Author: Thaddeus Cesari

The Webb team continues to commission the 17 science instrument modes. This week we asked Nathalie Ouellette of the Université de Montréal to give more detail about the modes of the Near-Infrared Imager and Slitless Spectrograph (NIRISS), Canada’s scientific instrument on Webb.

“NIRISS will be able to capture both images and spectra from different types of celestial objects in near-infrared light, at wavelengths up to 5.0 microns. The NIRISS team has developed four instrument modes to collect different kinds of data that are well-suited for different targets and scientific objectives.

Single Object Slitless Spectroscopy (SOSS)

“The SOSS mode on NIRISS allows the Webb telescope to obtain high-precision spectra from one bright object at a time. This mode is optimized to carry out time-series observations, which are ideal for studying a phenomenon that changes over the length of a typically hours-long observation, such as an exoplanet transiting in front of its host star.

“Using a technique called transit spectroscopy, the NIRISS instrument can collect a spectrum of an exoplanet’s atmosphere, which contains different markers that allow astronomers to determine its composition, temperature, potential habitability signatures, and other important characteristics.

Wide Field Slitless Spectroscopy (WFSS)

“The WFSS mode on NIRISS allows Webb to obtain spectra but for thousands of objects, such as galaxies, at the same time over the detector’s entire field of view (4.84 arcmin²). The spectra of thousands of galaxies will enable measurement of their distances, ages, and other physical parameters to trace how galaxies evolve over the lifetime of the universe. In the simulated example shown in the figure, the galaxy cluster acts like a cosmic lens that magnifies and stretches the images of faint background galaxies, so they can be studied in even greater detail.

“Since NIRISS can collect so many spectra at a time using the WFSS mode, individual spectra can overlap if their sources are too close. There are thus two orthogonal grisms, GR150C and GR150R, that can produce spectra horizontally and vertically, respectively, which helps to disentangle blended spectra from different galaxies.

Aperture Masking Interferometry (AMI)

“The AMI mode on NIRISS allows Webb to study objects that are very close together on the sky, using a special technique called interferometry. A mask inside the instrument allows light from only certain parts of the primary mirror to pass through. Astronomers can increase the resolution of the telescope by a factor of nearly 2.5 by looking at the patterns created as the carefully chosen beams of light interfere with each other. This allows two objects that are close to each other that would otherwise look like a single blurred point, like an exoplanet orbiting a star, to appear as two distinct points of light in a Webb image. The mask blocks out a large portion of the light, so the observed objects must be bright in order to detect them. The AMI mode will be used to observe exoplanets, brown dwarfs, and protoplanetary disks. This is the first time that such a mask is being used in space.

NIRISS Imaging

“Because of the importance of near-infrared imaging to Webb’s scientific success, NIRISS includes an imaging capability that functions as a backup to NIRCam imaging. This capability can be used in parallel, with NIRCam and NIRISS simultaneously taking images of two closely separated fields of view, imaging a larger area of an extended source.”

— Nathalie Ouellette, Webb outreach scientist, Université de Montréal

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— Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

As NASA’s James Webb Space Telescope moves through the final phases of commissioning its science instruments, we have also begun working on technical operations of the observatory.  While the telescope moves through space, it will constantly find distant stars and galaxies and point at them with extreme precision to acquire images and spectra. However, we also plan to observe planets and their satellites, asteroids, and comets in our solar system, which move across the background stars of our galaxy. Webb needs to be able to lock on to these objects and track them with sufficient precision to obtain images and spectra. The Webb team recently completed the first test to track a moving object. The test verified that Webb could conduct moving target science! As we move forward through commissioning, we will test other objects moving at various speeds to verify we can study objects with Webb that move throughout the solar system.

Today, we asked Heidi Hammel, Webb interdisciplinary scientist for solar system observations, to tell us about her plans for studying Earth’s nearest neighbors:

“I am really excited about Webb’s upcoming first year of science operations! I lead a team of equally excited astronomers eager to begin downloading data. Webb can detect the faint light of the earliest galaxies, but my team will be observing much closer to home. They will use Webb to unravel some of the mysteries that abound in our own solar system.

