From Euclid's fine guidance sensors intermittently losing track of stars to stray sunlight getting in the way of observing the Universe and X-rays appearing in instrument images, the issues raised do not threaten Euclid's mission but could impact how it carries out its work.

After months of late nights and determination from engineering teams at ESA's mission control, mission scientists and industry, with some tweaking to observation schedules, data processing and by altering the directions Euclid will point in, its looking good for the detective.

All was well, until...

Euclid commissioning – the period after launch when a mission’s instruments and subsystems are deployed, turned on, tested and calibrated – began well. Teams at ESA’s mission control worked 24 hours a day in 12-hour shifts through intense weeks of manoeuvres, tests and calibrations; flying the mission out to Lagrange point 2, its telescope mirrors were de-iced, its NISP and VIS instruments were woken up and saw ‘first light’ and its telescope mirror was focussed. Euclid’s first test images were mesmerising.

However, it is rare, if ever, that all goes perfectly throughout the life of a space mission. After all, we can only perform tests on Earth – space is a different world. To operate a spacecraft, engineering and science teams will always need to detect and quickly resolve issues as they arise, and there are a multitude of ways a mission can go wrong. It’s why months are spent in arduous simulations before launch – you never know quite what will happen. 

Now it’s not an outlook we recommend for daily life, but it's the nature of spacecraft operations to laser focus on the negative. For now, goodbye to Euclid’s better-than-expected optical quality and seamless trip to ‘L2’, time to investigate what’s not quite right. 

Lost guide stars likely found

Euclid is one of the most precise missions ever to be launched, providing razor-sharp images and deep spectra of our Universe, looking back 10 billion years. It will produce a grand survey of one-third of the entire sky. Every 75 minutes for its mission duration of six years, the telescope must point at a new field on the sky with extreme accuracy and stability. 

To do this, the spacecraft has a Fine Guidance Sensor (FGS); a completely new development in Europe made of optical sensors that pick out and lock onto stars found by ESA's Gaia mission, using them as guides to navigate and determine exactly where the telescope needs to point in the sky. This information is fed into the ‘Attitude and Orbit Control System’ which controls Euclid’s orientation and orbital motion. 

While most systems are performing well, there have been intermittent cases of the Fine Guidance Sensor failing to lock onto faint stars. In orbit, Euclid detects the true sky under real space conditions, something which is very hard to simulate before launch. Plus, cosmic rays from the Sun and the galaxy pollute observations, make the job of the FGS a real challenge. 

Euclid's commissioning phase was extended in order to look into the issue, delaying the all-important ‘performance verification’. Teams have since been working on a software fix that has now been uploaded onto the spacecraft and is undergoing thorough testing. 

“The issue of Euclid's fine guidance is something we’ve all been concerned about. Teams at ESA's technical heart (ESTEC), mission control (ESOC), Astronomy Centre (ESAC) and industry have been working day and night, tirelessly for months, and I can’t thank them enough for their determination to resolve the issue,” says Euclid Operations Director Andreas Rudolph.

“I’m relieved to say that initial tests are looking good. We’re finding 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.” 

The updated software has already passed with flying colours on a spacecraft simulator and ‘test bench’ (Euclid replica) at mission control, then performed perfectly in orbit and will next be tested under the control of the Science Operations Centre at ESA’s ESAC Astronomy Centre in Spain. 

“Obviously, this is where we will have the real test of truth, as only the science images can provide us with absolute certainty that Euclid’s pointing is performing well,” cautions Euclid Project Manager, Giuseppe Racca. 

“However, all evidence so far makes us very optimistic. We will continue to keep our fingers tightly crossed, but the restart of the performance verification phase gets nearer every day.”

Dark detective sees unwanted ‘stray light’

While Euclid’s faint, guide stars appear to be found, its next, (smaller) issues come from our nearest star. 

Euclid is located at Lagrange point 2 in a unique orbit ‘behind’ Earth. Here, Euclid has its ‘back’ to the Sun, so all the sensitive parts of its telescope are protected from sunlight by a dedicated sunshield. However, it was known that a thruster bracket lay outside the shadow of the sunshield and would receive direct sunlight.

It seems a small amount of sunlight is reflecting off the bracket towards the VISible (VIS) instrument that is protected by many layers of insulation. However, because of the extreme sensitivity of the VIS instrument, the current theory is that enough light is still getting through this insulation, with stray light being detected in test observations when VIS is turned at specific angles.

 

Euclid scans the night sky using a 'step-and-stare' method, combining separate measurements on an area of approximately 0.5 deg² - twice the size of the full Moon – to form the largest cosmological survey ever conducted in the visible and near-infrared.
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The majority of VIS’s observations showed no significant stray light interference, but at particular angles, about 10% of observations were affected. Science, engineering and industrial teams spent weeks deciphering which angles let too much of this unwanted light in and have re-designed and optimised Euclid’s survey to constrain each pointing’s orientation in the sky.

While this will not affect Euclid’s ability to take the precise images required, it could impact the efficiency of the survey – something that is still under investigation. 

Vis-à-vis the Sun

Sunlight isn’t the only solar issue Euclid faces. If this were an Agatha Christie novel, we’d start to wonder what the Sun is trying to hide from our cosmic detective. Enter: solar flares – sudden eruptions of electromagnetic radiation from the Sun’s surface, made up of light across the entire spectrum, including X-rays.  

