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xx James Webb Update
« Thread started on: May 2nd, 2017, 11:38am »

World's Largest Space Telescope Graduates Goddard Testing, Heads to Texas

By Sarah Lewin, Staff Writer
May 2, 2017

The James Webb Space Telescope is on the road again. After passing its final test at NASA's Goddard Space Flight Center in Greenbelt, Maryland, the megatelescope is ready for the next stop on its trip to space: further testing at Johnson Space Center in Houston.

The final mirrors for the giant space observatory arrived at Goddard in 2014, and the telescope's construction was finally completed in November 2016, after more than 20 years of construction. Since then, the instrument has endured a battery of testing to ensure that it can withstand the rigors of launch and deep space. Webb is slated to launch in 2018, when it will become the world's largest telescope to fly to space.
NASA highlighted some of Webb's testing in a new video:

http://www.space.com/36686-james-webb-space-telescopes-testing-at-nasa-goddard-highlighted-video.html

The final test at Goddard checked the curvature of the telescope's mirrors to see whether they had become warped during the year of intensive testing. To test this, engineers precisely measured the interference patterns of lasers reflected off of the mirrors, and then compared the measurements to those taken before environmental testing began last year, NASA officials said in a statement. The telescope's mirrors came out unchanged by the many stresses of simulated spaceflight.

"The Webb telescope is about to embark on its next step in reaching the stars as it has successfully completed its integration and testing at Goddard," Bill Ochs, NASA's Webb telescope project manager, said in a statement. "It has taken a tremendous team of talented individuals to get to this point from all across NASA, our industry, and international partners and academia.

"It is also a sad time, as we say goodbye to the Webb telescope at Goddard, but [we] are excited to begin cryogenic testing at Johnson," he added.

Once it arrives at Johnson, the telescope will be put to the ultimate test: The entire scope's optics will be tested in a vacuum in the space center's massive Chamber A and cooled to 11 degrees above absolute zero (minus 440 degrees Fahrenheit, or minus 262 degrees Celsius). Afterward, the telescope will continue on to Northrop Grumman Aerospace Systems in Redondo Beach, California, for its final testing and assembly. It will then go to French Guiana for launch.

Webb will probe the cosmos from a spot in space called Lagrange Point 2, located directly behind Earth from the sun's perspective, where the telescope can use one shield to protect itself from both the sun's and Earth's thermal emissions. From there, Webb will gather infrared views of the universe's first galaxies and of planets around distant stars; the use of infrared light will allow Webb to peer through interstellar dust for a better view.

NASA has positioned Webb as the Hubble Space Telescope's successor; the new instrument has seven times the collecting area of the famed Hubble instrument, and is chilled cold enough to collect infrared light that Hubble cannot. That will allow Webb to see even farther into space's outer reaches.

http://www.space.com/36684-james-webb-telescope-graduates-goddard-testing.html

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xx Re: James Webb Update
« Reply #1 on: May 13th, 2017, 11:16am »

Two James Webb instruments are best suited for exoplanet atmospheres

Date: May 9, 2017
Source: Penn State

The best way to study the atmospheres of distant worlds with the James Webb Space Telescope, scheduled to launch in late 2018, will combine two of its infrared instruments, according to a team of astronomers.

"We wanted to know which combination of observing modes (of Webb) gets you the maximum information content for the minimum cost," says Natasha Batalha, graduate student in astronomy and astrophysics and astrobiology, Penn State, and lead scientist on this project.

"Information content is the total amount of information we can get from a planet's atmospheric spectrum, from temperature and composition of the gas -- like water and carbon dioxide -- to atmospheric pressures."

Batalha and Michael Line, assistant professor, School of Earth and Space Science, Arizona State University, developed a mathematical model to predict the quantity of information that different Webb instruments could extract about an exoplanet's atmosphere.

Their model predicts that using a combination of two infrared instruments -- the Near Infrared Imager and Slitless Spectrograph (NIRISS) and the G395 mode on the Near Infrared Spectrograph (NIRSpec) -- will provide the highest information content about an exoplanet's atmosphere.

