The James Webb telescope, which will be launched into space on December 18, will take with it a double instrument of Canadian design, one of the parts of which, called NIRISS, is the result of research carried out by astronomer René Doyon started exactly 20 years ago. We can therefore imagine that the emotion will be at its height for this researcher from the University of Montreal when the rocket containing the telescope takes off.
“At the start of the project, there was not to be a Canadian instrument in the James Webb telescope. The Canadian Space Agency (CSA) was to supply only the precision guidance detector (” fine guidance sensor ”, Or FGS). We have often argued for the benefit of integrating a fourth instrument into the telescope, but we were refused. We were told that we could, however, contribute to one of the three scientific instruments already planned [soit le NIRCam, fourni par la NASA ; le NIRSpec, fourni par l’Agence spatiale européenne (ESA) ; et le MIRI, fourni par la NASA et l’ESA] », Relates René Doyon, principal researcher and coordinator of the entire Canadian contribution to the telescope.
Thus in November 2001, the principal investigator of the instrument NIRCam, the astronomer of the University of Arizona Marcia J. Rieke, requested the collaboration of René Doyon because of his expertise in infrared astronomy acquired at the Mont-Mégantic Observatory, which was a pioneer in this field, to design a module of the instrument supposed to include adjustable filters that would allow the study of the very first galaxies to appear in the Universe.
But a year later, when the choice was made to reduce the size of the telescope to 6.5 meters, NASA decided, to simplify procedures, that the NIRCam instrument would only be developed by Americans, eliminating the blow the Canadian contribution.
“There is nothing simple”
“We were disappointed! But with my colleague John Hutchings, from the National Research Council of Canada (NRC), who was the principal investigator of the FGS guidance system, we thought that nothing prevented us from putting the adjustable filters in the back of the FGS and thus to make an instrument containing the two technologies. This is what was proposed to the ASC, which accepted, and there was born the Canadian instrument which was called at the time “TFI” for “tunable filter imager”, Says Mr. Doyon.
“This optical element presented many technical challenges, however. Problems arose during the vibration and shock testing and made us doubt the ability of this adjustable filter to withstand launch. The ASC then asked us to consider reconfiguring the instrument, ”he continues.
In 2009, Hutchings and Doyon therefore reconfigured their instrument to make it more robust. “The changes were relatively minor, but when it comes to space technologies, there is nothing simple,” says the astronomer.
The new instrument was then proposed to the Webb Science Working Group, the scientific committee of the James Webb project, which finally accepted it on June 9, 2011. NASA only requested that the instrument be delivered to it by July 31, 2012 at the latest. “It was a pretty intense year, but we got there [avec l’aide précieuse des ingénieurs de la société Honeywell]. And the instrument we have now [dénommé “NIRISS” pour “near-infrared imager and slitless spectrograph”] is probably much more powerful than the one we had originally designed, ”emphasizes Mr. Doyon.
The instrument is currently on board the telescope alongside three others. It therefore contains on one side the Precision Guidance Detector (FGS), which is a camera through which the telescope can point a target with very high precision. The FGS will make it possible to correct the movements (in particular the vibrations) of the telescope. To do this, it “will measure very precisely the position of the star aimed at 16 times per second, and a small mirror inside the telescope will tilt in order to correct its movements. This instrument will be able to detect an angular movement of the telescope equivalent to the thickness of a hair seen from a kilometer. This will make it possible to obtain stable and very precise images, ”explains Mr. Doyon.
Search for life
On the other side of the instrument, which is the size of a dishwasher, is a slitless near infrared imager and spectrograph, the NIRISS. It is “an infrared camera aimed mainly at spectroscopy, which allows light to be dispersed and thus to obtain information on the speed of the objects observed and their chemical composition. This instrument will offer three different modes of observation ”, specifies the astronomer.
A first mode will make it possible in particular to observe bright stars endowed with a planetary system whose planets “transit”, that is to say pass, in front of their star. “When a planet passes in front of its star, you can easily detect a drop in the star’s luminosity, which makes it possible to measure the radius of the planet. And depending on the wavelengths that will be detected, we will know if this planet has an atmosphere, ”notes the researcher.
More particularly, we will measure the spectrum of the star before the passage of the planet as well as during its passage, and by comparing the two spectra, we will obtain a residual spectrum which will be that of the planet’s atmosphere. This technique makes it possible to determine the chemical composition of the atmosphere of exoplanets. “Most of the spectrum that we measure comes from the star itself, but we look for the part that is filtered by the small ring that surrounds the planet. This ring is the atmosphere of the planet. By performing spectroscopy during transit, we can measure the spectral signature of all the molecules present in the atmosphere of this planet, including that of water, CO2 and methane, and that is an important step towards the search for life elsewhere in the Universe ”, underlines Mr. Doyon.
Distant galaxies
The second mode of observation of NIRISS will be used to scan the far reaches of the Universe. We will try to detect the first galaxies that ignited a few hundred million years after the big bang. The instrument will achieve this using the slitless spectroscopy technique. “When we want to obtain a spectrum, we generally isolate the light of the object that we want to analyze using a small slit which makes it possible to avoid contamination of the signal by the brightness of the surrounding sky” , explains Mr. Doyon.
The NIRSpec instrument (near-infrared spectrograph) which is on board the telescope is equipped with a slit allowing the observation of up to a hundred objects at a time. “But to know where to point and put the slit, you first have to take pictures. The advantage of a slitless instrument like the NIRISS is that it allows us to take a spectrum of whatever is in our field of view, we don’t need to choose where to aim, and that’s best in first, because we do not really know where a galaxy will appear. By taking a spectrum without a slit, we have a better chance of being able to detect very distant galaxies, ”says Mr. Doyon. And when we have detected these galaxies with the NIRISS, we will then have recourse to the NIRSpec, which will allow them to be studied in more detail, because the spectra obtained with the NIRSpec are more sensitive and of better quality.
The five galaxy clusters that Canadian researchers plan to observe with the NIRISS are likely to act as gigantic gravitational lenses that will make them detect galaxies far away, because “these galaxy clusters contain dark matter which deflects light. , as Einstein taught us. Thus, if there is a galaxy very far away, in the background of the cluster of galaxies, its light will be amplified by the latter, and will then appear in our field of vision, ”explains Mr. Doyon.
Planets difficult to observe
The third mode of observation of the NIRISS consists of the insertion in the beam of the instrument of a small mask which will block the light of 11 of the 18 hexagonal segments of the mirror of the telescope and which will retain the light of only 7 of them. them, which will make it possible to detect exoplanets orbiting very close to their star by interferometry.
More than 4,500 exoplanets have been discovered so far, and the vast majority are planets that are very close to their star. “Taking images of these planets is very difficult, no terrestrial telescope, not even Hubble has ever managed to see them, because they are in what is called the blind spot of telescopes, underlines the astronomer. The NIRISS observation mode called AMI [pour “aperture masking interferometry”, interférométrie de masquage d’ouverture] will allow us to go and probe regions very close to the star, where we will perhaps detect new exoplanets, ”he points out.
This third mode will also make it possible to observe stars relatively close to us, as well as young stars.
In return for this important contribution from Canada to the James Webb Telescope, Canadian researchers will be entitled to 450 hours of observation that they can use as they see fit during the first two years of the telescope’s operation, which is a great deal. great privilege, because access to this space telescope will be greatly coveted by all researchers around the world.