Fullscreen HTML5Med Res (??MB)HTML4

BY D R DAVID MADDISON Image source: www.flickr.com/photos/nasawebbtelescope/37988427785/ The James Webb Space Telescope (JWST) is the newest and most advanced space telescope. Launched on December 25th, 2021 on an Ariane 5 rocket from French Guiana, it officially entered service on July 12th, 2022. While much has been said and written about it in the press, this article will concentrate on the amazing technology behind it. T he James Webb Space Telescope has the largest mirror of any telescope launched into space. It can see ‘back in time’ right up to the time of the first star and galaxy formation after the Big Bang (the presumed beginning of the universe). It also has more light gathering ability than any other space telescope, allowing it to see very faint objects. It can see in the infrared, meaning it can image objects that are not visible using the visible light spectrum. The JWST mission objectives are to explore the early universe, examine the evolution of galaxies over time, examine the star life cycle and look for and examine other planets. That includes our own minor planets, 14 Silicon Chip Kuiper Belt objects and the suspected Planet Nine in our solar system. The JWST project is led by NASA in collaboration with the European Space Agency (ESA) and the Canadian Space Agency (CSA). Their academic and industry partners include Who was James Webb? The telescope was named after James E. Webb, NASA’s second director, from 1961 to 1968. He oversaw the Mercury, Gemini and Apollo missions with a total of 75 launches. Image source: NASA Australia's electronics magazine the University of Arizona, Ball Aerospace, L3Harris Technologies, Lockheed Martin, Northrop Grumman and The Space Telescope Science Institute (see https://jwst.nasa.gov/content/ meetTheTeam/team.html). Design started in 1996, and in 1999, there was an expectation of a US$1 billion cost and a 2007 launch. The JWST has cost NASA US$9.7 billion ($14.1 billion) over the last 24 years, the ESA €700 million (A$1.02 billion) and the CSA C$200 million (A$224 million). JWST vs Hubble The JWST is a successor to the Hubble Space Telescope (HST), which entered service on May 20th, 1990 and siliconchip.com.au is still in operation today. However, the JWST does not replace the HST. Hubble has an uncertain remaining lifetime partly because NASA’s Space Shuttle fleet was retired in 2011, so there is no longer any way to service it. Despite that, it will be kept in service as long as possible. A fundamental difference between the JWST and the HST is the size of the primary mirror. The HST has a 2.4m diameter mirror while JWST’s is 6.5m in diameter (see Fig.1). They are also designed to image different light wavelength ranges (Fig.2). The HST has an effective light gathering area of 4m2 and the JWST 25m2, so the JWST has 6.25 times the light gathering capability of the HST. The HST was designed for the visible and ultraviolet part of the light spectrum, plus some infrared, while the JWST is designed to work mainly in the infrared. All objects with a temperature above absolute zero (-273°C) emit infrared radiation, making them visible to JWST as long as they give off enough infrared light. Specifically, HST images wavelengths of 100nm to 800nm with some parts of the infrared spectrum from 0.8μm (800nm) to 2.5μm, while the JWST images from 0.6μm to 28μm. The infrared spectrum extends from 0.75μm to a few hundred microns, so the JWST works mainly in that area with a small capability in the visible range from 600-750nm (orange is 590 to 620nm and red is 620 to 750nm). As infrared radiation comes from all objects, it is essential to keep the JWST as cool as possible. Hence its vast multi-layer sunshield, its remote orbit away from the Earth and the Moon and onboard cooling systems. It must be kept below -223°C to keep it from interfering with itself from self-emission of infrared. The electronics onboard operate at higher temperatures than that, though. JWST can detect objects 100 times fainter than the HST. JWST can also see objects as old as 180 million years after the Big Bang, compared to 400 million years for HST. Physical structure The JWST consists of four major sections (see Fig.3): 1. The spacecraft bus, which is like a chassis but also houses the following subsystems: • Electrical Power Subsystem siliconchip.com.au Fig.1: a size comparison of the Hubble and JWST primary mirrors. Source: https://jwst.nasa.gov/ content/observatory/ote/mirrors/ Fig.2: a comparison of the light spectrum coverage of the HST & JWST. Source: www.nasa.gov/content/goddard/hubble-vs-webb-on-the-shoulders-of-a-giant THE JAMES WEBB SPACE TELESCOPE Science Instrument Module (ISIM) Houses all of Webb's cameras and science instruments Primary Mirror 18 hexagonal segments made of the metal beryllium and coated with gold to capture faint infrared light Optical Telescope Element (OTE) Secondary Mirror Reflects gathered light from the primary mirror into the science instruments Trim Flap Helps stabilise the satellite Multilayer Sunshield Five layers shield the observatory from the light and heat of the Sun and Earth Solar Power Array Always facing the Sun, panels convert