“One of the questions I get asked frequently is why we need a powerful telescope like Webb to study our nearby solar system. We planetary scientists use telescopes to complement our in situ missions (missions that we send to fly by, orbit, or land on objects). One example of this is how Hubble was used to find the post-Pluto target for the New Horizons mission, Arrokoth. We also use telescopes when we don’t have in situ missions planned – like for the distant ice giants Uranus and Neptune or to make measurements of large populations of objects, such as hundreds of asteroids or Kuiper Belt Objects (small ice worlds beyond the orbits of Neptune, including Pluto), since we can only send missions to just a few of these.

“The Webb team has already used an asteroid within our solar system to run engineering tests of the ‘moving target’ (MT) capability. The engineering team tested this capability on a small asteroid in the Main Belt: 6481 Tenzing, named after Tenzing Norgay, the famous Tibetan mountain guide who was one of the first people to reach the summit of Mount Everest. Bryan Holler, at the Space Telescope Science Institute, had a choice of about 40 possible asteroids to test the MT tracking, but, as he told our team: “Since the objects were all virtually identical otherwise, picking the one with a name linked to success seemed like a no-brainer.”  We like that sort of thing.

“My role with Webb as an ‘Interdisciplinary Scientist’ means that my program uses all of the capabilities of this forefront telescope! We need all of them to truly understand the solar system (and the universe!).

“Our solar system has far more mysteries than my team had time to solve. Our programs will observe objects across the solar system: We will image the giant planets and Saturn’s rings; explore many Kuiper Belt Objects; analyze the atmosphere of Mars; execute detailed studies of Titan; and much more! There are also other teams planning observations; in its first year, 7% of Webb’s time will be focused on objects within our solar system.

“One exciting and challenging program we plan to do is observe ocean worlds. There’s evidence from the Hubble Space Telescope that Jupiter’s moon Europa has sporadic plumes of water-rich material. We plan to take high-resolution imagery of Europa to study its surface and search for plume activity and active geologic processes. If we locate a plume, we will use Webb’s spectroscopy to analyze the plume’s composition.

“I have a soft spot in my heart for Uranus and Neptune. Indeed, it was the lack of a mission to these very distant worlds that got me involved in Webb so many decades ago. The Uranus team hopes to definitively link the chemistry and dynamics of the upper atmosphere (detectable with Webb) to the deeper atmosphere that we have been studying with other facilities over many decades. I’ve spent the past 30 years using the biggest and best telescopes humanity has ever built to study these ice giants, and we will now add Webb to that list.

“We have been planning for Webb observations for over twenty years, and that has gone into overdrive now that we are launched, deployed, and focused! I’ll note that nearly all of my team’s solar system data will be freely available to the broad planetary science community immediately. I made that choice to enable more science discoveries with Webb in future proposals.

“I am gratified to have been able to work with the team for all this time, and I especially want to give a shout out to the thousands of people who collectively have enabled this amazing facility for the astrophysics and planetary communities. Thank you! Ad astra!”

—Heidi Hammel, vice president for science, Association of Universities for Research in Astronomy (AURA)

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–Stefanie Milam, Webb deputy project scientist for planetary science, NASA’s Goddard Space Flight Center

–Jonathan Gardner, Webb deputy senior project scientist, NASA Goddard

With the telescope optics and instruments aligned, the Webb team is now commissioning the observatory’s four powerful science instruments. There are 17 different instrument “modes” to check out on our way to getting ready for the start of science this summer. Once we have approved all 17 of these modes, NASA’s James Webb Space Telescope will be ready to begin scientific operations!

In this post we’ll describe the 17 modes, and readers are encouraged to follow along as the Webb team checks them off one by one on the Where is Webb tracker. Each mode has a set of observations and analysis that need to be verified, and it is important to note that the team does not plan to complete them in the order listed below. Some of the modes won’t be verified until the very end of commissioning.

For each mode we have also selected a representative example science target that will be observed in the first year of Webb science. These are just examples; each mode will be used for many targets, and most of Webb’s science targets will be observed with more than one instrument and/or mode. The detailed list of peer-reviewed observations planned for the first year of science with Webb ranges from our solar system to the most distant galaxies.

1.  Near-Infrared Camera (NIRCam) imaging. Near-infrared imaging will take pictures in part of the visible to near-infrared light, 0.6 to 5.0 micrometers wavelength. This mode will be used for almost all aspects of Webb science, from deep fields to galaxies, star-forming regions to planets in our own solar system. An example target in a Webb cycle 1 program using this mode: the Hubble Ultra-Deep Field.