NIRISS is a versatile camera and spectrograph that will observe infrared wavelengths similar to those the Hubble Telescope covers. NIRISS, according to Batalha and Line, should be combined with the G395 mode on NIRSpec, which will observe targets in longer infrared wavelengths at Webb's highest resolution.

Three main characteristics affect how much information an instrument can extract -- resolution, maximum observable brightness, and wavelength range. These combined determine the total observable fraction of the information content of a planet's atmospheric spectrum.

Both NIRISS and NIRSpec will observe near-infrared wavelengths, the region of the electromagnetic spectrum in which the stars that exoplanets orbit around shine brightest. NIRISS is poised to measure a strong signature of water and NIRSpec can do the same for methane and carbon dioxide, three chemical compounds that provide a substantial amount of information about an atmosphere.

Batalha and Line tested each of ten likely observing methods on its own and in every possible combination with the other methods to determine which would maximize the total information content.

They retrieved the information from a set of simulated planets with temperatures and compositions that cover the range of previously observed exoplanet atmospheres. By comparing the retrievable information content in each planet's atmosphere, Batalha and Line found that this one combination of NIRISS and NIRSpec modes gives the most information regardless of the exoplanet's temperature or composition. The researchers published these results in The Astronomical Journal.

"We won't know a planet's temperature ahead of time," says Batalha. "If you're going to do a shot in the dark observation, you have the greatest chance of getting the information you want with this combination of instruments."

As an exoplanet crosses between its host star and Earth's telescopes, some of the star's light passes through the exoplanet's atmosphere. The exo-atmosphere leaves its fingerprint in the star's light -- the planet's transmission spectrum -- from which astronomers can learn about the exo-atmosphere's temperature, chemical composition and structure. The researchers' information content analysis focuses on the information retrievable from the transmission spectrum of a planet.

While Webb will not launch until late 2018, but astronomers are already planning the first set of observations they would like from the telescope.

"If we can strategize now," says Batalha, "by the time the first cycle of formal proposals comes around we can ensure that we are picking the best modes for larger proposals and not waste valuable observing time. This way everyone starts on an even playing field with the science."

While they highlight two NIRISS and NIRSpec modes as the best combination for observing most exo-atmospheres, Batalha and Line explain that the other modes will still be useful to observe different features of exo-atmospheres that the astronomers have not tested for, like clouds, haze and atmospheres hot enough to emit their own light.

"In the future," Batalha says, "there will be a push to characterize the first Earth 2.0. If we don't nail this down now and master the art of characterizing exo-atmospheres, we will never accurately characterize Earth 2.0."
________________________________________
Story Source:
Materials provided by Penn State.

https://www.sciencedaily.com/releases/2017/05/170509145211.htm

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xx Re: James Webb Update
« Reply #2 on: Aug 26th, 2017, 5:51pm »

Aug. 24, 2017

NASA’s Webb Telescope Will Study Our Solar System’s “Ocean Worlds”

NASA’s James Webb Space Telescope will use its infrared capabilities to study the “ocean worlds” of Jupiter’s moon Europa and Saturn’s moon Enceladus, adding to observations previously made by NASA’s Galileo and Cassini orbiters. The Webb telescope’s observations could also help guide future missions to the icy moons.

Europa and Enceladus are on the Webb telescope’s list of targets chosen by guaranteed time observers, scientists who helped develop the telescope and thus get to be among the first to use it to observe the universe. One of the telescope’s science goals is to study planets that could help shed light on the origins of life, but this does not just mean exoplanets; Webb will also help unravel the mysteries still held by objects in our own solar system (from Mars outward).

Continue
https://www.nasa.gov/feature/goddard/2017/nasa-s-webb-telescope-will-study-our-solar-system-s-ocean-worlds
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xx Re: James Webb Update
« Reply #3 on: Oct 28th, 2017, 09:30am »

James Webb Space Telescope's laser-focused sight

Date: October 26, 2017
Source: NASA/Goddard Space Flight Center

Summary:
About 1 million miles away from the nearest eye surgeon, NASA's James Webb Space Telescope will be able to perfect its own vision while in orbit.

