industry. Despite the work and travel restrictions a concerted effort by all parties allowed to minimize the delays by implementing distributed and sequential integration activities of the instrument units and remote monitoring."

Now that the instruments have been delivered to Airbus, they will be integrated first with the telescope, and next with the rest of the payload module. It has been a long journey getting this far. Euclid was selected for implementation in 2011, having already undergone almost five years of studies. While there is still a lot of hard work and testing ahead, the delivery of the instruments and telescope means that the spacecraft can really begin to come together.

"Finalement, we have something in front of our eyes, " says Luis Miguel. "It's not just paper anymore. It's a fantastic piece of hardware; beautiful in a way."

Integrating the payload module will last several months as it is painstaking work to get everything bolted together, precisely aligned and electronically talking. The instrument's control units have already been mechanically and electrically integrated to the payload module. These tests have verified that the instruments can be properly powered by the spacecraft, can talk to the onboard computers, and can transmit the science data that will then be downloaded to ground through the spacecraft antennas.

Once the telescope has been integrated with the rest of the payload module, it will be shipped to Centre Spatial de Liège, La Belgique, for 'end-to-end' testing in a thermal vacuum chamber that can simulate the conditions of space as well as possible on Earth. This test is scheduled to take place in February and March 2021.

Once that test shows that everything is working as expected, the payload module will be shipped to the prime contractor Thales Alenia Space (TAS), in Torino, Italie. TAS has been building the service module, which contains essential systems such as power, propulsion and communications.

The service module's main structure recently passed its structural and thermal tests and is now ready to have the various systems integrated inside. TAS will begin by laying down the pipelines for the propulsion systems, and the cabling for other distributed systems. Flight electronics, including computers, power units, and attitude control units, are already mounted on their own structural panels and these will now be installed inside the main structure. Integration is due to be complete in the third quarter of the year, at which time tests will be performed.

TAS will then integrate the payload module with the service module to form the final, finished spacecraft. Puis, another round of tests will ensure that everything is working together properly. At that point, the spacecraft is essentially finished, and ready for launch.

Launch is currently scheduled for the second half of 2022 from Europe's spaceport, Kourou, French Guiana.