2.  NIRCam wide field slitless spectroscopy. Spectroscopy separates the detected light into individual colors. Slitless spectroscopy spreads out the light in the whole instrument field of view so we see the colors of every object visible in the field. Slitless spectroscopy in NIRCam was originally an engineering mode for use in aligning the telescope, but scientists realized that it could be used for science as well. Example target: distant quasars.

3.  NIRCam coronagraphy. When a star has exoplanets or dust disks in orbit around it, the brightness from a star usually will outshine the light that is reflected or emitted by the much fainter objects around it. Coronagraphy uses a black disk in the instrument to block out the starlight in order to detect the light from its planets. Example target: the gas giant exoplanet HIP 65426 b.

4.  NIRCam time series observations – imaging. Most astronomical objects change on timescales that are large compared to human lifetimes, but some things change fast enough for us to see them. Time series observations read out the instruments’ detectors rapidly to watch for those changes. Example target: a pulsing neutron star called a magnetar.

5.  NIRCam time series observations – grism. When an exoplanet crosses the disk of its host star, light from the star can pass through the atmosphere of the planet, allowing scientists to determine the constituents of the atmosphere with this spectroscopic technique. Scientists can also study light that is reflected or emitted from an exoplanet, when an exoplanet passes behind its host star. Example target: lava rain on the super-Earth-size exoplanet 55 Cancri e.

6.  Near-Infrared Spectrograph (NIRSpec) multi-object spectroscopy. Although slitless spectroscopy gets spectra of all the objects in the field of view, it also allows the spectra of multiple objects to overlap each other, and the background light reduces the sensitivity. NIRSpec has a microshutter device with a quarter of a million tiny controllable shutters. Opening a shutter where there is an interesting object and closing the shutters where there is not allows scientists to get clean spectra of up to 100 sources at once. Example target: the Extended Groth Strip deep field.

7.  NIRSpec fixed slit spectroscopy. In addition to the microshutter array, NIRSpec also has a few fixed slits that provide the ultimate sensitivity for spectroscopy on individual targets. Example target: detecting light from a gravitational-wave source known as a kilonova.

8.  NIRSpec integral field unit spectroscopy. Integral field unit spectroscopy produces a spectrum over every pixel in a small area, instead of a single point, for a total of 900 spatial/spectral elements. This mode gives the most complete data on an individual target. Example target: a distant galaxy boosted by gravitational lensing.

9.  NIRSpec bright object time series. NIRSpec can obtain a time series spectroscopic observation of transiting exoplanets and other objects that change rapidly with time. Example target: following a hot super-Earth-size exoplanet for a full orbit to map the planet’s temperature.

10.  Near-Infrared Imager and Slitless Spectrograph (NIRISS) single object slitless spectroscopy. To observe planets around some of the brightest nearby stars, NIRISS takes the star out of focus and spreads the light over lots of pixels to avoid saturating the detectors. Example target: small, potentially rocky exoplanets TRAPPIST-1b and 1c.

11.  NIRISS wide field slitless spectroscopy. NIRISS includes a slitless spectroscopy mode optimized for finding and studying distant galaxies. This mode will be especially valuable for discovery, finding things that we didn’t already know were there. Example target: pure parallel search for active star-forming galaxies.

12.  NIRISS aperture masking interferometry. NIRISS has a mask to block out the light from 11 of the 18 primary mirror segments in a process called aperture masking interferometry. This provides high-contrast imaging, where faint sources next to bright sources can be seen and resolved for images. Example target: a binary star with colliding stellar winds.

13.  NIRISS imaging. Because of the importance of near-infrared imaging, NIRISS has an imaging capability that functions as a backup to NIRCam imaging. Scientifically, this is used mainly while other instruments are simultaneously conducting another investigation, so that the observations image a larger total area. Example target: a Hubble Frontier Field gravitational lensing galaxy cluster.

14.  Mid-Infrared Instrument (MIRI) imaging. Just as near-infrared imaging with NIRCam will be used on almost all types of Webb targets, MIRI imaging will extend Webb’s pictures from 5 to 27 microns, the mid-infrared wavelengths. Mid-infrared imaging will show us, for example, the distributions of dust and cold gas in star-forming regions in our own Milky Way galaxy and in other galaxies. Example target: the nearby galaxy Messier 33.