Though the Webb telescope will focus on stars and galaxies approximately 13.5 billion light-years away, its sight goes through a similar process as you would if you underwent laser vision correction surgery to be able to focus on an object 10 feet across the room. In orbit at Earth's second Lagrange point (L2), far from the help of a terrestrial doctor, Webb will use its near-infrared camera (NIRCam) instrument to help align its primary mirror segments about 40 days after launch, once they have unfolded from their unaligned stowed position and cooled to their operating temperatures.

Laser vision correction surgery reshapes the cornea of the eye to remove imperfections that cause vision problems like nearsightedness. The cornea is the surface of the eye; it helps focus rays of light on the retina at the back of the eye, and though it appears to be uniform and smooth, it can be misshapen and pockmarked with dents, dimples, and other imperfections that can affect a person's sight. The relative positioning of Webb's primary mirror segments after launch will be the equivalent of these corneal imperfections, and engineers on Earth will need to make corrections to the mirrors' positions to bring them into alignment, ensuring they will produce sharp, focused images.

These corrections are made through a process called wavefront sensing and control, which aligns the mirrors to within tens of nanometers. During this process, a wavefront sensor (NIRCam in this case) measures any imperfections in the alignment of the mirror segments that prevent them from acting like a single, 6.5-meter (21.3-foot) mirror. An eye surgeon performing wavefront-guided laser vision correction surgery (a process that was improved by technology developed to shape Webb's mirrors) similarly measures and three-dimensionally maps any inconsistencies in the cornea. The system feeds this data to a laser, the surgeon customizes the procedure for the individual, and the laser then reshapes and resurfaces the cornea according to that procedure.

Engineers on Earth will not use a laser to melt and reshape Webb's mirrors (feel free to give a sigh of relief); instead, they will use NIRCam to take images to determine how much they need to adjust each of the telescope's 18 primary mirror segments. They can adjust the mirror segments through extremely minute movements of each segment's seven actuators (tiny mechanical motors) -- in steps of about 1/10,000th the diameter of a human hair.

The wavefront sensing and control process is broken into two parts -- coarse phasing and fine phasing.

During coarse phasing, engineers point the telescope toward a bright star and use NIRCam to find any large offsets between the mirror segments (though "large" is relative, and in this case it means mere millimeters). NIRCam has a special filter wheel that can select, or filter, specific optical elements that are used during the coarse phasing process. While Webb looks at the bright star, grisms in the filter wheel will spread the white light of the star out on a detector. Grisms, also called grating prisms, are used to separate light of different wavelengths. To an observer, these different wavelengths appear as parallel line segments on a detector.

"The light from each segment will interfere with adjacent segments, and if the segments are not aligned to better than a wavelength of light, that interference shows up like barber pole patterns," explained Lee Feinberg, optical telescope element manager for the Webb telescope at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The analysis of the barber pole patterns tell the engineers how to move the mirrors."

During fine phasing, engineers will again focus the telescope on a bright star. This time, they will use NIRCam to take 18 out-of-focus images of that star -- one from each mirror segment. The engineers then use computer algorithms to determine the overall shape of the primary mirror from those individual images, and to determine how they must move the mirrors to align them. These algorithms were previously tested and verified on a 1/6th scale model of Webb's optics, and the real telescope experienced this process inside the cryogenic, airless environment of Chamber A at NASA's Johnson Space Center in Houston. Engineers will go through multiple fine-phasing sessions until those 18 separate, out-of-focus images become a single, clear image.

After the engineers align the primary mirror segments, they must align the secondary mirror to the primary, then align both the primary and secondary mirrors to the tertiary mirror and the science instruments. Though the engineers complete the initial alignment with NIRCam, Feinberg explained they also test the alignment with Webb's other instruments to ensure the telescope is aligned "over the full field."

The entire alignment process is expected to take several months, and once Webb begins making observations, its mirrors will need to be checked every few days to ensure they are still aligned -- just as someone who underwent laser vision correction surgery will schedule regular eye doctor visits to make sure their vision is not degrading.
________________________________________
Story Source:
Materials provided by NASA/Goddard Space Flight Center. Note: Content may be edited for style and length.

https://www.sciencedaily.com/releases/2017/10/171026135248.htm

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