The James Webb Space Telescope is nearing completion of the first phase of the months-long process of aligning the observatory’s primary mirror using the Near Infrared Camera (NIRCam) instrument.
The team’s challenge was twofold: confirm that NIRCam was ready to collect light from celestial objects, and then identify starlight from the same star in each of the 18 primary mirror segments. The result is an image mosaic of 18 randomly organized dots of starlight, the product of Webb’s unaligned mirror segments all reflecting light from the same star back at Webb’s secondary mirror and into NIRCam’s detectors.
What looks like a simple image of blurry starlight now becomes the foundation to align and focus the telescope in order for Webb to deliver unprecedented views of the universe this summer. Over the next month or so, the team will gradually adjust the mirror segments until the 18 images become a single star.
“The entire Webb team is ecstatic at how well the first steps of taking images and aligning the telescope are proceeding. We were so happy to see that light makes its way into NIRCam,” said Marcia Rieke, principal investigator for the NIRCam instrument and regents professor of astronomy, University of Arizona.
This image mosaic was created by pointing the telescope at a bright, isolated star in the constellation Ursa Major known as HD 84406. This star was chosen specifically because it is easily identifiable and not crowded by other stars of similar brightness, which helps to reduce background confusion. Each dot within the mosaic is labeled by the corresponding primary mirror segment that captured it. These initial results closely match expectations and simulations. Credit: NASA
During the image capturing process that began Feb. 2, Webb was repointed to 156 different positions around the predicted location of the star and generated 1,560 images using NIRCam’s 10 detectors, amounting to 54 gigabytes of raw data. The entire process lasted nearly 25 hours, but notedly the observatory was able to locate the target star in each of its mirror segments within the first six hours and 16 exposures. These images were then stitched together to produce a single, large mosaic that captures the signature of each primary mirror segment in one frame. The images shown here are only a center portion of that larger mosaic, a huge image with over 2 billion pixels.
“This initial search covered an area about the size of the full Moon because the segment dots could potentially have been that spread out on the sky,” said Marshall Perrin, deputy telescope scientist for Webb and astronomer at the Space Telescope Science Institute. “Taking so much data right on the first day required all of Webb’s science operations and data processing systems here on Earth working smoothly with the observatory in space right from the start. And we found light from all 18 segments very near the center early in that search! This is a great starting point for mirror alignment.”
Lee Feinberg, Webb optical telescope element manager at NASA’s Goddard Space Flight Center, explains the early stages of the mirror alignment process.
Each unique dot visible in the image mosaic is the same star as imaged by each of Webb’s 18 primary mirror segments, a treasure trove of detail that optics experts and engineers will use to align the entire telescope. This activity determined the post-deployment alignment positions of every mirror segment, which is the critical first step in bringing the entire observatory into a functional alignment for scientific operations.
NIRCam is the observatory’s wavefront sensor and a key imager. It was intentionally selected to be used for Webb’s initial alignment steps because it has a wide field of view and the unique capability to safely operate at higher temperatures than the other instruments. It is also packed with customized components that were designed to specifically aid in the process. NIRCam will be used throughout nearly the entire alignment of the telescope’s mirrors. It is, however, important to note that NIRCam is operating far above its ideal temperature while capturing these initial engineering images, and visual artifacts can be seen in the mosaic. The impact of these artifacts will lessen significantly as Webb draws closer to its ideal cryogenic operating temperatures.
“Launching Webb to space was of course an exciting event, but for scientists and optical engineers, this is a pinnacle moment, when light from a star is successfully making its way through the system down onto a detector,” said Michael McElwain, Webb observatory project scientist, NASA’s Goddard Space Flight Center.
This “selfie” was created using a specialized pupil imaging lens inside of the NIRCam instrument that was designed to take images of the primary mirror segments instead of images of space. This configuration is not used during scientific operations and is used strictly for engineering and alignment purposes. In this case, the bright segment was pointed at a bright star, while the others aren’t currently in the same alignment. This image gave an early indication of the primary mirror alignment to the instrument. Credit: NASA
Moving forward, Webb’s images will only become clearer, more detail-laden, and more intricate as its other three instruments arrive at their intended cryogenic operating temperatures and begin capturing data. The first scientific images are expected to be delivered to the world in the summer. Though this is a big moment, confirming that Webb is a functional telescope, there is much ahead to be done in the coming months to prepare the observatory for full scientific operations using all four of its instruments.
