Month: February 2022

Webb continues on its path to becoming a focused observatory. The team has successfully worked through the second and third out of seven total phases of mirror alignment. With the completion of these phases, called Segment Alignment and Image Stacking, the team will now begin making smaller adjustments to the positions of Webb’s mirrors.

After moving what were 18 scattered dots of starlight into Webb’s signature hexagonal formation, the team refined each mirror segment’s image by making minor adjustments, while also changing the alignment of Webb’s secondary mirror. The completion of this process, known as Segment Alignment, was a key step prior to overlapping the light from all the mirrors so that they can work in unison.

Once Segment Alignment was achieved, the focused dots reflected by each mirror were then stacked on top of each other, delivering photons of light from each segment to the same location on NIRCam’s sensor. During this process, called Image Stacking, 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.

“We still have work to do, but we are increasingly pleased with the results we’re seeing,” said Lee Feinberg, optical telescope element manager for Webb at NASA’s Goddard Space Flight Center. “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.”

Although Image Stacking put all the light from a star in one place on NIRCam’s detector, the mirror segments are still acting as 18 small telescopes rather than one big one. The segments now need to be lined up to each other with an accuracy smaller than the wavelength of the light.

The team is now starting the fourth phase of mirror alignment, known as Coarse Phasing, where NIRCam is used to capture light spectra from 20 separate pairings of mirror segments. This helps the team identify and correct vertical displacement between the mirror segments, or small differences in their heights. This will make the single dot of starlight progressively sharper and more focused in the coming weeks.

This week, as the Webb team continues to make progress in aligning the telescope, other successful activities include the calibration of the NIRISS filter wheel and pupil wheel tuning for NIRCam. There are hundreds of activities like these planned during the commissioning process, and each is as important as the next to ensure that Webb can achieve its ambitious science goals. One such goal – detecting the earliest galaxies – also requires a lot of planning and theory to prepare for the observations. L.Y. Aaron Yung, a postdoc at NASA’s Goddard Space Flight Center, tells us more about the important theoretical work that helps plan for and then analyze galaxy surveys:

“This summer, Webb will start searching for galaxies in the distant universe. These highly anticipated observations are the key to unlocking the secrets in galaxy evolution and our universe’s history. Depending on the specific science goal of an observing program, the best-suited survey configurations can vary a lot.

“For instance, galaxy surveys going after the faintest and most distant galaxies require long exposure times (e.g., NGDEEP, PRIMER), but surveys for large-scale cosmological structure would require large survey areas (e.g., COSMOS-Web). Inputs from physically motivated simulations are essential to developing optimal observing strategies to achieve the specific scientific goals.

“To create a simulated universe, we first lay the foundation with dark matter concentrations, or halos, extracted from cosmological simulations. Dark matter accounts for 85% of the matter in the universe and has a dominant effect on the spatial distributions of galaxies across the universe. We then simulate the galaxies forming inside these dark matter halos based on astrophysical processes we learned from past observations.

“This figure illustrates an example of a portion of a simulated universe arranged in the shape of a cone traced by our sightlines. Because light travels at a finite speed, the light that originated in the early universe has travelled billions of years before finally reaching us today. This effectively allows us to look back in time and see the universe billions of years into its past.

“Our simulated universe serves as the basis to create mock observing fields that are statistically similar to the observed universe. The physically motivated models have been shown to match the galaxies observed by Hubble (e.g., the Hubble Ultra-Deep Field), and we use them to provide predictions for galaxies beyond Hubble’s capabilities.

“We process the simulated universe further into realistic mock images by adding effects from scientific instruments and survey configurations. These data products are used to support the development of the Webb data reduction pipeline and will inform the interpretation of future observations when they become available.

“Webb will detect, for the first time in human history, galaxy populations forming shortly after the big bang, and theory is paving the way for the search. In turn, Webb observations will refine our understanding of galaxies and the history of our universe.”

–Dr. L. Y. Aaron Yung, NASA postdoctoral program fellow, NASA Goddard

As Webb continues its commissioning activities on the way to normal operations, we will start to preview anticipated science on this blog in addition to providing updates on the latest observatory activities.