15.  MIRI low-resolution spectroscopy. At wavelengths between 5 and 12 microns, MIRI’s low-resolution spectroscopy can study fainter sources than its medium-resolution spectroscopy. Low resolution is often used for studying the surface of objects, for example, to determine the composition. Example target: Pluto’s moon Charon.

16.  MIRI medium-resolution spectroscopy. MIRI can do integral field spectroscopy over its full mid-infrared wavelength range, 5 to 28.5 microns. This is where emission from molecules and dust display very strong spectral signatures. Example targets: molecules in planet-forming disks.

17.  MIRI coronagraphic imaging. MIRI has two types of coronagraphy: a spot that blocks light and three four-quadrant phase mask coronagraphs. These will be used to directly detect exoplanets and study dust disks around their host stars. Example target: searching for planets around our nearest neighbor star Alpha Centauri A.

— Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

NASA’s James Webb Space Telescope is now experiencing all seasons – from hot to cold – as it undergoes the thermal stability test. Meanwhile, activities are underway for the final phase of commissioning: digging into the details of the science instruments, the heart of Webb. To complete commissioning, we will measure the detailed performance of the science instruments before we start routine science operations in the summer.

Today, the lead commissioning scientist for Webb, Scott Friedman of the Space Telescope Science Institute (STScI), gives us all the details on this final phase of commissioning.

“With the telescope beautifully aligned and the observatory near its final cryogenic temperature, we are ready to begin the last group of activities before the science observations start: science instrument commissioning. Here I describe just a few of those activities.

“The instruments, the Near-Infrared Camera (NIRCam), Near-Infrared Spectrometer (NIRSpec), Near-Infrared Imager and Slitless Spectrometer (NIRISS), Mid-Infrared Instrument (MIRI), and the Fine Guidance Sensor (FGS) have been powered up and safely cooled. We have operated their mechanisms and detectors, including filter wheels, grating wheels, and the NIRSpec microshutter assembly. The Webb optics team used images of isolated stars taken with each of the instruments to align the primary and secondary mirrors of the observatory. But we have more work to do before Webb is fully ready to embark on the ambitious science observations that will reveal the secrets of the universe.

“We will now begin an extensive suite of calibrations and characterizations of the instruments using a rich variety of astronomical sources. We will measure the instruments’ throughput – how much of the light that enters the telescope reaches the detectors and is recorded. There is always some loss with each reflection by the mirrors of the telescope and within each instrument, and no detector records every photon that arrives. We will measure this throughput at multiple wavelengths of light by observing standard stars whose light emission is known from data obtained with other observatories combined with theoretical calculations.

“The astrometric calibration of each instrument maps the pixels on the detectors to the precise locations on the sky, to correct the small but unavoidable optical distortions that are present in every optical system. We do this by observing the Webb astrometric field, a small patch of sky in a nearby galaxy, the Large Magellanic Cloud. This field was observed by the Hubble Space Telescope to establish the coordinates of about 200,000 stars to an accuracy of 1 milli-arcsec (less than 0.3 millionths of a degree). Calibrating this distortion is required to precisely place the science targets on the instruments’ field of view. For example, to get the spectra of a hundred galaxies simultaneously using the NIRSpec microshutter assembly, the telescope must be pointed so that each galaxy is in the proper shutter, and there are a quarter of a million shutters!

“We will also measure the sharpness of the stellar images, what astronomers call the ‘point spread function.’ We already know the telescope is delivering to the instruments image quality that exceeds our prelaunch expectations, but each instrument has additional optics. These optics perform a function, such as passing the light through filters to get color information about the astronomical target or using a diffraction grating to spread the incoming light into its constituent colors. Measuring the point spread function within each instrument at different wavelengths provides an important calibration for interpreting the data.

“We will test target acquisition for each instrument. For some observations, it is sufficient to point the telescope using the position of a guide star in the Fine Guidance Sensor and know the location of the science target relative to that guide star. This places the science target to an accuracy of a few tenths of an arcsecond. However, in some cases more precision is necessary, approximately a hundredth of an arcsecond. For example, for coronagraphy, the star has to be placed behind a mask so its light is blocked, allowing the nearby exoplanet to shine through. In time series observations, we measure how an exoplanet’s atmosphere absorbs the stellar light during the hours it takes to pass in front of its star, allowing us to measure the properties and constituents of the planet’s atmosphere. Both of these applications require that the instrument send corrections to the telescope pointing control system to put the science target precisely in the correct location within the instrument’s field of view.