Quelle: NASA
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Update: 19.02.2022
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Webb’s Fine Guidance Sensor Is Guiding!
After starting the mirror alignment with Webb’s first detection of starlight in the Near-Infrared Camera (NIRCam), the telescope team is hard at work on the next steps for commissioning the telescope. To make more progress, the team needs to use another instrument, the Fine Guidance Sensor, to lock onto a guide star and keep the telescope pointed to high accuracy. We have asked René Doyon and Nathalie Ouellette of the Université de Montréal to explain how Webb uses its Canadian instrument in this process.
“After being powered on Jan. 28, 2022, and undergoing successful aliveness and functional tests, Webb’s Fine Guidance Sensor (FGS) has now successfully performed its very first guiding operation! Together with the Near-Infrared Imager and Slitless Spectrograph (NIRISS), the FGS is one of Canada’s contributions to the mission.
“To ensure Webb stays locked on its celestial targets, the FGS measures the exact position of a guide star in its field of view 16 times per second and sends adjustments to the telescope’s fine steering mirror about three times per second. In addition to its speed, the FGS also needs to be incredibly precise. The degree of precision with which it can detect changes in the pointing to a celestial object is the equivalent of a person in New York City being able to see the eye motion of someone blinking at the Canadian border 500 kilometers (311 miles) away!
“Webb’s 18 primary mirror segments are not yet aligned, so each star appears as 18 duplicate images. On Feb. 13, FGS successfully locked onto and tracked one of these star images for the first time. The FGS team was thrilled to see this ‘closed loop guiding’ working! From now on, most of the alignment process of the telescope mirrors will take place with FGS guiding, while NIRCam images provide the diagnostic information for mirror adjustments.”
–René Doyon, principal investigator for FGS/NIRISS, Université de Montréal; and Nathalie Ouellette, Webb outreach scientist, Université de Montréal
Quelle: NASA
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Update: 23.02.2022
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James Webb Space Telescope will study Milky Way's flaring supermassive black hole
Sagittarius A* flickers every hour, making it a challenging target for telescopes.
A multiwavelength composite image of the heart of the Milky Way, about where a supermassive black hole resides.(Image credit: NASA, ESA, SSC, CXC, STScI)
The James Webb Space Telescope will study the weirdly flickering black hole at the heart of our galaxy, the Milky Way, which has proved elusive for existing telescopes to explore.
Webb will join the efforts of numerous telescopes to understand the nature of the supermassive black hole called Sagittarius A*, whose tendency to flare up on an hourly basis makes it difficult to image.
Joining Webb investigators will be a team working with the Event Horizon Telescope. EHT, made up of eight ground-based radio telescopes, which captured the first-ever image of a black hole, M87*, back in 2019.
Although Sagittarius A* is closer than M87*, its flickering nature makes the Milky Way's own supermassive black hole a much more difficult target, Webb officials said in a statement.
"While M87’s core presented a steady target, Sagittarius A* exhibits mysterious flickering flares on an hourly basis, which make the imaging process much more difficult," Webb officials wrote in late 2021. "Webb will assist with its own infrared images of the black hole region, providing data about when flares are present that will be a valuable reference to the EHT team."
The flares happen as charged particles are accelerated around the black hole to higher energies, creating light emission.
Webb, which launched Dec. 25 and is in the midst of a months-long commissioning period, will eventually image Sagittarius A* in two infrared wavelengths from a perch in deep space unimpeded by stray light. Since EHT is on the ground, the hope is the data collected from Webb will complement the ground-based network data and create a cleaner, easy to interpret, image.
Collaborators expect that Webb and EHT working together will provide more information concerning what causes the flares, which in turn could provide insights for studying black holes, solar flares or particle and plasma physics more generally.
