[LINK] Webb Space Telescope

Fri Mar 18 21:00:11 AEDT 2022

NASA's Webb space telescope achieves near-perfect focus, whetting appetites for discoveries to come

BY WILLIAM HARWOOD  MARCH 16, 2022 / 4:38 PM / CBS NEWS
https://www.cbsnews.com/news/nasa-webb-space-telescope-near-perfect-focus/

After weeks of microscopic adjustments, NASA unveiled the first fully focused image from the James Webb Space Telescope Wednesday, a razor-sharp engineering photo of a nondescript star in a field of more distant galaxies that shows the observatory's optical system is working in near-flawless fashion.

The goal was to demonstrate Webb can now bring starlight to a near-perfect focus, proving the $10 billion telescope doesn't suffer from any subtle optical defects like the aberration that initially hobbled the Hubble Space Telescope.

The galaxies in the image were a bonus, whetting astronomers' appetites for discoveries to come.

"This is one of the most magnificent days in my whole career at NASA, frankly, and for many of us astronomers, one of the most important days that we've had," said NASA science chief Thomas Zurbuchen.

"Today we can announce that the optics will perform to specifications or even better. It's an amazing achievement."

[Photo 031622-star.jpg A nondescript star used to help align the James Webb Space Telescope's optics shows a near-perfect focus in a major milestone for the observatory. The image also shows dim, remote galaxies strewn across the field of view, demonstrating Webb's sensitivity to infrared light.]  NASA/STSCI

Engineers and scientists still must calibrate Webb's science instruments and make iterative adjustments to ensure they all receive perfectly focused light, but astronomers now know the telescope, the most complex — and expensive — science spacecraft ever built, will almost certainly work as advertised.

"I'm happy to say that the optical performance of the telescope is absolutely phenomenal, it is really working extremely well," Lee Feinberg, Webb optical telescope element manager at the Goddard Space Flight Center, told reporters.

"We said last fall that we would know that the telescope is working properly when we have an image of a star that looks like a star. And now we have that, and you're seeing that image."

The star in question, one of several used in the alignment process, is known as 2MASS J17554042+6551277. It was imaged in a 2,100-second exposure at the end of a long process to precisely align the 18 segments making up Webb's 21.3-foot-wide primary mirror.

Marshall Perrin, Webb deputy telescope scientist at the Space Telescope Science Institute in Baltimore, said the star selected for the image was a "pretty much generic, anonymous star in the sky that worked well for the kind of sensing measurements we needed to do."

"It's about 100 times fainter than the human eye could see," he added, giving viewers "a sense of how sensitive the telescope is."

The "rays" extending from the star were caused by the hexagonal shape of Webb's primary mirror segments and so-called diffraction spikes, the result of light bending around the three struts holding the telescope's secondary mirror in place.

Such artifacts are normal with large telescopes and bright, relatively nearby stars. But they do not form around the dimmer stars and galaxies Webb was designed to study.

[Photo 031622-selfie.jpg  A "selfie" showing the precisely aligned segments making up Webb's 21.3-foot-wide primary mirror (the three dark lines making an inverted Y are supports holding Webb's secondary mirror in place]. NASA/STSCI

In any case, the mirror segments were initially aligned to within about a millimeter of each other.

To align them so all 18 act together as a single mirror, "they need to be lined up to within a few nanometers (billionths of a meter) one another, it's a few hundred atomic diameters, the level of precision that we need here," said Perrin.

Using seven actuators on the back of each mirror segment — six to control tip and tilt and one to push or pull on the center of a segment, if required, to slightly change its shape — the 18 initially separate reflections of an alignment star were brought to a single point. One of Webb's instruments, the Near Infrared Camera, or NIRCam, was used to map out the reflections.

"We have now finished the fourth and fifth phases of the telescope alignment, we call those phases course phasing and fine phasing," Feinberg said. "And that's where we've made the primary mirror, all 18 mirror segments, (act as) a single primary mirror."

Perrin said the biggest surprise since Webb's launch was "just how closely it matched the models and predictions from the ground."

"It has been far closer to those predictions than then a lot of us had dared to hope," he said. "And we now have achieved what's called diffraction-limited alignment of the telescope, the images are focused together as finely as the laws of physics allow. This is as sharp an image as you can get from a telescope of this size."

Jane Rigby, the Webb operations project scientist at Goddard, agreed, saying "the telescope performance so far is everything that we dared hope."

"The goal here was to build a telescope 100 times more powerful than anything we've had before," she said.

"From the early engineering data that we have seen so far, we know that we're on track to meet those very demanding science requirements."

Scientists and engineers now must calibrate Webb's other science instruments and make the tiny adjustments necessary to ensure perfectly focused light reaches each one.

Rigby said based on results to date, the end of the commissioning phase and the transition to science observations remains on track for the late June-July timeframe.

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> Sent: Thursday, 17 February 2022 6:35 PM
> Subject: [LINK] Photons Received: Webb Sees Its First Star

James Webb Space Telescope:

NASA Blogs Home

“Photons Received: Webb Sees Its First Star – 18 Times”

https://blogs.nasa.gov/webb/2022/02/11/photons-received-webb-sees-its-first-star-18-times/

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.

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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