“A final example of our instrument commissioning activities is observations of moving targets. Most astronomical objects are so far away that they appear to be stationary on the sky. However, this is not true of the planets, satellites and rings, asteroids, and comets within our own solar system. Observing these requires that the observatory change its pointing direction relative to the background guide stars during the observation. We will test this capability by observing asteroids of different apparent speeds using each instrument.

“We are now in the last two months of Webb’s commissioning before it is fully ready for its scientific mission. We still have important properties and capabilities of the instruments to test, measure, and demonstrate. When these are complete, we will be ready to begin the great science programs that astronomers and the public alike have been eagerly awaiting. We are almost there.”

– Scott Friedman, lead commissioning scientist for Webb, STScI

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-Jonathan Gardner, Webb deputy senior project scientist, NASA Goddard

-Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard

 

Completion of NASA’s James Webb Space Telescope’s optical alignment has moved us into the final phase of commissioning the Science Instruments. During this final phase the Webb team and instrument scientists will test all the modes and operations for the four science instruments to measure their performance, calibration, and overall observatory operations.

While the mirrors are slowly cooling to their final operating temperatures, the Webb team is preparing for the thermal stability test. We asked Erin Smith, the Webb deputy observatory project scientist, to tell us about the hot and cold of this test.

“Webb’s five-layer sunshield keeps the telescope and science instruments cool and shielded from the Sun, Earth and moon. This protection allows Webb to make measurements of the infrared universe, which requires a cold telescope and cold instrument optics. However, as Webb points to different targets around the sky, the angle of the Sun on the sunshade changes, which changes the thermal profile of the observatory. These variations in temperature can induce small changes in the observatory, and affect Webb’s optical quality, pointing, observed backgrounds, and other parameters.

“The thermal stability exercise will measure these changes by moving between the extremes of Webb’s field of view, from the hot to the cold attitude, spending multiple days in the cold attitude, then slewing back to the hot attitude. During this time, the Webb team will measure the thermal stability, pointing performance and optical wavefront drift. In addition to measuring the performance of the observatory, the team will also check the thermal modeling used to predict observatory behavior.

“With the telescope shielded from the Sun, Webb observes an annulus, or donut, on the sky at any given time, called the “field of regard”. Over the course of each year, this annulus sweeps out the whole sky. Pitch is the angle towards (negative) or away (positive) from the Sun. Webb points between pitches of -5 and +45 degrees. The “hot” attitude is at 0 degrees, with the Sun squarely illuminating the sunshield. The “cold” attitude is +45 degrees, with the sunlight reduced by a factor of cosine(45 degrees), about 0.7.

“To begin the thermal stability test, the Webb team will point the observatory in the hot attitude at about 0 degrees pitch, and keep it there for a five days while it thermally stabilizes. The team will make baseline measurements of the pointing stability, optical wavefront error and any oscillations caused by the instrument electronics. Once this baseline has been established, the team will slew the observatory to the cold attitude, about +40 degrees pitch. Immediately after the slew, the team will use NIRCam’s suite of weak lenses for 24 hours to continuously measure any short-timescale effects on the wavefront. After this, the team will monitor the stability of the telescope every 12 hours, to measure the thermal stabilization of the telescope itself.

“The observatory will spend more than a week at the cold attitude, until the temperatures stabilize. Then Webb will slew back to the hot attitude, and the team will take high-cadence pointing stability data using both the FGS/NIRISS and NIRCam instruments. The MIRI instrument will also make observations at both attitudes, to understand how the changing thermal environment affects the mid-infrared background levels. During this long test, the Observatory will not sit idle; some instrument commissioning activities are compatible with the hot and cold pointings.

“When assembled together, the data from the thermal stability tests will allow the observatory team to better understand how the observatory behaves thermally. Although the changes are expected to be very small, Webb is so sensitive that they could make a difference as we optimize the telescope’s performance. This real-world calibration of the complicated thermal models used by Webb’s developers will help to inform future observing strategies and proposals.”

