Dix-sept modes de découverte : Activités finales de mise en service du télescope spatial Webb.

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Webb MIRI Spectroscopy Animation
Poster du télescope spatial Webb

Le télescope spatial James Webb de la NASA est une véritable merveille technologique. Le plus grand et le plus complexe des télescopes spatiaux jamais construits, le Webb est capable de recueillir la lumière qui voyage depuis 13,5 milliards d’années, soit presque depuis le début de l’univers. En fait, le Webb est une machine à remonter le temps, qui nous permet d’observer les premières galaxies qui se sont formées après le Big Bang. Parce qu’il capte la lumière infrarouge, il voit directement à travers les nuages de poussière géants qui bloquent la vue de la plupart des autres télescopes. Webb est 100 fois plus puissant que le télescope spatial Hubble. Crédit : NASA/JPL-Caltech

Une fois l’optique du télescope et les instruments alignés, l’équipe du Webb met en service les quatre puissants instruments scientifiques de l’observatoire. Il y a 17 “modes” d’instruments différents à vérifier pour être prêt à commencer les activités scientifiques cet été. Une fois que nous aurons approuvé ces 17 modes, ;” data-gt-translate-attributes=”[{” attribute=””>NASA’s James Webb Space Telescope will be ready to begin scientific operations!

In this article we’ll describe the 17 modes, and readers are encouraged to follow along as the Webb team checks them off one by one on the Where is Webb tracker. Each mode has a set of observations and analysis that need to be verified, and it is important to note that the team does not plan to complete them in the order listed below. Some of the modes won’t be verified until the very end of commissioning.

For each mode we have also selected a representative example science target that will be observed in the first year of Webb science. These are just examples; each mode will be used for many targets, and most of Webb’s science targets will be observed with more than one instrument and/or mode. The detailed list of peer-reviewed observations planned for the first year of science with Webb ranges from our solar system to the most distant galaxies.

1. Caméra proche infrarouge (NIRCam) pour l’imagerie. L’imagerie dans le proche infrarouge prendra des photos dans une partie du visible jusqu’au proche infrarouge, soit une longueur d’onde de 0,6 à 5,0 micromètres. Ce mode sera utilisé pour presque tous les aspects de la science de Webb, des champs profonds aux galaxies, des régions de formation d’étoiles aux planètes de notre propre système solaire. Un exemple de cible dans un programme du cycle 1 de Webb utilisant ce mode : le champ ultra-profond de Hubble.

2. Spectroscopie sans fente à grand champ NIRCam. La spectroscopie sépare la lumière détectée en couleurs individuelles. La spectroscopie sans fente étale la lumière dans tout le champ de vision de l’instrument, de sorte que nous voyons les couleurs de chaque objet visible dans le champ. La spectroscopie sans fente du NIRCam était à l’origine un mode technique utilisé pour aligner le télescope, mais les scientifiques ont réalisé qu’elle pouvait également être utilisée à des fins scientifiques. Exemple de cible : quasars distants.

3. Coronagraphie NIRCam. Lorsqu’une étoile a des exoplanètes ou des disques de poussière en orbite autour d’elle, la luminosité de l’étoile surpasse généralement la lumière qui est réfléchie ou émise par les objets beaucoup plus faibles qui l’entourent. La coronagraphie utilise un disque noir dans l’instrument pour bloquer la lumière de l’étoile afin de détecter la lumière de ses planètes. Exemple de cible : l’exoplanète géante gazeuse HIP 65426 b.

4. Observations de séries temporelles NIRCam – imagerie. La plupart des objets astronomiques changent sur des échelles de temps qui sont grandes comparées à la durée de vie humaine, mais certaines choses changent assez vite pour que nous puissions les voir. Les observations de séries temporelles lisent rapidement les détecteurs des instruments pour observer ces changements. Exemple de cible : Une étoile naine blanche pulsante appelée magnétar..

5. Observations de séries temporelles de NIRCam – grisme. Lorsqu’une exoplanet crosses the disk of its host star, light from the star can pass through the atmosphere of the planet, allowing scientists to determine the constituents of the atmosphere with this spectroscopic technique. Scientists can also study light that is reflected or emitted from an exoplanet, when an exoplanet passes behind its host star. Example target: lava rain on the super-Earth-size exoplanet 55 Cancri e.