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

And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute

The Webb team continues to make progress in aligning the observatory’s mirrors. Engineers have completed the first stage in this process, called “Segment Image Identification.” The resulting image shows that the team has moved each of Webb’s 18 primary mirror segments to bring 18 unfocused copies of a single star into a planned hexagonal formation.

This image mosaic (top), which shows 18 randomly positioned copies of the same star, served as the starting point for the alignment process. To complete the first stage of alignment, the team moved the primary mirror segments to arrange the dots of starlight into a hexagonal image array (bottom). Each dot of starlight is labeled with the corresponding mirror segment that captured it.
Credits: NASA (top); NASA/STScI/J. DePasquale (bottom)

With the image array complete, the team has now begun the second phase of alignment: “Segment Alignment.” During this stage, the team will correct large positioning errors of the mirror segments and update the alignment of the secondary mirror, making each individual dot of starlight more focused. When this “global alignment” is complete, the team will begin the third phase, called “Image Stacking,” which will bring the 18 spots of light on top of each other.

“We steer the segment dots into this array so that they have the same relative locations as the physical mirrors,” said Matthew Lallo, systems scientist and Telescopes Branch manager at the Space Telescope Science Institute. “During global alignment and Image Stacking, this familiar arrangement gives the wavefront team an intuitive and natural way of visualizing changes in the segment spots in the context of the entire primary mirror. We can now actually watch the primary mirror slowly form into its precise, intended shape!”

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

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

And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute

Lee esta nota en español aquí.

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.

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.

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.

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By Thaddeus Cesari, Webb science writer, NASA’s Goddard Space Flight Center, Greenbelt, Md.

While we have started the long process of aligning the telescope mirrors, almost all of the components on Webb’s cold side are still continuing to cool.

Webb’s giant sunshield keeps the telescope and cameras out of both direct sunlight and sunlight that is reflected from Earth and the Moon. Everything on the cold side of the sunshield is passively cooling, radiating heat into deep space. That will continue until the telescope and the three near-infrared (NIR) instruments reach a steady-state temperature, where the milliwatts of energy that get through the sunshield, plus heat generated by the instruments’ own electronics, exactly balances the loss of heat into space. We expect that the primary mirror will cool to below 50 kelvins (about -370 degrees Fahrenheit, or -223 degrees Celsius), and the NIR instruments will reach about 40 kelvins (about -388 degrees Fahrenheit, or -233 degrees Celsius).

Webb’s Mid-Infrared Instrument (MIRI) needs to be even colder. In addition to passive cooling, MIRI will be cooled by a closed-cycle gaseous-helium cryocooler, or refrigerator, down to a temperature below 7 kelvins (-447 degrees Fahrenheit, or -266 kdegrees Celsius). Unlike some previous cryogenic missions, which were cooled by boiling off liquid helium and venting it into space, MIRI’s cooler reuses its helium, just like the refrigerator in your kitchen continuously recycles its own coolant. The Webb team turned on the first stage of the MIRI cryocooler this week.

In the several weeks since Webb’s sunshield was deployed, Webb’s mirrors have been cooling, but they aren’t at their final temperatures yet. There is a variation of temperatures across the different segments, and the segments closer to the sunshield and spacecraft bus are warmer. We expect that these mirror segments will all cool by another 10 kelvins or so, but their final temperatures will still have a spread of 15 to 20 kelvins. The secondary mirror, hanging out on the end of its “spider” support structure, is already very cold.

Meanwhile, the NIR instruments are also cooling. Early in the cooldown process, the Webb team used heaters to keep the instruments warmer than the cold-side structures, to prevent water ice from forming on the optical surfaces. But that is all done now, and the instruments and their detectors are cooling nicely. Their current temperatures are about 75 kelvins (-325 degrees Fahrenheit, or -198 degrees Celsius); they will continue to cool for a few more weeks before reaching their final operating temperature.

The cooling of an infrared telescope is a precise and critical process to ensure the success of the instrumentation and, ultimately, the amazing science. We have learned from and improved upon many years of infrared missions. Webb’s historian, Robert W. Smith, explains a bit more about how Webb builds on the legacy of previous infrared observatories:

“Pioneering investigators examined various astronomical objects in the infrared from the year 1800 on. Infrared astronomy, however, began to take off only in the 1960s. Given the limitations imposed by the atmosphere, researchers experimented with telescopes on balloons and rockets.

