The James Webb Space Telescope (JWST) is a remarkable feat of engineering. Folded like an origami to fit into the rocket that launched it into space, it has successfully carried out a series of steps to reach its final destination and unfold itself to its full extent.
There remain some crucial steps before this marvelous instrument can begin to return its first scientifically useful images, however. The 6.5 meter (a little over 21 feet) light-gathering mirror of the JWST is not a monolithic object like that in the Hubble Space Telescope. Like many of the world’s largest ground based telescopes, it is segmented. There are eighteen individual mirrors which must be aligned to create a single image.
https://webb.nasa.gov/images/JWST-HST-primary-mirrors.jpg
That alignment process began here. JWST focused on a single star, and as you can see, each of the eighteen mirrors produced a separate image.
https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#/media/File:JWST_-_First_images_of_HD_84406_(segments_marked).png
Which image came from which mirror segment was determined by “wiggling” a segment and seeing which star image was affected. First the ground team moved the mirrors so that the image positions corresponded to the actual positions of the segments themselves.
https://blogs.nasa.gov/webb/wp-content/uploads/sites/326/2022/02/PostGlobalAlignment-1024×1024.jpeg
The eighteen images were then combined into one.
https://blogs.nasa.gov/webb/wp-content/uploads/sites/326/2022/02/PostImageStacking-1024×1024.jpeg
There is further fine tuning ahead that will create an even sharper pinpoint image.
So we’re almost done, right? Not quite. The telescope still has to be cooled to very low temperatures. Why? JWST is an infrared (IR) telescope, designed to see wavelengths of electromagnetic radiation that are longer than visible light.
https://spaceflightnow.com/wp-content/uploads/2021/12/hst-webb-spectrum.jpg
Objects that radiate mostly in IR are relatively cool, and in order to see them, so must be the telescope. Think of the Hubble Space Telescope that saw objects in visible light. If it was as hot as a star (Yes, I know it would vaporize, but this is a thought experiment!), its own glow would obscure and overwhelm the light from the star. Hubble is of course much cooler, and can capture sharp images of stars and other warm objects with ease.
Webb’s large mirror can gather a lot of light and can therefore see objects that are quite faint. It can see farther out into the universe, and therefore farther back in time. A galaxy that is 5 billion light years away is showing us what that galaxy was like 5 billion years ago. JWST will be able to reach back almost to the beginning of the universe, to just a few hundred million years after the Big Bang 13.8 billion years ago, a time when the first stars and we believe the first galaxies were forming.
But there is a catch. The light emitted from those stars started out as visible and ultraviolet wavelengths. The expansion of the universe has stretched them into the infrared region, and that is what Webb is designed to see.
Closer to home, infrared light is not scattered by interstellar dust clouds the way visible light is. Webb can peer into those clouds to see the processes of stellar and planetary formation.
The final temperatures required are 37 Kelvins (37 degrees Celsius above absolute zero) for most instruments, and this is possible with passive cooling. But one instrument requires an operating temperature of 7 Kelvins, and this requires a “cryocooler” to achieve. “Being a refrigerator and a “closed” system, the cryocooler does not consume coolant like an ice chest full of ice or a big container (a.k.a. dewar) of liquid helium does, and so its life is limited only by wear in its moving parts (the pumps) or the longevity of its electronics, all of which should last for many years.” (https://jwst.nasa.gov/content/about/innovations/cryocooler.html)
The Hubble Space Telescope has probably been the most popular scientific instrument of all time. The James Webb Space Telescope is a worthy successor!
Leave a Reply