The place do James Webb’s distinctive “spikes” come from?

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Less than three months after its preliminary launch, it lastly occurred: the James Webb Space Telescope revealed its first telescope alignment evaluation image. For the primary time, humanity’s newest, best flagship observatory has efficiently aligned, centered, and finely phased the sunshine from all 18 of its main mirror segments to efficiently produce a single picture of a “test star.” The outcomes are so good that even on this preliminary picture, faint stars and galaxies positioned within the distant background may also be seen, resolved, and even examined intimately. It’s a outstanding triumph, and as NASA themselves introduced, “every optical parameter that has been checked and tested is performing at, or above, expectations.”

Still, what you’ll discover, even on a cursory look on the picture, is that the primary “star” imaged has a sequence of spikes popping out of it: six giant ones and two smaller ones, all seen even zoomed out and at low decision. For comparability, the Hubble Space Telescope solely had 4 spikes connected to each star, and an upcoming 25-meter ground-based telescope, the Giant Magellan Telescope, would be the first observatory of its form to haven’t any spikes on its stars in any respect. Here’s what we will count on, so far as spikes go, from the James Webb Space Telescope, even at full, most energy.

Artist rending exhibiting mild reflecting off of the first and secondary mirrors of NASA’s James Webb Space Telescope after its deployment in house.

(Credit: NASA/Mike McClare)

The very first thing you need to understand concerning the James Webb Space Telescope is that, in contrast to Hubble, it’s not a single-dish mirror. Instead, there are 18 particular person segments to its main mirror, and the aim of an optimally-configured James Webb shall be to have all 18 of these segments perform as if they’re a single mirror, in a single airplane, with optical perfection.

What does “optical perfection” imply on this occasion?

It implies that every one of many 18 segments will make up a bit of an ideal mirror, all designed to have the cumulative mild putting it from a distant, noticed goal focus all the way down to a single level exactly within the telescope’s devices. This is a tremendously formidable job, requiring that we compensate for:

  • the spacing in between every section,
  • the sides, and specifically, the sharp corners, of every section,
  • the optical imperfections induced by the trusses that maintain the secondary mirror in entrance of the first mirror,
  • and the person variations each throughout every section and from segment-to-segment.

This picture reveals the scale and scale of every of the 18 main mirror segments which might be put in on the James Webb Space Telescope, with a human for comparability. In house, simply ~4 millimeters of house separates every of the hexagonal segments from each other, however 4 millimeters is big in comparison with the ~20 nanometer parabolic form that your complete airplane traced by all 18 mirrors should occupy.

(Credit: NASA/MSFC/David Higginbotham/Emmett Given)

Each one of many particular person segments themselves, again at varied factors within the twentieth century, would have made a spectacular, cutting-edge observatory all by itself. If you wish to focus your mild correctly, it’s essential keep away from the issue that originally plagued the Hubble Space Telescope: spherical aberration. Spherical shapes, for each lenses and mirrors, are simpler to fabricate than different curves, like parabolas, and the shapes of enormous lenses and mirrors might be distorted by gravity.

As a consequence, when Webb, which was constructed on Earth however operates in house, was launched, there was a fear that every little thing won’t fall into place correctly. Even although the mirrors had been checked and re-checked and re-re-checked time and again, there was all the time the concern that one thing can be off with the optics. If that was the case, then the mirrors can be unable to focus the distant starlight into the only picture that was desired, and we’d have to search out some method to compensate for the blurriness that will come up.

A simulation of spherical aberration in an optical system with a round, unobstructed aperture admitting a monochromatic level supply. You can see the intense distinction between mild that’s centered by not fairly sufficient (prime), mild that’s centered an excessive amount of (backside), and lightweight that’s centered optimally (middle). Hubble’s main mirror had issues with spherical aberration; Webb’s mirrors don’t.

(Credit: Mdf at English Wikipedia)

For the 18 mirror segments that James Webb possesses, there are three particular person designs that wanted to be manufactured six occasions every:

  1. the “A” segments, that are for the inside segments, the place 5 of the six hexagonal edges will border one other mirror section, however the innermost one will depart a spot for mild to be mirrored inside, onto the devices,
  2. the “B” segments, that are on the outdoors corners of the hexagon-shaped honeycomb, every have three edges that border one other mirror section, however three edges that comprise the surface border of the first mirror,
  3. and the “C” segments, which go between the “B” segments and possess 4 edges that border one other mirror section, however two edges that, together with the “B” segments, outline the surface border of the first mirror.