sunlight into electricity to power the observatory Earth-pointing Antenna Sends science data back to Earth and receives commands from NASA's Deep Space Network Star Trackers Small telescopes that use star patterns to target the observatory Spacecraft Bus Contains most of the spacecraft steering and control machinery, including the computer and reaction wheels Australia's electronics magazine Fig.3: the crucial parts of the JWST. Note how the telemetry components, which must face the Earth, are on the opposite side of the sun shade from the telescope. Source: www.nasa.gov/ mission_pages/webb/observatory/ December 2022  15 • Attitude Control Subsystem • Communication Subsystem • Command and Data Handling Subsystem • Propulsion Subsystem • Thermal Control Subsystem 2. The optical telescope element (OTE), comprising the various mirrors. 3. The Integrated Science Instrument Module (ISIM), containing the cameras and other instruments such as NIRCam, NIRSpec, NIRISS and MIRI. 4. The Sunshield. Size and weight Fig.4: the JWST primary mirror during assembly. The left & right sides are folded to fit inside the rocket. Source: https://jwst.nasa.gov/content/ observatory/ote/mirrors/ Fig.5: a mock-up of the JWST at Goddard Space Flight Center in Maryland, USA. Source: www.flickr.com/photos/ nasawebbtelescope/8518326611 Actuator JWST Primary Mirror Segment Strut When the center actuator moves up or down, it pulls or pushes on the six struts, which in turn correctly curves the mirror. The actuators are tiny mechanical motors that move the mirrors into proper alignment and curvature with each other. Each mirror has seven actuators – six at the hexapod ends and one in the center. Hexapod Beryllium Substrate Beryllium was chosen for the mirror's “skeleton” because it is strong and light, and will hold its shape in the extreme cold of space. The substrate was machined in a honeycomb pattern to remove excess material and thus decrease its weight, yet maintain its strength. When the actuators at the hexapod ends pull or push on the hexapod, it pulls or pushes the mirror into correct alignment with the other mirrors. Electronics Box Every mirror segment has one electronics box. This box sends signals to the actuators to steer, position and control the mirrors. The electronics boxes are located within the backplane – the structure that holds all the mirrors. Fig.6: the structure of a mirror segment, showing the six mirror actuators plus the central one to control its curvature. Three beryllium ‘whiffles’ are located between the hexapod and substrate, measuring 60cm long by 30cm wide, helping to spread the load. Source: https://jwst.nasa.gov/content/observatory/ote/mirrors/ 16 Silicon Chip Australia's electronics magazine According to the ESA, the launch mass of the observatory was 6200kg, including the observatory, on-orbit consumables and launch vehicle adaptor. Its overall height is about 8m. The 705kg mirror is 6.5m in diameter and the focal length of the telescope optics is 131.4m. The mirror The JWST mirror and the rest of the spacecraft were far larger than could be accommodated by the Ariane 5 launch vehicle, so it had to be folded for launch, as partially shown in Fig.4. This was particularly challenging for the mirror, given the high level of precision required. The mirror comprises 18 hexagonal gold-coated beryllium metal segments (Fig.6), each weighing about 20.1kg and 1.32m across, with a total diameter of 6.6m and a total area of 25m2. Each mirror segment forms a primary mirror segment assembly (PMSA), weighing 39.48kg with actuators and other accessories. 48g of gold is used to coat the mirror, about the volume of a marble and the mass of a golf ball. Gold is used because it is highly reflective in the infrared. The primary mirror segments each have six actuators to adjust their alignment, as does the secondary mirror. Primary mirrors also have a central actuator to adjust the mirror curvature. Each segment had to be aligned with an accuracy of 7nm or one ten-­ thousandth the thickness of a human hair. The actuators can move to positions as accurate as 1nm or one-millionth of a millimetre. In use, the mirrors are realigned every 10 to 14 days. There are a total of 132 actuators, including 126 for the primary mirror. The mirrors are ground to a mean surface siliconchip.com.au accuracy of better than 25 nanometres. Diffraction spikes Most images of stars make them look like a point of light or a disc with four or more radial spikes in a specific pattern. These spikes are called diffraction spikes – see Fig.7. They are a common phenomenon in reflector telescopes (like the JWST) and are partly related to the support vanes of the secondary mirror. They are also common in any camera or telescope aperture that is non-circular, including the iris diaphragm of a traditional camera. In the case of the JWST, they also derive from the fact that the primary mirror is not circular. They occur because light interacts and diffracts around the edges of the aforementioned structures. So the JWST has two sources of diffraction spikes. These are designed so that they do not overlap with each other and remain as narrow as possible. Fig.8 is a comparison of the