"We want to know how the universe works, because we are part of the universe. Black holes could hold clues to some of these big questions," study principal investigator Farhad Yusef-Zadeh, an astrophysicist at Northwestern University in Illinois, said in the same statement.
The first physical black hole ever discovered was spotted in 1971; the first EHT image of M87* in 2019 provided "direct visual proof that Einstein’s black hole prediction was correct," the press release stated.
Black holes, the team added, are a "proving ground" for Einstein's theory and the hope is this first collaboration between Webb and EHT will allow for more telescope time in space, in future years.
Quelle: SC
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Update: 27.02.2022
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James Webb Space Telescope is nearly halfway through its mirror alignment stages
This animation shows the “before” and “after” images from the James Webb Space Telescope's segment alignment phase, when the team corrected large positioning errors of its primary mirror segments and updated the alignment of the secondary mirror.(Image credit: NASA/STScI)
The team recently completed the third of seven planned steps to align the 18 hexagonal segments of Webb's mirror, marking nearly the halfway point in a complex, weeks-long process.
The second and third stages were respectively called segment alignment and image stacking, representing larger movements of the main mirror. Subsequent stages will make more minute adjustments to take an image of a distant star and gradually bring it to a single, precise point, NASA said in a statement Friday (Feb. 25).
"We still have work to do, but we are increasingly pleased with the results we’re seeing," Lee Feinberg, optical telescope element manager for Webb at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in the same statement. "Years of planning and testing are paying dividends, and the team could not be more excited to see what the next few weeks and months bring."
During the segment alignment stage, Webb engineers refined an initial image of a star rendered 18 times. Engineers made minor adjustments to the main mirror and changed the alignment of Webb's secondary mirror. These repositionings were key to "overlapping the light from all the mirrors so that they can work in unison," Webb officials said in the update.
During the image stacking stage, individual segment images are moved so they produce one unified image instead of 18 separate images. In this image, all 18 segments are stacked on top of each other. After future alignment steps, the image will be even sharper. (Image credit: NASA/STScI)
Then the third stage, image stacking, saw the focused dots reflected by each mirror stacked on top of one another. Photons of light from the individual segments were each rendered to the same location of a sensor on the telescope's near-infrared mirror (NIRCam).
"The team activated sets of six mirrors at a time and commanded them to repoint their light to overlap, until all dots of starlight overlapped with each other," Webb officials said of image stacking.
The NIRCam, seen here, will measure infrared light from extremely distant and old galaxies. (Image credit: NASA/Chris Gunn, CC BY)
Next will come the fourth phase of mirror alignment, called coarse phasing. That phase is already underway. NIRCam will be used to receive the light spectra (or wavelengths) from 20 pairings of the mirror segments. The process, Webb officials said, will allow engineers to correct small differences in heights between mirror segments.
"This will make the single dot of starlight progressively sharper and more focused in the coming weeks," NASA officials said, noting that the segments will gradually align to achieve an accuracy smaller than a single wavelength of light.
Quelle: SC
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Update: 6.03.2022
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James Webb Space Telescope instrument gets ready to probe the universe's chemistry
The Near-Infrared Spectrograph passed initial checkouts amid the observatory's mirror alignment activities.
The James Webb Space Telescope's NIRSpec (Near-Infrared Spectograph) during assembly at NASA's Goddard Space Flight Center in Maryland.(Image credit: NASA/Chris Gunn)
During the observatory's commissioning and ongoing mirror alignment, Webb's Near-Infrared Spectrograph (NIRSpec) team successfully finished initial check-out and characterization of three mechanisms that are key for the instrument to do its work.
"The NIRSpec team will continue their commissioning efforts. The whole team is very much looking forward to the start of science observations this summer," several NIRSpec representatives wrote in a Webb blog post Thursday (March 3).
Once operational, NIRSpec will split the light from targets that Webb observes into what scientists call spectra, measuring the amount of light with specific wavelengths. This fingerprint can teach astronomers about galaxies, exoplanets and other objects, shedding light on properties such as mass, temperature and chemical composition.