Euclid's detectors are being shielded from low-energy protons that could do them damage. However, it appears that at particular angles, X-rays emitted by the Sun during solar flares can occasionally reach the detectors, spoiling a portion of the images taken at that moment.  

Solar activity is currently high as the Sun gets close to the most active period in the current solar cycle, expected to peak in 2024-25. 
 
Analysis currently predicts that, depending on solar activity, Euclid could lose about 3% of its data if this problem were left untackled. However, now that the issue has been discovered teams are able to identify affected pixels and discount them in later analysis and are working on plans to repeat observations to eventually make up for any gaps in Euclid's cosmological survey.

Optimism for science results to come

It’s important to put the problems above in context. This commissioning period is the time when teams focus hard on uncovering any possible issue that could affect the mission – large or small.

Euclid will take remarkable images of our Universe and help piece together how dark energy and dark matter influence the parts of our world that we can see. Euclid's fine guidance seems close to being solved without any further impact on the mission. The Sun's stray light can be mitigated with clever reprogramming of Euclid's survey and, while the X-ray issue will have a marginal effect, teams are working hard to minimise this through repeat observations and data processing.

Stay tuned for updates, as we get closer to Euclid’s first images and start the quest to reveal the nature of dark matter and dark energy.

Quelle: ESA

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Update: 9.10.2023

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'Immense relief': Universe-mapping Euclid telescope fixes problem that threatened mission

ESA says a software patch restored stability to its new cosmic mapper — but slower operations could extend the mission.

A problem with the onboard software failing to recognize cosmic rays (which appear as faint dots in this image) meant that Euclid sometimes couldn’t lock into place, and produced images that appear to show swirling star trails.Credit: ESA/Euclid Consortium/TAS-I (CC BY-SA 3.0 IGO)

Shortly after launching on 1 July, the European space observatory Euclid started performing tiny, unexpected pirouettes. The problem revealed itself during initial tests of the telescope’s automated pointing system. If left unfixed, it could have severely affected Euclid’s science mission and led to gaps in its map of the Universe.

Now the European Space Agency (ESA) says that it has resolved the issue by updating some of the telescope’s software. The problem occurred when the on board pointing system mistook cosmic noise for faint stars in dark patches of sky, and directed the spacecraft to reorient itself in the middle of a shot.

Giuseppe Racca, Euclid’s mission manager at ESA in Noordwijk, the Netherlands, says that the updated pointing system will operate slightly slower than planned. As a result, the main mission, due to last 6 years, could take up to 6 months longer. Its scientific goals should not be affected, ESA says.

Mapping the Universe

Euclid is designed to carry out a deep survey of the Universe by mapping the positions of 1.5 billion galaxies in three dimensions, looking beyond the stars in the Milky Way. But to do so, it will often have to photograph some of the darkest patches of the sky, which have only very faint stars. Euclid must use the known positions of those stars — as previously mapped by another ESA mission, Gaia — to find the correct patch and continuously adjust its position to extremely high precision for more than 10 minutes at a time.

Initial tests of this system showed that in some cases the telescope was not pointing stably. Instead, it would wobble, producing test images in which some stars appeared to follow tiny looping trails.

ESA says that the Euclid team, together with its principal industrial contractor, Thales Alenia Space, was able to diagnose the problem quickly. The pointing system uses auxiliary sensors inside the telescope to take periodic 2-second exposures of the field of view. It then matches the stars it sees with those the Gaia catalogue, to make sure they are in the expected position. But the sensors also pick up noise from energetic particles such as cosmic rays, which continuously rain onto the probe from all directions, explains Giovanni Bosco, a physicist at Thales Alenia Space in Turin, Italy. Within 100 milliseconds, the onboard software has to filter through that noise and single out the real stars.

This didn’t always work out as planned, says Racca. “Sometimes it had too few stars, and it was getting confused. It was losing the guiding stars and then automatically started to look for them again.”

Bosco worked with the team at subcontractor Leonardo in Florence, Italy, to fix the problem by improving how the algorithms filter out cosmic noise. ESA has now tested the system and announced on 5 October that it is working as planned.

Rogue light

Another issue spotted in early imaging tests was that tiny amounts of stray light seemed to be entering the telescope — despite it being protected by a sunshield and wrapped in multiple layers of insulation. The problem was probably caused by a thruster that sticks out to one side of the spacecraft, where it is not protected by the sunshield, says Racca. When the telescope was oriented at certain angles, sunlight was ricocheting off a 1-square-centimeter area on the thruster — the only part of it that is not painted black — and bouncing from the back of the sunshield onto the side of the telescope. A small fraction of this light could be detected by Euclid’s super-sensitive cameras. The mission team found that the problem went away by simply adjusting the orientation of the probe by 2.5 degrees.

Racca says that the mission can now resume its planned commissioning stages, and expects that it will be able to begin its scientific work some time in November.

“When I heard about the problems and the solutions they were trying out, to me it sounded like this will work out,” says Anthony Brown, an astronomer at Leiden University in the Netherlands and senior member of the Gaia science team. Still, he adds, when a space mission can overcome problems, “it’s always an immense relief”.

Quelle: nature