As a consequence, the form of the James Webb Space Telescope’s main mirror makes up a form often called a tricontagon, or a 30-sided polygon. This is a really, very sophisticated geometrical form to cope with, and the technical achievements wanted to provide a high quality knowledge product are actually astronomical.

This picture reveals the 18 particular person segments that make up James Webb’s main mirror, and the three impartial units of mirrors, labeled with letters A, B, and C and numbers 1-6, that correspond to the put in place of every mirror on the at the moment deployed telescope.

(Credit: NASA/James Webb Space Telescope group)

In its unfocused state, the James Webb Space Telescope would merely be made up of 18 particular person mirrors, every with their very own form, their very own airplane of focus, and each would produce their very own picture for no matter object it was that we had been making an attempt to watch.

The aim is to have every of those 18 segments kind a single airplane, collectively, that has a parabolic form. At some 6.5 meters (round 21 toes) throughout, the variations within the airplane, each throughout every section and from segment-to-segment, needs to be proper round ~20 nanometers for optimum efficiency. That’s an unimaginable precision, by the best way; if the floor of the Earth had been as clean, over the Earth’s total dimension, as Webb’s precision must be for its optics, then the very best mountain and the deepest ocean trench would solely depart from sea stage by about 2 centimeters (lower than one inch), whole.

When Webb took its very first picture of a star, making an attempt to see what kind of picture the 18 segments produced, it was clear that lots of work remained forward of the group.

james webb spikes

This picture mosaic was created by pointing the telescope at a shiny, remoted star within the constellation Ursa Major often called HD 84406. This star was chosen particularly as a result of it’s simply identifiable and never crowded by different stars of comparable brightness, which helps to scale back background confusion. Each dot throughout the mosaic is labeled by the corresponding main mirror section that captured it. These preliminary outcomes carefully match expectations and simulations.

(Credit: NASA)

You’ll discover, while you view the preliminary (above) picture, you see 18 completely different sources of sunshine: one corresponding to every section of the first mirror. You’ll additionally discover that these sources seem everywhere, fairly than within the desired “honeycomb” configuration that the mirror segments themselves tackle. Finally, you’ll discover that every one among these sources doesn’t correspond to a single point-like supply, which is what you’d count on for a star, however that every one winds up distorted and unfold out over a quantity of house.

When you are taking a real level supply of sunshine and picture it by any form of optical system, you’re not going to get a “point” again once more. Instead, you’ll get a form distinctive to your tools, and that may be described by a mathematical equation often called a point-spread perform. We can know that we’re taking a look at a star, and that the star ought to seem as a single level of sunshine, however that’s not what we see. For James Webb, with its 18 hexagonal segments configured to make a tricontagon general, it ends in an extremely sophisticated point-spread perform that astronomers engaged on the telescope merely name, “the nightmare snowflake.”

Even with the substantial progress that was made to focus and align the person mirror segments right into a single airplane, after which to mix these 18 particular person pictures right into a single one which greatest represents the true level supply we’re observing, the “nightmare snowflake” clearly rears its ugly head.

This three-panel animation reveals the distinction between 18 unaligned particular person pictures, those self same pictures after every section had been higher configured, after which the ultimate picture the place the person pictures from all 18 mirror had been stacked and co-added collectively. The sample made by that star, often called the “nightmare snowflake,” might be improved upon with higher calibration.

(Credits: NASA/STScI, compiled by E. Siegel)

The particular person star you see on the finish, above, represents what occurs when all the mirrors are centered and phased collectively. But in all places you’ve got an edge, a spot, or one thing that blocks a portion of the sunshine from coming into your main mirror, you’re going to get a picture artifact, and that’s one thing we should be capable of efficiently appropriate for.

That includes tweaking the form of every particular person mirror by the usage of actuators, and doing so in a wavelength-dependent manner. It includes guaranteeing that every particular person mirror not solely makes its personal completely parabolic form, however that the shapes between mirrors all correspond to a unique portion of the identical parabola. And, even with gaps which might be roughly ~4 millimeters between the person mirror segments, that one single parabola must be excellent all the way down to a tolerance of ~20 nanometers.