With NIRSpec, Webb can take spectra of up to 100 galaxies at the same time, making observations much more efficient since collecting the light photons from such distant objects will take the observatory hundreds of hours.
NIRSpec's three key mechanisms are a filter wheel assembly, a grating wheel assembly and a refocus mechanism assembly, the Webb blog post stated.
Here is how the instrument will work: The grating wheel spreads light from a target of interest into its colors (wavelengths) to make a spectrum. The filter wheel reduces contamination by blocking wavelengths outside of what scientists are interested in looking at. Then the refocus mechanism will adjust and sharpen NIRSpec's focus.
Engineers checked out each of these assemblies separately, starting with the filter wheel assembly to make sure its eight positions in forward and reverse directions are working.
"At each position, we recorded a set of reference data," NIRSpec officials wrote. "This data showed us how well the wheel was moving, and how accurately it settled into each position. Between each ... position, we downloaded 'high-capacity buffer' data from the positioning sensors, and the NIRSpec team analyzed the data. The data showed that the wheel moved very well even in the first attempt."
Next, Webb engineers recorded the reference data of the grating wheel assembly and cycled the positions in much the same way, showing that everything was working correctly.
Then with the refocus mechanism assembly (RMA), engineers also did an initial data collection before commanding the mechanism to move forward "a few hundred steps from launch position," the blog post said.
"After the initial move," the NIRSpec team added in the blog, "we commanded the RMA mirrors to their previous best focus position. Successful completions of this test showed us that the RMA is a well-behaved and healthy mechanism."
Amid the instrument checkout, Webb mirror alignment is continuing in its fourth phase, focusing on "coarse phasing" that measures and corrects small height differences between individual mirror segments, the blog post noted. The telescope remains in excellent health after its Dec. 25 launch and has enough fuel for at least 20 years of operations.
Quelle: SC
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Update: 14.03.2022
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NASA to Discuss Progress as Webb Telescope’s Mirrors Align
NASA technicians use a crane to lift and move the James Webb Space Telescope, with its 21-foot primary mirror deployed, inside a clean room at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in April 2017.
Credits: NASA/Desiree Stover
NASA will hold a virtual media briefing at noon EDT Wednesday, March 16, to provide an update on the James Webb Space Telescope’s mirror alignment. The briefing will air live on NASA TV, the NASA app, and the agency’s website.
Participants will share progress made in aligning Webb’s mirrors, resulting in a fully focused image of a single star. NASA will make imagery demonstrating the completion of this milestone available on the agency’s website at 11:30 a.m., prior to the briefing.
Briefing participants include:
Thomas Zurbuchen, associate administrator, Science Mission Directorate, NASA Headquarters in Washington
Lee Feinberg, Webb optical telescope element manager, NASA’s Goddard Space Flight Center in Greenbelt, Maryland
Marshall Perrin, Webb deputy telescope scientist, Space Telescope Science Institute in Baltimore
Jane Rigby, Webb operations project scientist, Goddard
Erin Wolf, Webb program manager, Ball Aerospace in Broomfield, Colorado
To ask questions during the briefing, media must RSVP no later than two hours before the start of the event to Laura Betz at: laura.e.betz@nasa.gov. Media and members of the public may also ask questions on social media using #UnfoldtheUniverse.
In recent weeks, the Webb team successfully captured starlight through each of Webb’s 18 mirror segments. The team then refined and stacked those 18 individual dots of light on top of one another to form an initial alignment image of a single star. Since then, in stages of alignment called “coarse phasing" and "fine phasing,” engineers have made smaller adjustments to the positions of Webb’s 18 primary mirror segments so they act as a single mirror, producing a sharp and focused image of a single star.
Webb, an international partnership with ESA (European Space Agency) and the Canadian Space Agency, launched Dec. 25 from Europe’s Spaceport in Kourou, French Guiana. After unfolding into its final form in space and successfully reaching its destination 1 million miles from Earth, the observatory is now in the process of preparing for science operations. The Webb team will release the telescope’s first science images and data this summer after completing telescope alignment and preparing the instruments.