-Erin Smith, Webb deputy observatory project scientist, NASA Goddard

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-Jonathan Gardner, Webb deputy senior project scientist, NASA Goddard

-Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard

 

 

 

Alignment of NASA’s James Webb Space Telescope is now complete. After full review, the observatory has been confirmed to be capable of capturing crisp, well-focused images with each of its four powerful onboard science instruments. Upon completing the seventh and final stage of telescope alignment, the team held a set of key decision meetings and unanimously agreed that Webb is ready to move forward into its next and final series of preparations, known as science instrument commissioning. This process will take about two months before scientific operations begin in the summer.

The alignment of the telescope across all of Webb’s instruments can be seen in a series of images that captures the observatory’s full field of view.

“These remarkable test images from a successfully aligned telescope demonstrate what people across countries and continents can achieve when there is a bold scientific vision to explore the universe,” said Lee Feinberg, Webb optical telescope element manager at NASA’s Goddard Space Flight Center.

The optical performance of the telescope continues to be better than the engineering team’s most optimistic predictions. Webb’s mirrors are now directing fully focused light collected from space down into each instrument, and each instrument is successfully capturing images with the light being delivered to them. The image quality delivered to all instruments is “diffraction-limited,” meaning that the fineness of detail that can be seen is as good as physically possible given the size of the telescope. From this point forward the only changes to the mirrors will be very small, periodic adjustments to the primary mirror segments.

“With the completion of telescope alignment and half a lifetime’s worth of effort, my role on the James Webb Space Telescope mission has come to an end,” said Scott Acton, Webb wavefront sensing and controls scientist, Ball Aerospace. “These images have profoundly changed the way I see the universe. We are surrounded by a symphony of creation; there are galaxies everywhere! It is my hope that everyone in the world can see them.”

Now, the Webb team will turn its attention to science instrument commissioning. Each instrument is a highly sophisticated set of detectors equipped with unique lenses, masks, filters, and customized equipment that helps it perform the science it was designed to achieve. The specialized characteristics of these instruments will be configured and operated in various combinations during the instrument commissioning phase to fully confirm their readiness for science. With the formal conclusion of telescope alignment, key personnel involved with the commissioning of each instrument have arrived at the Mission Operations Center at the Space Telescope Science Institute in Baltimore, and some personnel involved with telescope alignment have concluded their duties.

Webb Telescope Completes Alignment Phase
Credit: NASA’s Goddard Space Flight Center

Though telescope alignment is complete, some telescope calibration activities remain: As part of scientific instrument commissioning, the telescope will be commanded to point to different areas in the sky where the total amount of solar radiation hitting the observatory will vary to confirm thermal stability when changing targets. Furthermore, ongoing maintenance observations every two days will monitor the mirror alignment and, when needed, apply corrections to keep the mirrors in their aligned locations.

By Thaddeus Cesari, NASA Goddard

The Mid-Infrared Instrument (MIRI) on NASA’s James Webb Space Telescope is now cooled by a gaseous helium cryocooler to under 7 kelvins. With the cooler in its final state, the Webb team is operating the MIRI instrument this week as part of seventh and final stage of the telescope alignment. When the instrument is operating, the detectors and electronics produce heat, which is balanced by the cryocooler to keep MIRI at a stable, and very cold, operating temperature. The near-infrared instruments also warm up during operations and have to dissipate heat, although for these instruments this is done with passive cooling; the heat from the detectors and electronics is radiated into deep space.

Now that the instruments are at their operating temperatures, the telescope mirrors will also continue cooling down to their final temperatures, but they are not quite there yet. The primary mirror segments and the secondary mirror are made of beryllium (coated with gold). At cryogenic temperatures, beryllium has a long thermal time constant, which means that it takes a long time to cool or to heat up. The primary mirror segments are still cooling, very slowly.

The secondary mirror, hanging out on the end of its support structure a long way from any heat sources, is the coldest mirror, currently at 29.4 kelvins. The 18 primary mirror segments range in temperature from 34.4 kelvins to 54.5 kelvins. An advantage of beryllium mirrors is that they don’t change shape with temperature the way glass mirrors would at these temperatures, so the temperature range does not affect the telescope alignment process.