“Nevertheless, the grand prize was an infrared telescope in space not limited to the five or so minutes observing time of a rocket flight. Efforts in the U.S., the Netherlands, and the United Kingdom led to the Infrared Astronomy Satellite (IRAS). Launched in 1983, IRAS surveyed the skies at a range of wavelengths and, during its ten-month lifetime, identified 350,000 infrared sources. The Infrared Space Observatory (ISO) followed IRAS in 1995. It became the first infrared space telescope to exploit arrays of detectors of the sort that had begun to revolutionize ground-based infrared astronomy in the years around 1990.

“Critical for the future of infrared space telescopes was the radical shift to radiative or passive cooling. The mirrors of infrared telescopes emit infrared radiation, and to observe the infrared signals emitted by astronomical sources, many of which are exceedingly faint, the mirrors need to be kept very cold. Both IRAS and ISO had kept their telescopes cold by placing them inside a Dewar filled with liquid helium. But adopting this approach seriously limited the size of the telescope that could fly. Tim Hawarden of the Royal Observatory, Edinburgh, began in the early 1980s to explore the idea of doing away with the Dewar. Instead, a telescope would be launched warm and cooled by radiating heat away to space.

“The first infrared space telescope to use passive cooling was NASA’s Spitzer Space Telescope, launched in 2003 into an Earth-trailing orbit. The primary mirror cooled passively to about 34 kelvins before liquid helium was used to get the observatory to less than 6 kelvins. The Herschel Space Observatory, an ESA (European Space Agency) project, had a passively cooled primary mirror (to 80 kelvins) with liquid helium cooled instruments. Herschel operated from 2009 to 2013 and orbited around the L2 Lagrange point, similar to Webb. Herschel’s 3.5-meter diameter mirror made it the largest infrared telescope before Webb.

“In 1989, at a workshop at the Space Telescope Science Institute, astronomers explored ideas for the ‘Next-Generation U-V-Visible-IR Telescope’ to succeed Hubble. These discussions led to the suggestion of an infrared optimized telescope, the ‘Next Generation Space Telescope,’ the vision of which was realized in the world’s largest and most powerful infrared observatory: Webb.”

-Robert W. Smith, professor of history, University of Alberta

Check back tomorrow for an exciting update about progress in the first weeks of Webb’s mirror alignment!

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

And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute

This week, the three-month process of aligning the telescope began – and over the last day, Webb team members saw the first photons of starlight that traveled through the entire telescope and were detected by the Near Infrared Camera (NIRCam) instrument. This milestone marks the first of many steps to capture images that are at first unfocused and use them to slowly fine-tune the telescope. This is the very beginning of the process, but so far the initial results match expectations and simulations.

A team of engineers and scientists from Ball Aerospace, Space Telescope Science Institute, and NASA’s Goddard Space Flight Center will now use data taken with NIRCam to progressively align the telescope. The team developed and demonstrated the algorithms using a 1/6th scale model telescope testbed. They have simulated and rehearsed the process many times and are now ready to do this with Webb. The process will take place in seven phases over the next three months, culminating in a fully aligned telescope ready for instrument commissioning. The images taken by Webb during this period will not be “pretty” images like the new views of the universe Webb will unveil later this summer. They strictly serve the purpose of preparing the telescope for science.

To work together as a single mirror, the telescope’s 18 primary mirror segments need to match each other to a fraction of a wavelength of light – approximately 50 nanometers. To put this in perspective, if the Webb primary mirror were the size of the United States, each segment would be the size of Texas, and the team would need to line the height of those Texas-sized segments up with each other to an accuracy of about 1.5 inches.