Webb will explore every phase of cosmic history – from within our solar system to the most distant observable galaxies in the early universe, and everything in between. Webb will reveal new and unexpected discoveries and help humanity understand the origins of the universe and our place in it.
NASA has a digital media kit, as well as image and video galleries, available online. The public also can follow Webb’s progress via a “Where is Webb?” interactive tracker.
Quelle: NASA
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Update: 16.03.2022
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NASA’s Webb Reaches Alignment Milestone, Optics Working Successfully
While the purpose of this image was to focus on the bright star at the center for alignment evaluation, Webb's optics and NIRCam are so sensitive that the galaxies and stars seen in the background show up. At this stage of Webb’s mirror alignment, known as “fine phasing,” each of the primary mirror segments have been adjusted to produce one unified image of the same star using only the NIRCam instrument. This image of the star, which is called 2MASS J17554042+6551277, uses a red filter to optimize visual contrast.
Credits: NASA/STScI
Following the completion of critical mirror alignment steps, NASA’s James Webb Space Telescope team expects that Webb’s optical performance will be able to meet or exceed the science goals the observatory was built to achieve.
On March 11, the Webb team completed the stage of alignment known as “fine phasing.” At this key stage in the commissioning of Webb’s Optical Telescope Element, every optical parameter that has been checked and tested is performing at, or above, expectations. The team also found no critical issues and no measurable contamination or blockages to Webb’s optical path. The observatory is able to successfully gather light from distant objects and deliver it to its instruments without issue.
Although there are months to go before Webb ultimately delivers its new view of the cosmos, achieving this milestone means the team is confident that Webb’s first-of-its-kind optical system is working as well as possible.
“More than 20 years ago, the Webb team set out to build the most powerful telescope that anyone has ever put in space and came up with an audacious optical design to meet demanding science goals,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington. “Today we can say that design is going to deliver.”
While some of the largest ground-based telescopes on Earth use segmented primary mirrors, Webb is the first telescope in space to use such a design. The 21-foot, 4-inch (6.5-meter) primary mirror – much too big to fit inside a rocket fairing – is made up of 18 hexagonal, beryllium mirror segments. It had to be folded up for launch and then unfolded in space before each mirror was adjusted – to within nanometers – to form a single mirror surface.
“In addition to enabling the incredible science that Webb will achieve, the teams that designed, built, tested, launched, and now operate this observatory have pioneered a new way to build space telescopes,” said Lee Feinberg, Webb optical telescope element manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
NASA’s Webb Reaches Alignment Milestone, Optics Working Successfully
Credits: NASA's Goddard Space Flight Center
With the fine phasing stage of the telescope’s alignment complete, the team has now fully aligned Webb’s primary imager, the Near-Infrared Camera, to the observatory’s mirrors.
“We have fully aligned and focused the telescope on a star, and the performance is beating specifications. We are excited about what this means for science,” said Ritva Keski-Kuha, deputy optical telescope element manager for Webb at NASA Goddard. “We now know we have built the right telescope.”
This new “selfie” was created using a specialized pupil imaging lens inside of the NIRCam instrument that was designed to take images of the primary mirror segments instead of images of the sky. This configuration is not used during scientific operations and is used strictly for engineering and alignment purposes. In this image, all of Webb’s 18 primary mirror segments are shown collecting light from the same star in unison.
Credits: NASA/STScI
Over the next six weeks, the team will proceed through the remaining alignment steps before final science instrument preparations. The team will further align the telescope to include the Near-Infrared Spectrograph, Mid-Infrared Instrument, and Near InfraRed Imager and Slitless Spectrograph. In this phase of the process, an algorithm will evaluate the performance of each instrument and then calculate the final corrections needed to achieve a well-aligned telescope across all science instruments. Following this, Webb’s final alignment step will begin, and the team will adjust any small, residual positioning errors in the mirror segments.