Currently, four of the 18 mirror segments are above 50 kelvins: at 52.6, 54.2, 54.4, and 54.5. These four mirror segments emit some mid-infrared light that reaches the MIRI detectors. Since all the mirror temperatures are now below 55 kelvins, it is expected that MIRI will be sensitive enough to perform its planned science, but any additional cooling of these mirrors will only enhance its performance. The Webb team hopes to see the mirrors cool by an additional 0.5 to 2 kelvins.

When we point the telescope at an astronomical target, the telescope and sunshield move together. The angle that the sunshield presents to the Sun is called the pointing “attitude.” The tiny amount of residual heat that makes its way through the five-layer sunshield to the primary mirror depends on this attitude, and since the mirror segment temperatures change very slowly, their temperatures depend on the attitude averaged over multiple days.

During commissioning, Webb is currently spending most of its time pointed at the ecliptic poles, which is a comparatively hot attitude. During science operations, starting this summer, the telescope will have a much more even distribution of pointings over the sky. The average thermal input to the warmest mirror segments is expected to go down a bit, and the mirrors will cool a bit more.

Later in commissioning, we plan to test the thermal dependence of the mirrors on the attitude. We will point Webb at a hot attitude for several days, and point Webb at a cold attitude for several days, in a process called the thermal slew. This will inform us how long it takes for the mirrors to cool down or heat up when the observatory is at these positions for any given amount of time.

Is Webb at its final temperature? The answer is: almost!

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–Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

The journey of commissioning the Webb telescope continues this week with the successful cooling of the Mid-InfraRed Instrument (MIRI), through the critical ‘pinch point,’ down to its final operating temperature of less than 7 kelvins (-447 degrees Fahrenheit, or -266 degrees Celsius). This was a precondition to completing the seventh and final stage of the mirror alignment process.  The next steps include initial check-outs of MIRI and continue on to the final stages of multi-instrument, multi-field alignment with all four science instruments.

Last week we shared the cool science on star and planet formation planned for Webb. Today, we get into details on how Webb will study planets around other stars, which are known as extrasolar planets, or exoplanets. Knicole Colón, Webb’s deputy project scientist for exoplanet science, takes us into the discovery space of exploring new worlds beyond our solar system. Dr. Colón brings a unique perspective as she is also the project scientist for the Transiting Exoplanet Survey Satellite (TESS), a mission that has found many exoplanet targets that Webb will observe.

“Over the last 30 years, astronomers have discovered over 5,000 extrasolar planets. These discoveries have revealed that exoplanets span a vast range of masses, sizes, and temperatures and orbit all types of stars, leading to extraordinarily diverse worlds.

“With its powerful spectroscopic and imaging capabilities across a wide infrared wavelength range, Webb is poised to revolutionize our knowledge of the composition of these worlds and of planet-forming disks. From small, potentially rocky exoplanets up to giant, gaseous ones, Webb will observe these worlds with the transit technique. Direct imaging techniques will be used to study young, giant exoplanets along with the environments in which planets form and evolve around stars, known as protoplanetary disks and debris disks.

“One specific exoplanet observation that will be done with Webb involves collecting observations over the course of a planet’s orbit to enable measurements of the atmospheric composition and dynamics. I am involved in a program to observe the gas giant HD 80606 b as part of Webb’s first year of observations. Because the orbit of HD 80606 b is extremely eccentric (non-circular) and long (111 days), the amount of energy received by the planet from its star ranges from approximately 1 to 950 times what Earth receives from the Sun! This results in extreme temperature variations, which are predicted to cause clouds to rapidly form and dissipate in the planet’s atmosphere on very short timescales. Our science team will probe these predicted cloud dynamics in real-time over the course of a continuous ~18 hour observation of HD 80606 b as it passes behind its star, using the NIRSpec instrument on Webb to measure thermal light from the planet’s atmosphere.

“Beyond gas giants, a number of Webb’s exoplanet targets in its first year of observations are small and orbit stars that are smaller and cooler than the Sun, known as M dwarfs. While exoplanet discovery began around 30 years ago, many of these small exoplanets around M dwarfs were just discovered in the last few years by surveys like TESS. Webb observations will start to reveal the diversity of atmospheres that exist on these small planets by searching for evidence of molecules like water, carbon dioxide, and methane in their atmospheres. Because M dwarfs are typically much more active than the Sun and have energetic stellar flares that could potentially strip the atmospheres off of these planets, Webb observations may even reveal that some of these small planets have no atmosphere at all.