Even at that, there are nonetheless the sides to reckon with, and the truth that there are helps for holding the secondary mirror in place that cross the airplane of the first mirror.

This new “selfie” was created utilizing a specialised pupil imaging lens within the NIRCam instrument that was designed to take pictures of the first mirror segments as an alternative of pictures of the sky. This configuration shouldn’t be used throughout scientific operations and is used strictly for engineering and alignment functions. Note the presence of the three struts that maintain the secondary mirror in place.

(Credit: NASA/STScI)

In the case of James Webb, particularly, there are three axes that the helps relaxation on, and so they trigger three units of inevitable spikes that may all the time seem in pictures of level sources, akin to stars.

This isn’t distinctive to the James Webb Space Telescope, by the best way. Any reflecting telescope the place the sunshine is mirrored again in entrance of the telescope and right into a secondary mirror, the place the secondary mirror then displays the sunshine right into a “hole” positioned within the main mirror itself (after which, into the devices), wants that secondary mirror held in place in entrance of the first mirror by one thing.

However these helps are configured then determines the form of the spikes that you just’ll see, with every distinctive assist creating its personal diffraction spikes which might be perpendicular to that assist construction itself.

James Webb spike

Comparison of diffraction spikes for varied strut preparations of a reflecting telescope. The interior circle represents the secondary mirror, whereas the outer circle represents the first, with the “spike” sample proven beneath. Webb may have a configuration most much like the rightmost illustration; Hubble’s was extra just like the next-to-last one.

(Credit: Cmglee/Wikimedia Commons)

The Hubble Space Telescope, whose pictures are arguably essentially the most ubiquitous and recognizable of all telescopes in human historical past, is configured in a typical manner for a reflecting telescope: with helps for the secondary mirror formed just like the “+” signal. Its perpendicular helps be certain that there shall be heavy diffraction spikes making a “+” form coming off of each supply that qualifies as point-like: the person stars it might probably see.

Other sources, nonetheless, akin to distant galaxies and nebulae, are what we all know of as prolonged sources, as their mild is unfold out over a bigger space on the sky. As a consequence, these spikes are non-existent, since mild arrives from greater than only a level, and that optical impact is successfully washed out over the massive angular space that the prolonged object supplies. In the (Hubble) picture under, for instance, you’ll be able to simply establish the factors of sunshine which might be stars contained inside our personal Milky Way by their diffraction spikes, whereas the fainter, extra distant, prolonged objects definitively don’t possess them.

Hickson compact group 31, as imaged by Hubble, is a spectacular “constellation”, however virtually as outstanding are the few stars from our personal galaxy seen, famous by the diffraction spikes. When considered by James Webb, the celebrities will possess six, not 4, diffraction spikes. In just one case, that of the GMT, will these spikes be absent.

(Credit: NASA, ESA, J. English (U. Manitoba), and the Hubble Heritage Team (STScI/AURA); Acknowledgement: S. Gallagher (U. Western Ontario))

As the fantastic phasing of the James Webb Space Telescope continues, we’ll proceed to see the “nightmare snowflake” evolve nearer and nearer in direction of its desired form: of merely six spikes coming off of each star, and of the extra distant, prolonged objects trying an increasing number of pristine.

On March 11, 2022, the James Webb Space Telescope group at NASA produced what’s been labeled as a Telescope Alignment Evaluation Image, and the outcomes are instantly obvious as spectacular. Instead of a snowflake with all kinds of artifacts emanating from it, and as an alternative of an prolonged level the place the sunshine is clearly unfold out over a big space, as an alternative the star itself seems to be crisp, collimated, and has six main diffraction spikes within the anticipated instructions. Taken solely with the NIRCam instrument, it’s already well-enough aligned to disclose background stars and galaxies, with lots of the background stars displaying their very own diffraction spikes as effectively.

James webb spikes

While the aim of this picture was to give attention to the brilliant star on the middle for alignment analysis, Webb’s optics and NIRCam are so delicate that the galaxies and stars seen within the background present up. At this stage of Webb’s mirror alignment, often called “fantastic phasing,” every of the first mirror segments have been adjusted to provide one unified picture of the identical star.