The team is on track to conclude all aspects of Optical Telescope Element alignment by early May, if not sooner, before moving on to approximately two months of science instrument preparations. Webb’s first full-resolution imagery and science data will be released in the summer.
Webb is the world's premier space science observatory and once fully operational, will help solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners at ESA (European Space Agency) and the Canadian Space Agency.
Quelle: NASA
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Update: 17.03.2022
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Webb reaches alignment milestone
Following the completion of critical mirror alignment steps, the NASA/ESA/CSA James Webb Space Telescope team expects that Webb’s optical performance will be able to meet or exceed the science goals the observatory was built to achieve.
On 11 March, the Webb team completed the stage of alignment known as “fine phasing”. At this key stage in the commissioning of Webb’s Optical Telescope Element, every optical parameter that has been checked and tested is performing at, or above, expectations. The team also found no critical issues and no measurable contamination or blockages to Webb’s optical path. The observatory is able to successfully gather light from distant objects and deliver it to its instruments without issue.
Although there are months to go before Webb ultimately delivers its new view of the cosmos, achieving this milestone means the team is confident that Webb’s first-of-its-kind optical system is working as well as possible.
With the fine phasing stage of the telescope’s alignment complete, the team has now fully aligned Webb’s primary imager, the Near-Infrared Camera, to the observatory’s mirrors.
Over the next six weeks, the team will proceed through the remaining alignment steps before final science instrument preparations. The team will further align the telescope to include the Near-Infrared Spectrograph, Mid-Infrared Instrument, and Near InfraRed Imager and Slitless Spectrograph. In this phase of the process, an algorithm will evaluate the performance of each instrument and then calculate the final corrections needed to achieve a well-aligned telescope across all science instruments. Following this, Webb’s final alignment step will begin, and the team will adjust any small, residual positioning errors in the mirror segments.
The team is on track to conclude all aspects of Optical Telescope Element alignment by early May, if not sooner, before moving on to approximately two months of science instrument preparations. Webb’s first full-resolution imagery and science data will be released in the summer.
Webb is the world's premier space science observatory and once fully operational, will help solve mysteries in our Solar System, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it.
Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).
Quelle: ESA
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Update: 10.04.2022
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Webb’s Mid-Infrared Instrument Cooldown Continues
“The Mid-Infrared Instrument (MIRI) and other Webb instruments have been cooling by radiating their thermal energy into the dark of space for the bulk of the last three months. The near-infrared instruments will operate at about 34 to 39 kelvins, cooling passively. But MIRI’s detectors will need to get a lot colder still, to be able to detect longer wavelength photons. This is where the MIRI cryocooler comes in.
By necessity, MIRI’s detectors are built using a special formulation of Arsenic-doped Silicon (Si:As), which need to be at a temperature of less than 7 kelvins to operate properly. This temperature is not possible by passive means alone, so Webb carries a “cryocooler” that is dedicated to cooling MIRI’s detectors. Credit: NASA/JPL-Caltech.
“Over the last couple weeks, the cryocooler has been circulating cold helium gas past the MIRI optical bench, which will help cool it to about 15 kelvins. Soon, the cryocooler is about to experience the most challenging days of its mission. By operating cryogenic valves, the cryocooler will redirect the circulating helium gas and force it through a flow restriction. As the gas expands when exiting the restriction, it becomes colder, and can then bring the MIRI detectors to their cool operating temperature of below 7 kelvins. But first, the cryocooler must make it through the ‘pinch point’ – the transition through a range of temperatures near 15 kelvins, when the cryocooler’s ability to remove heat is at its lowest. Several time-critical valve and compressor operations will be performed in rapid succession, adjusted as indicated by MIRI cryocooler temperature and flow rate measurements. What is particularly challenging is that after the flow redirection, the cooling ability gets better as the temperature gets lower. On the flip side, if the cooling is not immediately achieved due to, for example, larger than modeled heat loads, MIRI will start warming.