“With TESS and other surveys continuing to discover additional planets in our galaxy at a regular pace and Webb preparing to study the atmospheres of many of these newly discovered worlds, our exoplanet adventures are in many ways just beginning.”

– Knicole Colón, Webb’s deputy project scientist for exoplanet science, NASA’s Goddard Space Flight Center

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Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard

Jonathan Gardner, Webb deputy senior project scientist, NASA Goddard

 

The ongoing success of the multi-instrument optics alignment for NASA’s Webb telescope’s near-infrared instruments has moved the attention of the commissioning team to chill as we carefully monitor the cooling of the Mid-InfraRed Instrument (MIRI) down to its final operating temperature of less than 7 kelvins (-447 degrees Fahrenheit, or -266 degrees Celsius). We are continuing other activities during this slow cooldown which include monitoring the near-infrared instruments. As MIRI cools, other major components of the observatory, such as the backplane and mirrors, also continue to cool and are approaching their operational temperatures.

Last week, the Webb team did a station-keeping thruster burn to maintain Webb’s position in orbit around the second Lagrange point. This was the second burn since Webb’s arrival at its final orbit in January; these burns will continue periodically throughout the lifetime of the mission.

In the last few weeks, we have been sharing some of Webb’s anticipated science, beginning with the study of the first stars and galaxies in the early universe. Today, we will see how Webb will peer within our own Milky Way galaxy at places where stars and planets form. Klaus Pontoppidan, the Space Telescope Science Institute project scientist for Webb, shares the cool science planned for star and planet formation with Webb:

“In the first year of science operations, we expect Webb to write entirely new chapters in the history of our origins – the formation of stars and planets. It is the study of star and planet formation with Webb that allows us to connect observations of mature exoplanets to their birth environments, and our solar system to its own origins. Webb’s infrared capabilities are ideal for revealing how stars and planets form for three reasons: Infrared light is great at seeing through obscuring dust, it picks up the heat signatures of young stars and planets, and it reveals the presence of important chemical compounds, such as water and organic chemistry.

“Let us look at each reason in more detail. We often hear that infrared light passes through obscuring dust, revealing newborn stars and planets that are still embedded in their parental clouds. In fact, mid-infrared light, as seen by MIRI, can pass through 20 times thicker clouds than visible light. Because young stars are formed quickly (by cosmic standards, anyway) – in as little as a few 100,000 years – their natal clouds have not had time to disperse, hiding what is going on in this critical stage from visible view. Webb’s infrared sensitivity allows us to understand what happens at these very first stages, as gas and dust are actively collapsing to form new stars.

“The second reason has to do with the young stars and giant planets themselves. Both begin their lives as large, puffy structures that contract over time. While young stars tend to get hotter as they mature, and giant planets cool, both typically emit more light in the infrared than at visible wavelengths. That means that Webb is great at detecting new young stars and planets and can help us understand the physics of their earliest evolution. Previous infrared observatories, like the Spitzer Space Telescope, used similar techniques for the nearest star-forming clusters, but Webb will discover new young stars across the galaxy, the Magellanic Clouds, and beyond.

“Finally, the infrared range (sometimes called the “molecular fingerprint region”) is ideal for identifying the presence of a range of chemicals, in particular water and various organics. All four of Webb’s science instruments can detect various important molecules using their spectroscopic modes. They are particularly sensitive to molecular ices present in cold molecular clouds before stars are formed, and NIRCam and NIRSpec will, for the first time, comprehensively map the spatial distribution of ices to help us understand their chemistry. MIRI will also observe warm molecular gas near many young stars where rocky, potentially habitable planets may be forming. These observations will be sensitive to most bulk molecules and will allow us to develop a chemical census at the earliest stages of planet formation. It is no surprise that a significant number of Webb’s early scientific investigations aim to measure how planetary systems build the molecules that may be important for the emergence of life as we know it.

“We will be keeping a close eye on MIRI as it cools down. As the only mid-infrared instrument on Webb, MIRI will be particularly important for understanding the origins of stars and planets.”

–Klaus Pontoppidan, Webb project scientist, Space Telescope Science Institute

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By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

And Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard

“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.

“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).

“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