As its scientific campaign began, researchers observed a drop in brightness on the Euclid telescope’s mirrors. Worrying? Don’t panic, it’s just frost. But this is no small feat for such a sensitive device which must map the structure of the Universe.
This is not entirely new, even in the small world of space telescopes. Despite the precautions during preparation, despite the clean rooms and their protected atmosphere, there remains a very low level of humidity confined during the launch in the bowels of the optical part of the Euclid telescope. These water molecules, in weightlessness and very cold conditions, freeze and tend to deposit on the extraordinarily clean optics of the telescope.
It is a tiny layer, just a few particles of frost, but it can disrupt the measurements of instruments whose sensitivity is extremely sensitive. “ We compared the light received by the VIS instrument during the calibration stages, with measurements taken by Euclid and its cousin the Gaia telescope “, explains Mischa Schirmer, one of the people responsible for the instrument. “ There are stars in the Universe whose intensity can vary, but a majority are stable for millions and millions of years. So when our measurements showed that they were receiving fewer photons, we knew it wasn’t them, it was us. » Boring, 1.5 million kilometers from Earth.
No “windshield defrost” function
It took several weeks to identify in which specific location (and then on which mirror) a tiny layer of frost was forming and disrupting the measurements. There are ways to evacuate the atmospheric gases present in the optical parts of a space vehicle, in particular during its initial calibration, which simply consists of heating (gently) each organ of the telescope so that the ice sublimates into gas , before ventilating it in space.
The Euclid telescope has already completed this step, but there is still a little frost remaining which is reconstituted. This is a particular problem, because unlike a “classic” optical satellite, Euclid has started its scientific mission. It would therefore be better to avoid heating up the telescope as a whole, because the sensors must then be recalibrated. Which would amount to repeating months of measurements…
One little touch after another
However, the problem must be addressed. And limit any thermal change. To do this, the teams will “play” with heating cycles targeted on the mirrors, which are not the most sensitive parts (these are obviously the sensors), first one by one, then more if necessary, until the optical measurements show an improvement in the measurements.
This is of course a delicate compromise. The frost does not prevent Euclid from doing the majority of his surveys well. However, for the most precise of them, which are important in order to succeed in its mission of mapping and observing the structure of the Universe (in particular the observation of extremely distant galaxies, thanks to gravitational lensing effects) , it will be necessary to successfully defrost, then degas as many water molecules as possible.
The more complex the sensors and ambitions of a mission, the more complex the problems…
Source: ESA press release
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