“Once the cryocooler overcomes the remaining heat loads, it will settle into its lower-power steady science operation state for the rest of the mission. This pinch point event has been extensively practiced in the cryocooler testbed at NASA’s Jet Propulsion Laboratory (JPL), which manages the MIRI cryocooler, as well as during Webb testing at the agency’s Goddard Space Flight Center and Johnson Space Center. Performing it on orbit will be supported by the operations team comprised of personnel from JPL, Goddard, and the Space Telescope Science Institute. The MIRI cryocooler was developed by Northrop Grumman Space Systems. MIRI was developed as a 50/50 partnership between NASA and ESA (European Space Agency), with JPL leading the U.S. efforts and a multi-national consortium of European astronomical institutes contributing for ESA.”
– Konstantin Penanen and Bret Naylor, cryocooler specialists, NASA JPL
“MIRI stands out from Webb’s other instruments because it operates at much longer infrared wavelengths, compared to the other instruments that all begin with an ‘N’ for ‘near-infrared.’ MIRI will support the instrument suite to explore the infrared universe with depth and detail that are far beyond anything that has been available to astronomers to date.
“The imager promises to reveal astronomical targets ranging from nearby nebulae to distant interacting galaxies with a clarity and sensitivity far beyond what we’ve seen before. Our grasp on these glittering scientific treasures relies on MIRI being cooled to a temperature below the rest of the observatory, using its own dedicated refrigerator. Exoplanets at temperatures similar to Earth will shine most brightly in mid-infrared light. MIRI is therefore equipped with four coronagraphs, which have been carefully designed to detect such planets against the bright glare of their parent stars. The detailed colors of exo-giant planets (similar to our own Jupiter) can then be measured by MIRI’s two spectrometers to reveal chemical identities, abundances, and temperatures of the gases of their atmospheres (including water, ozone, methane, ammonia, and many more).
MIRI is inspected in the giant clean room at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in 2012. Credit: NASA/Chris Gunn.
“Why so cold? MIRI’s state-of-the-art light sensitive detectors that are tuned to work in the mid-infrared are blind unless they are cooled below 7 kelvins (-266 degrees Celsius, or -447 degrees Fahrenheit). For contrast, a standard domestic freezer cools its contents to about 255 kelvins (-18 degrees Celsius, or -0.7 degrees Fahrenheit). At higher temperatures, any signal that may be detected from the sky is lost beneath the signal from its own internally generated ‘dark current.’ Even if the detectors are cooled, Webb images would still be swamped by the glow of thermal infrared light emitted by MIRI’s own mirrors and aluminum structure if they are to get warmer than 15 kelvins (-258 degrees Celsius, or -433 degrees Fahrenheit). The engineering solution was to stand MIRI off from the instrument mounting structure behind Webb’s primary mirror like a high-tech metal spider on six carbon fibre legs. These insulate MIRI from the much hotter telescope (where 45 kelvins, or -228 degrees Celsius/-379 degrees Fahrenheit, qualifies as hotter). The instrument’s body is also swathed in a shiny aluminum-coated thermal blanket, which reflects the radiant heat of its surroundings.
“Getting this instrument cold is one of the last major challenges faced by Webb before the MIRI team can truly relax, and passing through the cooler’s ‘pinch point’ will be the most daunting step in this challenge. At that time, the cooler will have pulled out almost all of the heat left in MIRI’s 100 kilograms (220 pounds) of metal and glass from that tropical launch day morning, three months ago. MIRI will be the last of Webb’s four instruments to open its eyes on the universe.”
– Alistair Glasse, Webb-MIRI Instrument Scientist, UK Astronomy Technology Centre and Macarena Garcia Marin, MIRI Instrument and Calibration Scientist, ESA
Quelle: NASA
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Update: 16.04.2022
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Webb’s coldest instrument reaches operating temperature
With help from a cryocooler, Webb's Mid-Infrared Instrument has dropped down to just a few degrees above the lowest temperature matter can reach and is ready for calibration.
The James Webb Space Telescope will see the first galaxies to form after the Big Bang, but to do that its instruments first need to get cold – really cold. On 7 April, Webb’s Mid-Infrared Instrument (MIRI) – a joint development by ESA and NASA – reached its final operating temperature below 7 kelvins (minus 266 degrees Celsius).
