Euclid Early commissioning test image, NISP instrument

One busy summer for Euclid: now almost ready for science

From guidance errors to problems related to our Sun, the European Space Agency faced a few issues during commissioning and performance verification of Euclid

It’s been a busy summer for the Euclid space telescope. The spacecraft, designed to shed light onto the elusive dark matter, was launched on July 1, 2023, aboard a SpaceX Falcon 9. 

The launch went well, and mission specialists at the European Space Agency started to set the stage for the next part of the mission: the journey to the planned orbit, and the preparation of instruments for observation. 

The commissioning phase to ready the various spacecraft sensors and detectors started immediately, while Euclid was still traveling to its final orbit around the L2 Lagrange Point. Its two primary sensors, VIS and NISP were turned on without issues, while the telescope was still unfocused.

Focusing operations were completed in the following days, and by July 28, Euclid had reached the established orbit around L2. Euclid was ready for the first light, and images arrived on July 31. Both VIS (as VISible Instrument) and NISP (Near-Infrared Spectrometer and Photometer) were working as expected. The “performance verification” phase commenced soon after, but with it, some issues were also discovered.


Stray lights get in the way

VIS has a much larger field of view compared to other space observatories, aiming to observe and catalogue galaxies taking into account the effects of gravitational lensing. This could help uncover the nature of dark matter and dark energy, and their influence on stars and galaxies in the universe.

Specialists at ESA soon noted that the instrument was seeing unpredicted light patterns in a portion of the image sensors. The issue occurred only at certain orientations of the spacecraft and was ultimately attributed to our Sun

Euclid VIS Stray Light appearing during instrument testing
The stray lights observed during testing. Credits: ESA

Euclid’s location (as GAIA and the JWST’s), was chosen to offer an unobstructed view of a large portion of the sky. The light coming from the Sun but also the Earth and Moon, would be so bright for Euclid’s extremely sensitive instruments, that observations would be compromised. Hence keeping it “behind them” rids ESA of this issue, just by placing a sunshield on one side of the observatory. 

However, it was already predicted that for certain observation angles, a thruster bracket would have been illuminated by the Sun. The effect of this small component turned out to be much more severe than expected. Engineering teams solved the problem by redesigning the survey pattern and therefore “avoiding” those angles, finding a compromise between precise imaging and a slightly less efficient pattern. This fix is temporary and teams are still investigating a better solution.

Keeping an eye on solar flares

The Sun is also responsible for copious emissions of X-rays, along with other EM spectrum radiations, during solar flares. While both sensors of Euclid are shielded from dangerous protons, there are another few angles in which the detectors have part of their field of view spoiled by x-rays.

On Sept. 6, 2017, the Sun released the most powerful solar flare recorded since 2008. Our nearest star has been lively all week, emitting three flares – two of which were "X-class," the most intense kind. NASA’s Solar Dynamics Observatory, which watches the Sun constantly, captured this image of the largest flare.
On Sept. 6, 2017, the Sun released the most powerful solar flare recorded since 2008. Credits: NASA/GSFC/SDO

With a predicted intensification of solar activity in the next 2 years, this could mean a 3% loss of useful data if not mitigated. A change in the survey observation schedule is being studied, while the affected pixels are for now being discarded.


Trouble with finding guide stars

The most relevant issue Euclid had to face was a completely different one. Its guidance system, in particular its Fine Guidance Sensor, which has to “figure out” the orientation of the spacecraft with extreme precision, had a hard time finding and locking onto known stars in the sky.

Euclid's sky survey pattern.
Euclid scanning the sky. Credits: ESA

The database of stars has to be accurate and very detailed. The tiny changes in position need to have a precise guide so as not to steer the spacecraft too far from its needed orientation. The database is being provided by ESA’s GAIA star surveyor, whose mission is to create a new, much more comprehensive stellar census.

The Milky Way blazes above the European Southern Observatory (ESO), composed with an ESA GAIA space surveyor render, launched in 2013
Artist’s rendition of GAIA. Credits: ESA/Medialab

The star tracker was intermittently failing to find its guide stars, and this concerning problem prompted ESA to extend the commissioning phase of the spacecraft, while teams at ESTEC, ESOC, and ESAC looked into the issue. With newly developed software, uploaded to the spacecraft on September 18, scientists are seeing “many more stars in all our tests, and while it’s too early to celebrate and more observations are needed, the signs are very encouraging” said Euclid Operations Director Andreas Rudolph.

Issues aside, the image quality is mesmerizing and it confirms the hard work of ESA is paying off. And while we’re waiting for official science images, we keep wondering what new discoveries on dark matter and dark energy will Euclid help uncover.


Marco Guardabasso

Marco Guardabasso

Aerospace Engineering student with a passion for space, photography and arranging music.

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