diffraction spikes between the HST and JWST. Fig.7: the contribution and shape of diffraction spikes from the combination of the JWST struts and mirror shape. Source: https://webbtelescope.org/contents/ media/images/01G529MX46J7AFK61GAMSHKSSN Sunshield Apart from the mirror, the sunshield is the most prominent feature with a deployed size of 21.2m x 14.2m or about the size of a tennis court – see Figs.5 & 9. It shields the telescope from heat and light from the Sun, Earth and Moon. It is made of thin aluminium and doped-silicon coated plastic called Kapton E, with five separate layers each 0.025mm thick. ‘Rip stop’ structures are built into the shield material to prevent a tear catastrophically propagating through an entire layer. You can check the sunshield and instrument temperatures at the website: siliconchip.au/link/abgu At the time of writing, the layer on the sun side has a temperature of 13°C and 50°C (measured at two locations), while the innermost layer has temperatures between -231°C and -236°C. The five instruments were at temperatures from -235°C to -267°C. Fig.8: a comparison of the refractive image spikes between the HST and JWST. Scientific instruments The Integrated Science Instrument Module (ISIM) is behind the main mirror and holds the four scientific instruments plus the Fine Guidance Sensor, a camera for aligning the observatory. It also has power supplies, computers and instrument cooling – see Fig.10. siliconchip.com.au Fig.9: the JWST’s sunshield comprises five layers of Kapton film. Source: https://jwst.nasa.gov/content/ observatory/sunshield.html Australia's electronics magazine Fig.10: the ISIM compartment. Source: www.flickr.com/photos/ nasawebbtelescope/30785200072/ December 2022  17 ► Fig.11: the NIRCam configuration. Source: www. astro.princeton.edu/~jgreene/ast303/NIRCampocket-guide.pdf Fig.12: the NIRCam instrument (near-infrared camera). Source: www.flickr.com/photos/ nasawebbtelescope/albums/72157627248683106 The four scientific instruments are the Near-Infrared Camera (NIRCam), Near-Infrared Spectrograph (NIRSpec), Near-infrared Imager and Slitless Spectrograph (NIRISS) and MidInfrared Instrument (MIRI) As their names imply, these all work in the infrared. See Fig.13 for their specific wavelength ranges. We will go over each instrument in detail: NIRCam (0.6-5μm wavelength range, Figs.11 & 12) is an infrared camera that has 10 mercury-cadmium-­ telluride (HgCdTe) detector arrays, each with four megapixels (4MP; 2048 × 2048 pixels). It is also used for “wavefront sensing” to align the mirror segments. In addition, it has coronographs to block light from stars with associated exoplanets (planets outside our solar system). It operates at -236°C while its electronics operate at 17°C. NIRSpec (0.6-5.3μm, Figs.16 & 18) is a spectrometer that can be used to analyse the chemical composition of objects. It has several operating modes, including the ability to take spectra of 100 objects simultaneously. The instrument runs at -235°C. Fig.14: there are four arrays, each containing 62,000 shutters (measuring 0.1 × 0.2mm). Source: https://jwst.nasa.gov/content/about/ innovations/microshutters.html Multiple simultaneous spectra are taken with the aid of 248,000 micro shutters (see Fig.14). They can be individually opened or closed to allow light from the objects of interest through to the spectrometer via gratings and a prism to split up the light into its component wavelengths – see https://w.wiki/5hex NIRISS (0.6-5um, Fig.17) is used for imaging and spectroscopy. It is combined with the Fine Guidance Sensor (FGS) used to guide the telescope. The FGS (Fig.19) finds pre-selected guide stars from a database and uses those for guidance. Together, the instrument is known as the FGS-NIRISS (Fig.15); they are optically separate but contained within one assembly. NIRISS was built by the Canadian Space Agency. The detector for NIRISS is a 2048 × 2048 pixel (4MP) HgCdTe array with 18 × 18μm pixels. NIRISS is used for near-infrared imaging, wide-field slitless spectroscopy, single object slitless spectroscopy and aperture masking interferometry. Fig.13: the JWST instrument light detection wavelength ranges, mainly in the infrared part of the spectrum. Source: www.nasa.gov/mission_pages/webb/ news/geithner-qa.html 18 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.16: a schematic view of the NIRSpec instrument. Source: https://w. wiki/5gyc Fig.15: a photograph of the completed FGS-NIRISS assembly. Source: John Brebner, Communications Research Centre Spectroscopy enables the chemical composition and physical structure of distant objects to be determined from their emission spectra. Slitless spectroscopy is used in sparsely-populated star fields to determine the spectrum of many objects at once. Aperture masking interferometry is used to resolve closely spaced objects such as a binary star system. MIRI (4.9-28.8μm, Fig.20 overleaf) is a camera and imaging spectrometer. It can see longer wavelength infrared light than the other instruments and thus needs to be kept much colder. It operates at -267°C and has its own ‘cryocooler’ cooling system, shown in Fig.21 (also overleaf). The cooling system is spread over several