May 16, 2022

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Wondering about the six rays out of a JWST test image?  Here's why this happens

Wondering about the six rays out of a JWST test image? Here’s why this happens

In the Space Telescope Science Institute (STSI) In Baltimore, Maryland, NASA engineers are busy aligning the mirrors and instruments on James Webb Space Telescope (JWST). In the meantime, the expedition team has given us another glimpse of what it considers this observatory – the successor to the venerable observatory Hubble Space Telescope – You will see once it is fully turned on. The last joke is a “Telescope Alignment Assessment Image” of a distant star that appears red and spiky!

This milestone marks the completion of the fifth stage of preparation, known as the “micro-stages,” in which mission controllers modified each part of Webb’s primary mirror sections to produce a uniform image using only the near-infrared camera (NIRCam). This image focused on a bright star in the center of the JWST alignment. This star is known as 2 Diamonds J17554042 + 6551277 It is located about 2,000 light-years from Earth.

The sensitivity of the Webb and NIRCam optics (and a red filter that improves visual contrasts) meant that background galaxies and stars were visible as well. But while the stars and galaxies in the background are billions of years away (and slightly distorted), the foreground star is rising in appearance. This is known as Diffraction spikes (or “spider”), which refers to artifacts created by a telescope’s secondary mirror or aperture.

Image of an alignment evaluation star, called 2MASS J17554042 + 6551277. Credit: NASA/STScI

according to Dr. Christopher S. BirdAssistant Professor of Physics at West Texas A&M University:

Some telescopes have a large primary mirror that focuses the incoming light beam onto a secondary mirror or sensor mounted above the primary mirror. The secondary mirror diverts the light from the telescope so that it can be seen or processed further. Or, alternately, a sensor mounted above the primary mirror transforms the image into an electrical signal that is connected to a computer.”

The key to diffraction heights, Bird writes, is that the secondary mirror (or sensor) is held in place above the primary mirror by support rods (also known as struts or rotors), which block the incoming light. As starlight enters the telescope and heads toward the primary mirror, some of it bypasses the support rods and deflects slightly. This diffraction eventually shifts the light into the final image, forming a “spider” corresponding to the position of the support rods (not the original image).

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“For stars and other bright point sources of light, this shifting light pattern takes the form of radial spikes,” Bird adds. “When the support rods of a telescope’s secondary mirror are built in a nice, symmetrical cross pattern, the diffraction heights in the star image take on the same cross pattern.”

One look at the JWST secondary mirror shows that it does not correspond to a crossed or six-sided “spider” diffraction. However, diffraction can also be caused by the telescope’s aperture edge, through which incoming light must also pass. Because the apertures of the lenses in most telescopes and cameras are circular, they typically create diffraction rings rather than generally very faint spikes – known as an “antenna pattern”.

The primary Webb mirror intercepts the red and infrared light that travels through space and reflects it back onto a smaller secondary mirror. Source: IMAGE: STScI, Andi James (@STScI)

As Baird explained, diffraction spikes can also occur due to hexagonal slits, which is consistent with James Webb mirror slices:

“If the aperture is not circular but has another shape, then both rings and spikes can result from only the aperture. These polygonal apertures also cause diffraction spikes. Thus, the diffraction heights seen in images captured by lens-based cameras are not caused by rods The support but due to the non-circular aperture. In contrast, telescopes usually have circular apertures and thus create images with diffraction spikes caused by the support rods.”

This is common in segmented primary mirrors, which are common in terrestrial observatories. Examples include Kik telescopesThe Gran Telescopio, Canary Islands (GTC), and Hobby Eberle telescope (HET), and large south african telescope (SALT) and Large Sky Area Multi-Object Fiber Spectroscopic Telescope (Lamost) in China. With its 6.5 m (21 ft 4 in) primary mirror (made up of 18 beryllium mirror hexagons), Webb is the first space telescope to use such a design.

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Although there are months before that web Beginning scientific processes and offering new views of the universe, this image is a major milestone. Indicates that stage 5 is completed and that Webb’s primary imaging device and optical system are operating as expected. As Webb’s Deputy Director of the Optical Telescope, Ritva Keski Koha, noted in a recent NASA press releaseIt boosted the mission team’s confidence in the telescope.

“We aligned the telescope and focused it entirely on a star, and the performance was above spec,” she said. “We are excited about what this means for science. We now know we built the right telescope.” Over the next six weeks, the team will continue with the remaining alignment steps before making final preparations for the scientific instrument.

The team is currently in the sixth stage of preparation, where they will take measurements at multiple field points and extend the alignment to the rest of the instrumentation. near infrared spectrometer (NIRSpec), mid infrared instrument (Mary) and Close to InfraRed Imager and Slitless Spectrograph (Nerys). At this point, the algorithm will evaluate the performance of each instrument and then calculate the final corrections needed to achieve a well-coordinated telescope across all scientific instruments.

Then, Webb’s final alignment step will begin, and the team will adjust any small errors left in position in the mirror segments. Thomas Zurbuchen, associate director of NASA’s Science Mission Directorate (SMD) in Washington, DC:

“More than 20 years ago, the Webb team set out to build the most powerful telescope anyone has ever put in space and created a daring optical design to achieve demanding scientific goals. Today we can say that the design will succeed.”

The team is on track to complete all aspects optical telescope element (OTE) by early May before moving on to the last two months of scientific instrument preparations (Phase 7). Preparations are expected to be complete by this summer, when the first full-resolution web images and scientific data will be released. So get ready for more amazing photos like these!

Speaking of photos, check out #JWSTArt PresentationsAny advantages? JWST inspired art.

In-depth reading: NASAAnd the West Texas A&M University