A change in attitude

The problem was that a tiny amount of unwanted sunlight was reaching Euclid's visible instrument (VIS) at specific angles, even with the spacecraft’s sunshield (its back) turned towards the Sun.

“The original plan was for Euclid to keep its sunshield facing the Sun. But soon after launch, disturbing light from the Sun was detected in the test images,” explains Ismael Tereno of the University of Lisbon, Portugal, and lead of the Euclid survey operations support team, operated with the support of the Portuguese Space Agency.

“After intense troubleshooting, science, engineering, and industrial teams found that for this light to disappear, Euclid needed to observe with a different orientation (attitude) with respect to the Sun. This meant that the original survey design would no longer work. We had to quickly come up with a new strategy, implement it, and test it,” adds João Dinis, also from the University of Lisbon, Portugal who together with Ismael was responsible for the survey (re-)design.

To minimise the effect of ‘stray’ sunlight, the teams found out that Euclid needs to observe with a more restricted rotation angle, such that the sunshield is not directly facing the Sun, with a small but impactful tilt in one direction. With this new restricted attitude, parts of the sky could not be reached from any point in Euclid’s orbit around L2. “It proved very difficult to find a good survey solution, and we had to go back to the drawing board,” recalls Ismael.