regions of the ISIM, all of which are at different temperatures. Like NIRCam (0.6-5μm), it has four coronagraphs to block starlight when ► Fig.18: a photograph of the NIRSpec instrument. Source: https://w. wiki/5gyd Pick-Off Mirror Kinematic Mounts (3 pairs) Collimator Pupil / Filter Wheel Fig.17: a schematic view of the NIRISS instrument (Nearinfrared Imager and Slitless Spectrograph). Source: https://w. wiki/5gye Camera Detector Guider POM Guider Relay TMA Fig.19: the FGS optical assembly; this is the other side of the NIRISS assembly shown in Fig.17. Source: https://w.wiki/5gyf Guider ICP-1 Optical Alignment Cuber (1 / 2) Fine Focus Mechanism with fold mirror Guider SIDECar ASICs (2) Detector Assembly with 2 FPAs Kinematic Mounts (2 of 3) siliconchip.com.au Australia's electronics magazine December 2022  19 Fig.20: a schematic view of the MIRI (Mid Infrared Instrument). Source: https://w.wiki/5gyg ► Table 1 – JWST instruments and their detectors NIRCam HgCdTe H2RG (0.6-2.5μm) 8 HgCdTe H2RG (0.6-5μm) 2 NIRSpec FGS/NIRISS 2 3 Arsenic doped silicon (5-2.8μm) MIRI 3 Shows the number of different types of IR detectors used in each instrument. observing exoplanets. It uses arsenic-­ doped silicon arrays as its infrared sensors. Infrared detectors The infrared detectors are essential for the operation of the scientific instruments described above. There are two types, 4MP HgCdTe arrays for the 0.6-5μm ‘near-infrared’ and arsenic-­ doped silicon (Si:As) detectors of about 1MP for the 5-28μm ‘mid-­ infrared’ wavelength range. These are all extremely sensitive as they must detect incredibly faint light. HgCdTe sensors can be tuned to the wavelength range of interest by adjusting the Hg-to-Cd ratio; two different compositions are used, one for 0.6-2.5μm and the other for 0.6-5μm. See Table 1 for the specific detectors used in each instrument. The basic layout of one of the detectors is shown in Fig.22, while an actual detector is shown in Fig.23. There is an HgCdTe or Si:As absorber layer on a silicon readout chip. When a light photon strikes the absorber, one or more electron-hole pairs are created. The electrons and holes move under the influence of an electric field and can be sensed by the readout circuitry. Folding the observatory With such a large mirror and sunshield, the spacecraft could not fit in any rocket and so needed to be folded. That is why the mirror has multiple segments. The sunshield, mirror, solar panels and antenna were all folded – see Figs.24 & 25. According to Mike Menzel of NASA, the unfolding process involved hundreds of possible “single points of failure”. JWST has 344 known possible single-points-of-failure, about 80% related to the unfolding process. There were 144 release mechanisms for the unfolding process, all of which had to work perfectly. Naturally, all such mechanisms got special attention during design, assembly and testing to ensure they would work. There were also contingency plans for any deployment failure that might have occurred, some as simple as re-­sending a command. The most important thing that had to work first was the solar array deployment. For a video of the unfolding (deployment) sequence, see the video titled “James Webb Space Telescope Deployment Sequence (Nominal)” at https:// youtu.be/RzGLKQ7_KZQ Fig.21: the cooling arrangement for MIRI; it is kept at -267°C or 6K. Source: https://w.wiki/5gyi 20 Silicon Chip Australia's electronics magazine siliconchip.com.au The JWST’s orbit JWST orbits a Lagrange point. These are five special points in space in the Earth-Moon system. At the L4 and L5 Lagrange points, there is an equal gravitational pull from the Earth, Moon and Sun, meaning that (in theory) an object can remain there indefinitely (asteroids are known to accumulate there). The L1, L2 and L3 points are only metastable; for an object to stay there, it must expend minimal fuel for station-­ keeping. Objects can reside there for a long time, but not indefinitely. Unlike most satellites but like some other space telescopes, the JWST orbits in a ‘halo orbit’ around the L2 Lagrange Point (see Fig.26). It is way beyond the orbit of the Moon, 1,500,000km from the Earth. In contrast, Hubble orbits the Earth at an altitude of only 550km. The reason for orbiting L2 is to avoid the heat radiated from the Sun, Earth and Moon, which would swamp its sensitive infrared instruments. JWST can maintain the same orientation, so its sunshield will continue to protect the telescope. JWST’s view will never be blocked by the shadow of the Earth or the Moon, unlike Hubble, which is in Earth’s shadow every 90 minutes. JWST takes six months to complete its halo orbit. In this orbit, JWST is in continuous contact with NASA’s Deep Space Network with stations in Australia, Spain and California. For more on the orbit, see the video titled “Animation: The James Webb Space Telescope’s Orbit” at https:// youtu.be/6cUe4oMk69E Fig.22: a schematic view of an infrared detector sensor used in several JWST instruments. Source: https://jwst.nasa.gov/content/about/ innovations/infrared.html Fig.23: an infrared detector as used in the NIRCam instrument. Light is collected on the mauve HgCdTe film. Source: https://jwst.nasa.gov/content/ about/innovations/infrared.html Fig.24: this shows how the JWST was folded inside the fairing of the Ariane 5 launch vehicle. Source: https://jwst.nasa.gov/content/about/launch.html# postLaunchDeployment Comparing images from JWST and HST ► siliconchip.com.au ► The ‘Deep Field’ images shown in Figs.27 & 28 (overleaf) are the same area of space taken using the JWST and HST. In astronomy, Deep Field means a very long exposure. The JWST Deep Field image was its first, taken with its Near-Infrared Camera (NIRCam) using several wavelengths and a 12.5 hour exposure time. It shows the galaxy cluster SMACS 0723 which, due to its great mass, acts as a ‘gravitational lens’, distorting light from galaxies behind it into a circular pattern. The HST image needed to be exposed for weeks. Despite that, it shows much less detail due to its smaller mirror and inability to image objects in the infrared. Australia's electronics magazine Fig.25: the deployment sequence of JWST. LV is launch vehicle, UPS is Unitized Pallet Structure, PMBA is Primary Mirror Backplane Assembly and SMSS is Secondary Mirror Support Structure. Source: https://w.wiki/5gyj Fig.26: this shows the Lagrange points around the Earth-Sun-Moon system and the location of the JWST in a halo orbit around L2. Source: https://jwst. nasa.gov/content/about/orbit.html December 2022  21 Fig.27: the first and iconic Deep Field image from the JWST. Source: www. nasa.gov/image-feature/goddard/2022/ nasa-s-webb-delivers-deepestinfrared-image-of-universe-yet Fig.28: a Deep Field image from the HST taken in 2017 of the same area shown in Fig.27. Source: https:// archive.stsci.edu/prepds/relics/color_ images/smacs0723-73.html The area of sky covered by these images is equivalent to a grain of sand held at arm’s length. smallness. The known laws of physics cannot describe the singularity but do more-or-less apply for periods starting 10-43 seconds after the Big Bang. It is important to realise that it was not an explosion in the conventional sense, but a sudden expansion of the very fabric of space-time itself for reasons not fully understood. It might have been due to some sort of quantum fluctuation. Light and objects cannot travel faster than the speed of light, but space itself expanded much faster than the speed of light during the early ‘inflationary’ Looking back in time One objective of the JWST mission is to ‘look back in time’ at the early universe. What does that mean? To understand, we first have to consider the beginning of the universe. According to accepted theories of cosmology, the universe started in a ‘Big Bang’; it came into being suddenly from a ‘singularity’ of infinite temperature and density and infinite Waves Imprint Characteristic Polarization Signals Density Waves Earliest Time Visible with Light 0 −32 10 s 1 µs Cosmic Microwave Background Nuclear Fusion Ends Nuclear Fusion Begins Inflation Big Bang Protons Formed Quantum Fluctuations Radius of the Visible Universe Free Electrons Scatter Light 0.01 s 3 min 380,000 yrs Redshift Just as a vehicle-mounted siren appears to rise in frequency as it approaches and falls in frequency as it moves away, so too does light. A light source such as a star or galaxy moving away from us shifts toward a lower frequency which is also a longer wavelength, pushing it toward the red end of the spectrum. This is called redshift. The opposite, blueshift, occurs for objects moving towards us. In 1929, Edwin Hubble discovered that all galaxies were moving away from us and each other, ie, the Modern Universe { Neutral Hydrogen Forms Inflation Generates Two Types of Waves History of the Universe Gravitational Waves phase of the Big Bang, before 10-32 seconds had elapsed and where the early universe grew to enormous size in an unimaginably tiny fraction of a second, going through several phases as shown in Fig.29. Because of the ongoing inflation of the universe, objects can be more light years away than the universe’s age. We can currently look as far back in time as the cosmic microwave background 380,000 years after the Big Bang (but not with the JWST, as explained below). In future, it may be possible to look back in time even further than that by detecting so-called primordial gravitational waves, which current detectors cannot sense (see our article on Gravitational Waves in the October 2021 issue – siliconchip.au/ Article/15063). There are also density waves, like shock waves, which correspond to the regions of differing matter density in the universe that led to the formation of stars and galaxies. A consequence of the Big Bang is that all parts of the universe are moving away from each other, like dots painted on the surface of a balloon as it is inflated. We see these objects as they were long ago, not as they are now, because of the time it takes light to travel to us. 13.8 Billion yrs Age of the Universe Fig.29: a timeline of events during the universe’s formation, showing how the radius of the universe is thought to have changed with time. Note the gravitational and density waves. Source: https://w.wiki/5gyk (CC BY-SA 3.0). 