The fine tuning of the eighteen hexagons of the 6.5 m reflector of the James Webb Space Telescope (JWST) is continuing. A new image shows the progress made with a perfectly focused star and, in the background, several visible galaxies.
1.5 million km from the Earth, at the Lagrange point L2, the James Webb Space Telescope (JWST, or also more simply “the Webb”) continues its commissioning procedure by receiving instructions from its controllers from the Space Telescope Science Institute (STScI) located in Baltimore in the United States.
A Star in the Dragon
Previous steps had already enabled the eighteen 1.3 m-wide hexagons which constitute the 6.5 m primary reflector of the Webb to align. On 16 May, NASA announced that the eighteen hexagons were surveying the universe acting as a single reflector with the image below of the star referenced as 2MASS J17554042+6551277 as a benchmark.
The star chosen for this new test is different from the previous test. Located this time in the Draco constellation, it is referenced as 2MASS J17554042+6551277, with 2MASS as the name of the catalogue it is in (2MASS means 2 Micron All-Sky Survey, this is a survey of the sky in the 2 micrometres wavelength).
Of a magnitude of 10.9, this star cannot be seen with the naked eye (the limit in perfect conditions is 6) or with binoculars. The very visible branches are explained by the hexagonal shape of the segments and were expected. They are, therefore, not a defect, but a consequence of the Webb’s optical design. This image, above all, shows that NASA’s telescope, to which the European Space Agency (ESA) and the Canadian Space Agency (CSA) are linked, is showing “a performance which surpasses the specifications” to use the words of Ritva Keski-Kuha, who works at the American agency’s Goddard centre.
We can see many galaxies in the background which is testimony to the excellent sensitivity of the Webb and the NIRCam camera used. These galaxies are, of course, considerably further from us than the star in the Draco constellation. Indeed, they are located billions of light years away according to this APOD publication by NASA.
And, As Always, the Selfie of the Reflector!
As it says above, the image was realised using the NIRCam instrument. which means the precise alignment of the eighteen hexagons is suitable for this camera. But the Webb is equipped with other instruments, namely, the MIRI imager and spectrometer, the NIRSpec spectrometer, the NIRISS near infrared imager and FGS, responsible for directing. In the coming weeks, STScI teams will still be improving the alignment of the hexagons so that the adjustment is as effective for these instruments in addition to NIRCam. The precision required is about 50 nanometres … To give an idea, if the 6.5 m reflector of the JWST was enlarged to the size of the United States, the hexagons would be fine-tuned to 4 cm!
Pending this work, we can look at a new selfie of the main reflector.
Finally, let us remember that all the Webb instruments work in infrared, because this wavelength allows us to go back in time, about 200 million years after the Big Bang, during the formation of the first stars and galaxies. Infrared also “passes” through gas clouds and reveals the mechanisms of nebulae where new suns are born. Studying the composition of the atmosphere of exoplanets (worlds which orbit other stars than our own) will also be a major scientific objective of JWST.