(Credit: NASA/STScI)

This isn’t, nonetheless, “as good as it gets” for James Webb. If you have a look at this with a really cautious eye and take note of particulars, you’ll discover many issues about this star and the rays that come off of it. For instance:

  • Each of the six main diffraction spikes has a set of maybe five-to-seven main streaks, fairly than all being aligned right into a single spike.
  • Between the spikes, there are smaller rays of sunshine that come off and stop us from viewing objects which might be too near the luminous star.
  • In the horizontal airplane, the place there needs to be no diffraction spikes, we’ve an additional, fainter, however nonetheless substantial set of spikes: a seventh and eighth spike, each of which needs to be eradicated.
  • And that in case you look at the opposite stars or galaxies revealed within the picture, you’ll be able to see that they’re not pristine both, however fairly have distortions according to Webb not but being optimally aligned and configured.

In the picture under, I’ve highlighted a few of the particulars that the group will work to enhance over the approaching months. Even although progress has been spectacular over the primary three months of Webb’s commissioning, try to be heartened to know that there are nonetheless months forward of it earlier than science operations start, giving the group a possibility to iron out as many of those particulars as attainable earlier than we begin utilizing the observatory’s capabilities to show us as a lot as attainable concerning the Universe.

This close-up of the earlier picture, launched by NASA on March 16, reveals particulars within the central diffraction spike and between them, in addition to distorted particulars in a few of the fainter background objects which were revealed. There continues to be room for enchancment, however progress to this point has been spectacular.

(Credit: NASA/STScI; annotations by E. Siegel)

However, you shouldn’t assume that each telescope, and even each reflecting telescope, will all the time be caught with this “diffraction spike” downside. Right now, on the primary units of pictures we’re seeing from James Webb, there are lots of extra spikes and options than we should always see when calibration is full. At that time, there needs to be solely the six main spikes and nothing else; the extra options needs to be completely minimized. The solely cause a star ought to seem bigger than a single level, excepting the spikes, needs to be if it’s shiny sufficient to saturate the detector itself.

Moreover, there’s already a world-class telescope underneath development that needs to be the primary of its form to provide pictures with none diffraction spikes. The Giant Magellan Telescope, slated for completion maybe in direction of the top of the 2020s, goes to be roughly 25 meters in diameter, making it the second-largest optical telescope on the planet behind the (additionally underneath development) 39 meter European Extremely Large Telescope. But in contrast to its bigger counterpart, which, like Webb, shall be made up of enormous numbers of hexagonal segments tiled collectively, the Giant Magellan Telescope will solely be made up of seven giant, round mirrors, all put in upon the identical telescope mount.

The 25-meter Giant Magellan Telescope is at the moment underneath development, and would be the best new ground-based observatory on Earth. The spidar arms, seen holding the secondary mirror in place, are specifically designed in order that their line-of-sight falls instantly between the slender gaps within the GMT mirrors.

(Credit: Giant Magellan Telescope/GMTO Corporation)

As a results of the Giant Magellan Telescope’s distinctive configuration, the three assist struts that may maintain the secondary mirrors in place will exist within the gaps between the first mirror segments; they won’t impede the sunshine that reaches and displays off of the telescope mirrors in any respect! Although there shall be different picture artifacts that come up, specifically a set of round beads that seem alongside ring-like paths (Airy rings), merely observing the identical object for about quarter-hour or longer will fill these beads in, creating our first cutting-edge pictures of stars utilizing a reflecting telescope with none diffraction spikes in any respect.

The six spikes coming off of James Webb’s greatest configuration picture to this point will enhance and slender with time, and the remaining spikes and picture artifacts needs to be improved upon within the coming months. Although Webb has typically been referred to as the successor to Hubble, it will likely be observing primarily within the infrared: the identical units of wavelengths beforehand considered by one other six-spiked space observatory: NASA’s Spitzer. Sure, Webb will all the time possess these spikes, however because of intelligent engineering, there shall be different telescopes that gained’t have them in any respect. We’re going to be getting not solely a complete new view of the Universe, however a wholly novel expertise in visualizing it. With every passing day, the potential of science and discovery with James Webb solely will get an increasing number of thrilling.

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Ethan Siegel

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