22 Silicon Chip Australia's electronics magazine siliconchip.com.au universe was expanding. He saw that the redshift of fainter and presumably more distant galaxies was greater than brighter, closer galaxies. Hence, he concluded that the more distant the galaxy, the faster it is receding and that the universe must be expanding. The rate at which the universe is expanding is determined by the Hubble constant, which is about 65km/s for every three million light years an object is away from us. One light year is the distance light travels in a year, about 9,461,000,000,000,000km. It was also concluded that the higher the redshift of a galaxy (the same as saying the more distant it is), the earlier in its life we see it. In other words, we see it the way light first left the object millions or billions of years ago. The object might not even exist now, but we wouldn’t know that and would have to wait millions or billions of years to find out. The redshift can be so far toward and beyond the red end of the spectrum that it is beyond the visible light spectrum, ie, in the infrared. We can tell how far the light spectrum has been redshifted by reference to specific markers within the spectra corresponding to known molecular and atomic absorption lines. These characteristic spectral patterns correspond to specific elements or compounds – see Fig.30. In extreme cases, ie, the most distant objects, the entire spectrum becomes invisible as it has entirely shifted into the infrared. The Big Bang happened 13.8 billion years ago, and the first stars are now believed to have formed 100 million years after the Big Bang and the first galaxies about one billion years after the Big Bang. The JWST seeks to detect some of the very first stars and galaxies. Redshift is denoted by the letter z, corresponding to the fractional change Fig.30: two example spectra with absorption lines; our Sun below and a supercluster of distant galaxies above. The upper absorption lines are all shifted towards the red end of the spectrum due to redshift as the cluster is moving away from us rapidly. Source: www.ctaobservatory.org/redshiftwhy-does-distance-matterto-cta/ siliconchip.com.au Why doesn’t JWST have ‘selfie’ cameras? The JWST doesn’t have any cameras for viewing itself because they would be an unnecessary source of unwanted heat. Heat would be conducted along the connecting wires and struts even if they were turned off. It was a matter that the designers did carefully consider. Also, onboard sensors can detect most malfunctions. The telescope does have a limited capability to take a selfie of the primary mirror. A ‘selfie’ image of the primary mirror of the JWST taken during initial mirror alignment procedures. Source: https://blogs.nasa.gov/ webb/2022/02/11/photons-receivedwebb-sees-its-first-star-18-times/ in wavelength. For example, if light were emitted at 120nm (nanometres) and observed at 150nm, the redshift factor z would be 0.25 (150 ÷ 120 − 1). While the HST can see objects no further back than 400 million years after the Big Bang (redshift of z ≈ 11.1), JWST can detect objects even earlier at 180 million years after the Big Bang (redshift z ≈ 20). The earliest stars are now thought to be from 100 to 180 million years after the Big Bang (redshift of z ≈ 30 to z ≈ 20), and the earliest galaxies from 270 million years after the Big Bang (redshift of z ≈ 15). Imaging in the infrared The ability to image in the infrared has several advantages plus some challenges. Important advantages are: 1. Being able to see through dust and gas clouds, as they tend to block visible light but are transparent to IR. 2. Being able to see very distant objects where the redshift causes them to be invisible in the visible light spectrum. 3. Infrared radiation is absorbed in Earth’s atmosphere, so an IR space telescope can see things that are very difficult or impossible to image from the Earth’s surface. 4. Objects such as planets, local asteroids and debris discs around other solar systems being formed emit more strongly in the infrared than in visual wavelengths. One of the most significant challenges is that the telescope has to be kept as cool as possible because all matter radiates in the infrared in proportion to its temperature. The colder something is held, the less infrared radiation emanates from it. We all know that metal objects glow when very hot, but you might not realise that they emit light before being heated; we just can’t see it because it is infrared. If the telescope and its instruments were not kept cool, the instruments would not be able to detect infrared radiation from the universe because they would be swamped by radiation from the telescope itself. The use of infrared telescopes is limited on Earth because water vapour in the atmosphere absorbs infrared radiation. Such telescopes are placed on Can the JWST be seen with other telescopes? Researchers at the Virtual Telescope Project (www.virtualtelescope.eu) managed to image the JWST as a single small dot of light. The JWST imaged with an amateur Planewave 17in (43cm) f/6.8 telescope with a 300s exposure. Source: www.virtualtelescope. eu/2022/01/25/james-webb-spacetelescope-a-new-image-24-jan-2022/ Australia's electronics magazine December 2022  23 Fig.31: our atmosphere almost completely absorbs