Scott Acton and Chanda Walker of Ball Aerospace, along with Lee Feinberg of NASA Goddard, walk through the basic steps below:

“With deployment of the mirror segments now complete, and the instruments turned on, the team has begun the numerous steps required to prepare and calibrate the telescope to do its job. The telescope commissioning process will take much longer than previous space telescopes because Webb’s primary mirror consists of 18 individual mirror segments that need to work together as a single high-precision optical surface. The steps in the commissioning process include:

1. 1. 1. Segment Image Identification
      2. Segment Alignment
      3. Image Stacking
      4. Coarse Phasing
      5. Fine Phasing
      6. Telescope Alignment Over Instrument Fields of View
      7. Iterate Alignment for Final Correction

1. Segment Image Identification

First, we need to align the telescope relative to the spacecraft. The spacecraft is capable of making extremely precise pointing moves, using “star trackers.” Think of star trackers as a GPS for spacecraft. At first, the position of the spacecraft from the star trackers does not match the position of each of the mirror segments.

We are pointing the telescope at a bright, isolated star (HD 84406) to capture a series of images that are then stitched together to form a picture of that part of the sky. But remember, we don’t have just one mirror looking at this star; we have 18 mirrors, each of which is initially tilted towards a different part of the sky. As a result, we’ll actually capture 18 slightly shifted copies of the star – each one out of focus and uniquely distorted. We refer to these initial star-copies as “segment images.” In fact, depending on the starting positions of the mirrors, it may take multiple iterations to locate all 18 segments in one image.

One by one, we will move the 18 mirror segments to determine which segment creates which segment image. After matching the mirror segments to their respective images, we can tilt the mirrors to bring all the images near a common point for further analysis. We call this arrangement an “image array.”

2. Segment Alignment

After we have the image array, we can perform Segment Alignment, which corrects most of the large positioning errors of the mirror segments.

We begin by defocusing the segment images by moving the secondary mirror slightly. Mathematical analysis, called Phase Retrieval, is applied to the defocused images to determine the precise positioning errors of the segments. Adjustments of the segments then result in 18 well-corrected “telescopes.” However, the segments still don’t work together as a single mirror.

3. Image Stacking

To put all of the light in a single place, each segment image must be stacked on top of one another. In the Image Stacking step, we move the individual segment images so that they fall precisely at the center of the field to produce one unified image. This process prepares the telescope for Coarse Phasing.

The stacking is performed sequentially in three groups (A-segments, B-segments, and C-segments).

4. Coarse Phasing

Although Image Stacking puts all the light in one place on the detector, the segments are still acting as 18 small telescopes rather than one big one. The segments need to be lined up with each other with an accuracy smaller than the wavelength of the light.

Conducted three times during the commissioning process, Coarse Phasing measures and corrects the vertical displacement (piston difference) of the mirror segments. Using a technology known as Dispersed Fringe Sensing, we use NIRCam to capture light spectra from 20 separate pairings of mirror segments. The spectrum will resemble a barber pole pattern with a slope (or angle) determined by the piston difference of the two segments in the pairing.

5. Fine Phasing

Fine Phasing is also conducted three times, directly after each round of Coarse Phasing, and then routinely throughout Webb’s lifespan. These operations measure and correct the remaining alignment errors using the same defocusing method applied during Segment Alignment. However, instead of using the secondary mirror, we use special optical elements inside the science instrument which introduce varying amounts of defocus for each image (-8, -4, +4, and +8 waves of defocus).

6. Telescope Alignment Over Instrument Fields of View

After Fine Phasing, the telescope will be well aligned at one place in the NIRCam field of view. Now we need to extend the alignment to the rest of the instruments.

In this phase of the commissioning process, we make measurements at multiple locations, or field points, across each of the science instruments, as shown below. More variation in intensity indicates larger errors at that field point. An algorithm calculates the final corrections needed to achieve a well-aligned telescope across all science instruments.

7. Iterate Alignment for Final Correction

After applying the Field of View correction, the key thing left to address is the removal of any small, residual positioning errors in the primary mirror segments. We measure and make corrections using the Fine Phasing process. We will do a final check of the image quality across each of the science instruments; once this is verified, the wavefront sensing and controls process will be complete.

As we go through the seven steps, we may find that we need to iterate earlier steps as well. The process is flexible and modular to allow for iteration. After roughly three months of aligning the telescope, we will be ready to proceed to commissioning the instruments.”

—Scott Acton, Webb lead wavefront sensing and control scientist, Ball Aerospace; Chanda Walker, Webb wavefront sensing and control scientist, Ball Aerospace; and Lee Feinberg, Webb optical telescope element manager, NASA’s Goddard Space Flight Center