“I am delighted that after so many years of hard work by the MIRI team the instrument is now cold and ready for the next steps. That the cooler worked so well is a major achievement for the mission,” said Gillian Wright, European principal investigator for MIRI and Director of the UK Astronomy Technology Centre (ATC).
Along with Webb’s three other instruments, MIRI initially cooled off in the shade of Webb’s tennis-court-size sunshield, dropping to about 90 kelvins (minus 183 C). But dropping to less than 7 kelvins required an electrically powered cryocooler. Last week, the team passed a particularly challenging milestone called the “pinch point,” when the instrument goes from 15 kelvins (minus 258 C) to 6.4 kelvins (minus 267 C).
“The MIRI cooler team has poured a lot of hard work into developing the procedure for the pinch point,” said Analyn Schneider, project manager for MIRI at NASA’s Jet Propulsion Laboratory in Southern California, USA. “The team was both excited and nervous going into the critical activity. In the end it was a textbook execution of the procedure, and the cooler performance is even better than expected.”
The low temperature is necessary because all four of Webb’s instruments detect infrared light – wavelengths slightly longer than those that human eyes can see. Distant galaxies, stars hidden in cocoons of dust, and planets outside our Solar System all emit infrared light. But so do other warm objects, including Webb’s own electronics and optics hardware. Cooling down the four instruments’ detectors and the surrounding hardware suppresses those infrared emissions. MIRI detects longer infrared wavelengths than the other three instruments, which means it needs to be even colder.
Another reason Webb’s detectors need to be cold is to suppress something called dark current, or electric current created by the vibration of atoms in the detectors themselves. Dark current mimics a true signal in the detectors, giving the false impression that they have been hit by light from an external source. Those false signals can drown out the real signals astronomers want to find. Since temperature is a measurement of how fast the atoms in the detector are vibrating, reducing the temperature means less vibration, which in turn means less dark current.
MIRI’s ability to detect longer infrared wavelengths also makes it more sensitive to dark current, so it needs to be colder than the other instruments to fully remove that effect. For every degree the instrument temperature goes up, the dark current goes up by a factor of about 10.
Once MIRI reached a frigid 6.4 kelvins, scientists began a series of checks to make sure the detectors were operating as expected. Like a doctor searching for any sign of illness, the MIRI team looks at data describing the instrument’s health, then gives the instrument a series of commands to see if it can execute tasks correctly.
This milestone is the culmination of work by scientists and engineers at multiple institutions in addition to JPL, including Northrop Grumman, which built the cryocooler, and NASA’s Goddard Space Flight Center, which oversaw the integration of MIRI and the cooler to the rest of the observatory.
“We spent years practicing for that moment, running through the commands and the checks that we did on MIRI,” said Mike Ressler, project scientist for MIRI at JPL. “It was kind of like a movie script: Everything we were supposed to do was written down and rehearsed. When the test data rolled in, I was ecstatic to see it looked exactly as expected and that we have a healthy instrument.”
There are still more challenges that the team will have to face before MIRI can start its scientific mission. Now that the instrument is at operating temperature, team members will take test images of stars and other known objects that can be used for calibration and to check the instrument’s operations and functionality. The team will conduct these preparations alongside calibration of the other three instruments, delivering Webb’s first science images this summer.
“I am immensely proud to be part of this group of highly motivated, enthusiastic scientists and engineers drawn from across Europe and the USA,” said Alistair Glasse, MIRI instrument scientist at the ATC in Edinburgh, Scotland. “This period is our ‘trial by fire’ but it is already clear to me that the personal bonds and mutual respect that we have built up over the past years is what will get us through the next few months to deliver a fantastic instrument to the worldwide astronomy community.”
MIRI is part of Europe’s contribution to the James Webb Space Telescope (Webb) mission. It is a partnership between Europe and the USA; the main partners are ESA, a consortium of nationally funded European institutes, the Jet Propulsion Laboratory (JPL) and NASA's Goddard Space Flight Center (GSFC).
Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).