infrared energy. That is why infrared observations are best made from space. Source: https://w.wiki/5gym high mountain tops with dry environments or are airborne on aircraft or balloons. Regardless, superior infrared observations can be made from space – see Fig.31. Looking back further in time While the JWST looks back in time as far as is possible to see with infrared light, to about 180 million years after the Big Bang, we have looked back further in time using microwaves with the Wilkinson Microwave Anisotropy Probe (WMAP) to about 375,000 years after the Big Bang. This was a time before star and galaxy formation; microwaves were evidence of the “afterglow” of the Big Bang – see Fig.32. The time between 375,000 and 400 million years after the Big Bang is known as the “Cosmic Dark Age”, as there were (previously) believed to be no stars or other light sources to generate light. In fact, the end of the Cosmic Dark Age at 400 million years is now disputed. The JWST has found galaxies younger than that (see below). The most distant galaxy At the time of writing, the most distant and earliest galaxy is believed to be the candidate object named CEERS93316, discovered using the JWST in July 2022 – see Figs.33 & 34. It is believed to have formed just 235.8 million years after the Big Bang. It was previously believed that the first Fig.32: looking back in time with microwaves. The cosmic microwave background was imaged by the Wilkinson Microwave Anisotropy Probe (WMAP) and depicted as the afterglow pattern in this diagram. The JWST sees back in time to the first stars. Source: https://map.gsfc.nasa.gov/media/060915/index.html 24 Silicon Chip Australia's electronics magazine Fig.33: the galaxy CEERS-93316. It mightn’t look like much, but it is the most distant object yet observed by the JWST. Source: www.ed.ac.uk/ news/2022/edinburgh-astronomersfind-most-distant-galaxy galaxies formed 400 million years after the Big Bang. Light from this object has travelled for 13.55 billion years, and the distance to the object is 34.68 billion light years due to the universe’s expansion. The red shift is z ≈ 16.7. Imaging exoplanets JWST will be able to observe certain young, hot planets via a technique called direct imaging as well as other methods. JWST will also be able to detect oxygen and organic molecules in exoplanet atmospheres, which are possible indicators of life. Limited ability for service The JWST was not intended to be serviced. Once its fuel is depleted or there is a major system failure, the mission will be terminated. The minimum planned mission time is five years, so service is not expected to be needed, but compare that to Hubble, which has exceeded its design lifetime by a substantial margin and has been in orbit for 32 years. But the HST was designed to be serviced and was placed in an orbit accessible by the Space Shuttle. In contrast, the JWST is in a very hard-to-access orbit. There is no present way to service the JWST, but there are very limited provisions for a possible manned or robotic servicing operation. Details on that are hard to find. Among these provisions are a refuelling adaptor and, according to Space. com, a docking ring (see their 2007 article at siliconchip.au/link/abgo). Alternatively, the interface ring used to attach the JWST to the Ariane 5 siliconchip.com.au Fig.34: a timeline from the Big Bang to the present. The letter z refers to the amount of redshift. The more redshift, the more distant the object and the older it is. Source: https://w.wiki/5gyn launch vehicle could be used to grapple the spacecraft. The JWST has been engineered with multiple redundant systems so that if one fails, others can take over, minimising the need for servicing. The goal is for a ten-year lifespan, ie, twice the planned mission duration. Ultimately, if there are no significant failures, the fuel supply for station keeping will be the limiting factor. Because of an excellent initial rocket burn and trajectory, it used much less fuel for mid-course correction than expected, and it is hoped that there is enough fuel left for perhaps 20 years of operation. Electrical power The JWST has a solar array to provide 2kW of electrical power. JWST stores power from the array in lithium-­ ion batteries, specifically, Enersys ABSL types in an 8S44P (series and parallel) 28V, ~66Ah configuration. Propulsion & attitude keeping The propulsion system uses thrusters that run on hydrazine fuel (N2H4, 159L tank capacity) with dinitrogen tetroxide oxidiser (N2O4, 79.5L tank capacity). There are four Secondary Combustion Augmented Thrusters (SCATs) in two pairs. One pair was used to propel the JWST into orbit, while the other is for station keeping. It also has eight monopropellant Rocket Engines (MRE-1) – see Fig.35. These use hydrazine decomposition (without oxidiser) and are for attitude control and momentum unloading of the reaction wheels. Slewing and then pointing the telescope in the desired direction is done Fig.35: a schematic view of the JWST propulsion system. “GHe” stands for gaseous helium used to pressurise the fuel tanks. Original source: Hammann, Jeff, JWST Propellant Budget Document, Northrup Grumman, July 19th, 2013 (D40258). siliconchip.com.au Australia's electronics magazine December 2022  25 by the Attitude Control System (ACS) and the JWST Fine Guidance Sensor (FGS). The ACS also is responsible for Delta-V (orbit correction), momentum unloading (see below), antenna pointing, avoiding pointing at the Sun and controlling observatory “safe modes”. The spacecraft flight software receives data from various sensors, instructions from the Integrated Science Instrument Module (ISIM) and JWST ground control and processes them to send data to either the reaction wheels or the thrusters. The sensors include sun sensors, two star trackers and gyroscopes. The star trackers choose appropriate stars from a catalog, track their positions and compare them with the commanded position. During exposures (taking pictures), the Fine Guidance System (FGS) observes the guide star and makes measurements every 64ms. That data is sent to the ACS, which corrects any pointing error using reaction wheels and the Fine Steering Mirror (FSM). Momentum management and reaction wheels Reaction wheels are used in spacecraft, including the JWST, to control their attitude (orientation with respect to a fixed object). They are essentially motorised flywheels. When the wheel spins up or down, the spacecraft reacts by rotating in the opposite direction. The JWST has six reaction wheels – see Fig.36. Their use saves spacecraft fuel and they can also be used for tiny and accurate attitude adjustments, more so than rocket thrusters. They can only be used to rotate a spacecraft, not to move it. Interesting links 1. 2. 3. 4. 5. Build a paper model of the JWST: siliconchip.au/link/abgq Make a model of the JWST mirror: siliconchip.au/link/abgr Links for accessing data from JWST and instrument documentation: siliconchip.au/link/abgs Details of all the deployment operations: siliconchip.au/link/abgt 43-part playlist of time-lapse videos of the JWST being built and tested: siliconchip.au/link/abgp Photons from the Sun constantly hit the JWST sunshield. Since photons can exert a small force, this causes a force to be applied to the telescope. The centre of pressure of the sunshield is not the same as the centre of mass of the telescope, so this force generates a torque, making the telescope want to rotate. The reaction wheels counter this rotation. As a result, angular momentum accumulates in the wheels (ie, they keep spinning faster). If this were not corrected, the wheels would exceed their design limits. Therefore, the thrusters are fired about 4-8 times per month to allow the reaction wheels to be spun down. This “momentum unloading” activity takes several hours. The JWST has a “momentum flap”, also known as a “trim flap”, to somewhat minimise the rotation due to photon pressure, saving fuel. What can JWST image? Every six months, the JWST can image almost anything in the celestial sphere as it orbits the Sun and the Earth. At any one instant, however, it can see anything with a 50° field of view. 39% of the celestial sphere is potentially accessible to it at once. The only areas permanently inaccessible are imaging of the Sun, Mercury, Venus, Earth and Moon as these are too Fig.36: a model RSI 50-220/451 reaction wheel, similar or the same as used on the JWST and built by Rockwell Collins Deutschland GmbH (formerly Teldix). It features integrated electronics, spins at up to 4500 RPM, is 347mm in diameter and 124mm high, weighs 9.5kg, runs on 100V DC and consumes under 20W. Source: https://artes.esa.int/ projects/htmod2 26 Silicon Chip Australia's electronics magazine hot and would overload its sensors, possibly damaging or destroying them. False image colours Images in visible light have the traditional colours of the visible light spectrum that we are used to, but pictures from the JWST are also coloured, even though they were taken in the infrared. Beyond the visible light spectrum, colour is meaningless; however, adding colour to images helps us interpret them, so, like visible light, colour in infrared images is based on the wavelength of the light detected. Colour is arbitrarily assigned to the various infrared wavelengths to convey additional information to us; otherwise, the images would be all in greyscale and only show intensity information. Data and comms Data from the JWST is sent to the ground via NASA’s Deep Space Network. The telescope can downlink a minimum of 57.2GB of data daily at 28Mbit/s. It has a solid-state data recorder to store up to 65GB of science data. Downlinks occur twice per day for four hours, and up to 28.6GB of recorded data is transmitted per downlink period. Comms occur over Ka-band (2740GHz) for the high-rate downlink of data and telemetry, and S-band (2-4GHz), which is used for command uplink, low-rate telemetry downlink and ranging. Micrometeroroid impacts As expected, the JWST mirror has suffered at least 19 micrometeoroid impacts at the time of writing, but these have not caused any significant performance degradation. One impact was larger than expected and required a readjustment of the mirror actuator to compensate for the damage. When the JWST passes through high-risk areas, its mirror will be turned away from the direction of travel. SC siliconchip.com.au