5 minute read
Aligned & Focused
We are one step closer to unravelling the mysteries of the universe with the James Webb Space Telescopes’ penultimate phase of commissioning completed. The multi-phase commissioning process for this freshly launched galactic eye in the sky officially started the second of blast off from the launch pad of the Kourou space port in French Guiana on Christmas day 2021.
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The first phase of spacecraft launch and deployment ran smoothly with no reported issues during the unfurling process. The tennis court sized sun shield was unfolded over a period of a week bringing the final steps of the deployment to a close on January 8th. The next crucial phase was to cool down the onboard instruments ready for mirror and scientific instrument commissioning operations to begin. This phase saw the Gaseous Helium Cryocooler chill down the science instruments to -266◦C, a temperature below the freezing point of nitrogen.
This incredibly low temperature will help optimise the detection of the faint heat signatures from infrared light with limited interference from onboard generated electronic heat. Astonishingly according to NASA the JWST is so sensitive to infrared light, it would be able to detect even the slightest heat of a bumblebee at the distance of the moon. Surprisingly the 18 hexagonal gold coated beryllium mirrors stretching 6.5 meters in diameter took a while longer than the instruments to cool down. This was mostly due to the fact beryllium has a long thermal time constant at cryogenic temperatures, meaning this particular material takes a long time to cool down and heat up. The operational target temperature for the mirrors is below - 223◦C.
By February the telescope was optimally cooled and the 3 month, 7 stage instrument and mirror alignment process could begin. This has been the most exciting instalment in the JWST story so far.
Aligned & Focused
Stage 1 was known as ‘Segment Image Identification’ and this stage resulted in NASA releasing the very first JWST image on February 11th , showing an array image comprised of 18 images of the same star. The lucky star chosen for this monumental step is know as HD 84406 , a sun like star situated 260 light years away in the constellation Ursa Major. This first fuzzy image mosaic was taken over 25 hours beginning on the Feb 2nd the telescope was pointed at 156 different positions around the location of the star generating 1,560 images in total using the NIRCam. The results of this imaging session validated that all 18 mirrors were working and had survived the journey thus far.
The next four phases were know as ‘Segment Alignment’, ‘Image Stacking’, ‘Course Phasing’ and ‘Fine Phasing’. These phases would allow NASA’s engineers to make the fine focus corrections to both primary and secondary mirrors required to bring the 18 fuzzy dots of light into sharply focused star dots that could be image stacked to form one crisp image. On March 11th after many weeks of primary mirror alignment the NASA team finished focusing the JWST. On the March 16th an image of 2MASS J17554042+6551277, was released. This star was chosen for its remoteness away from other stars so it could be easily identifiable. Excitingly, when the team imaged the star with the Near Infrared Camera not only did it image the focused target star it also picked up some background galaxies. This star is around 2,000 light- years away in the constellation Draco within our Milky Way Galaxy. However the background galaxies are estimated to be billions of lightyears beyond.
“We now have achieved what’s called ‘diffraction limited alignment’ of the telescope,” said Marshall Perrin, deputy project scientist for Webb at the Space Telescope Science Institute. “The mirrors are focused together as finely as the laws of physics allow, and this is the sharpest image you can get from a telescope of this size.”
The sixth phase known as Telescope Alignment Over Instrument Fields of View’ would see all scientific instruments calibrated. This phase looked at star fields as seen by NIRCam, NIRISS, FGS, and NIRSpec to see whether they were in focus. Optical error measurements were then taken by the engineering team to help with determining the final alignment corrections required as part of the final seventh stage of alignment. On April 28th NASA announced that the JWST can now capture sharp images of celestial objects with multiple instruments and released an image showing off it’s repertoire from all four of its science instruments exceeding all expectations for image quality. This was followed by NASA releasing a stunning comparison image showing the same image taken by the JWST vs the now retired Spitzer on May 9th.
The image taken by the MIRI, the Mid-Infrared Instrument, Webb’s coldest camera shows part of the Large Magellanic Cloud, a small satellite galaxy of the Milky Way next to the same image taken by the Spitzer Space Telescope. It really shows how incredibly clear and sharply focused the JWST is in comparison. Now that the JWST optics and instruments are aligned the project is entering its final overall stage of commissioning before the start of its first main science operations starting later this year. This final stage is know as science instrument commissioning and will take approximately two months to complete. There is a suite of 17 different scientific instrument modes to commission across the 4 parent instruments. Once these are checked off and ready, scientific operations and discovery can begin.
Each of the 17 modes are designed with specific analysis in mind. These modes will be selected by the scientists across the globe to best suit their selected fields of particular research. There was fierce competition for securing observing time during cycle 1, the first year of science with over 1,000 proposals submitted before the deadline back in November 2020. Scientists from 44 countries applied for a portion of the 6,000 general observing hours available in this first year. This equated to two thirds of the overall time, with the rest allocated to the Early Release Science Program and Guaranteed Time (GTO) programmes set by the Scientists that worked on developing JWST hardware and software components.
Future programs cover all areas of astronomy from examining chemistry in planet forming regions, analysing galaxy protocultures and drivers of cosmic reionisation to examining the physics behind black hole jets. NASA is scheduled to release the first images taken by the Telescope on July 12, I predict a wave of exciting new celestial discoveries, expanding upon previous knowledge of the cosmos and perhaps challenging the current known rules of astrophysics.
New image of the Large Magellanic Cloud. Image Credit: NASA/ESA/CSA/STSScI