NIRCam Sensor Array

A sensor array for the NIRCam instrument, designed and tested by Marcia Rieke’s research group at Steward Observatory. For the sensors to detect infrared light without too much noise in the data, Webb and its instruments must be kept as cool as possible. Credit: Marcia Rieke

6. Near-Infrared Spectrograph (NIRSpec) multi-object spectroscopy. Although slitless spectroscopy gets spectra of all the objects in the field of view, it also allows the spectra of multiple objects to overlap each other, and the background light reduces the sensitivity. NIRSpec has a microshutter device with a quarter of a million tiny controllable shutters. Opening a shutter where there is an interesting object and closing the shutters where there is not allows scientists to get clean spectra of up to 100 sources at once. Example target: the Extended Groth Strip deep field.

7. NIRSpec fixed slit spectroscopy. In addition to the microshutter array, NIRSpec also has a few fixed slits that provide the ultimate sensitivity for spectroscopy on individual targets. Example target: detecting light from a gravitational-wave source known as a kilonova.

8. NIRSpec integral field unit spectroscopy. Integral field unit spectroscopy produces a spectrum over every pixel in a small area, instead of a single point, for a total of 900 spatial/spectral elements. This mode gives the most complete data on an individual target. Example target: a distant galaxy boosted by gravitational lensing.

9. NIRSpec bright object time series. NIRSpec can obtain a time series spectroscopic observation of transiting exoplanets and other objects that change rapidly with time. Example target: following a hot super-Earth-size exoplanet for a full orbit to map the planet’s temperature.

10. Near-Infrared Imager and Slitless Spectrograph (NIRISS) single object slitless spectroscopy. To observe planets around some of the brightest nearby stars, NIRISS takes the star out of focus and spreads the light over lots of pixels to avoid saturating the detectors. Example target: small, potentially rocky exoplanets TRAPPIST-1b and 1c.

Webb MIRI Spectroscopy Animation

The beam of light coming from the telescope is then shown in deep blue entering the instrument through the pick-off mirror located at the top of the instrument and acting like a periscope.
Then, a series of mirrors redirect the light toward the bottom of the instruments where a set of 4 spectroscopic modules are located. Once there, the beam of light is divided by optical elements called dichroics in 4 beams corresponding to different parts of the mid-infrared region. Each beam enters its own integral field unit; these components split and reformat the light from the whole field of view, ready to be dispersed into spectra. This requires the light to be folded, bounced and split many times, making this probably one of Webb’s most complex light paths.
To finish this amazing voyage, the light of each beam is dispersed by gratings, creating spectra that then projects on 2 MIRI detectors (2 beams per detector). An amazing feat of engineering! Credit: ESA/ATG medialab

11. NIRISS wide field slitless spectroscopy. NIRISS includes a slitless spectroscopy mode optimized for finding and studying distant galaxies. This mode will be especially valuable for discovery, finding things that we didn’t already know were there. Example target: pure parallel search for active star-forming galaxies.

12. NIRISS aperture masking interferometry. NIRISS has a mask to block out the light from 11 of the 18 primary mirror segments in a process called aperture masking interferometry. This provides high-contrast imaging, where faint sources next to bright sources can be seen and resolved for images. Example target: a binary star with colliding stellar winds.

13. NIRISS imaging. Because of the importance of near-infrared imaging, NIRISS has an imaging capability that functions as a backup to NIRCam imaging. Scientifically, this is used mainly while other instruments are simultaneously conducting another investigation, so that the observations image a larger total area. Example target: a Hubble Frontier Field gravitational lensing galaxy cluster.

14. Mid-Infrared Instrument (MIRI) imaging. Just as near-infrared imaging with NIRCam will be used on almost all types of Webb targets, MIRI imaging will extend Webb’s pictures from 5 to 27 microns, the mid-infrared wavelengths. Mid-infrared imaging will show us, for example, the distributions of dust and cold gas in star-forming regions in our own Milky Way galaxy and in other galaxies. Example target: the nearby galaxy Messier 33.

15. MIRI low-resolution spectroscopy. At wavelengths between 5 and 12 microns, MIRI’s low-resolution spectroscopy can study fainter sources than its medium-resolution spectroscopy. Low resolution is often used for studying the surface of objects, for example, to determine the composition. Example target: Pluto’s moon Charon.

16. MIRI medium-resolution spectroscopy. MIRI can do integral field spectroscopy over its full mid-infrared wavelength range, 5 to 28.5 microns. This is where emission from molecules and dust display very strong spectral signatures. Example targets: molecules in planet-forming disks.

17. MIRI coronagraphic imaging. MIRI has two types of coronagraphy: a spot that blocks light and three four-quadrant phase mask coronagraphs. These will be used to directly detect exoplanets and study dust disks around their host stars. Example target: searching for planets around our nearest neighbor star Alpha Centauri A.

Written by Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

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