Summary
The review by Griggs, “Why stars look spiky in images from the James Webb Space Telescope (JWST)” (2022), informs the reader on how the JWST produces high-resolution images. Space telescopes are tasked with orbiting the solar system and collecting images in space. When capturing images with space telescopes, quality can be influenced by a diffraction spike. Diffraction spikes are the lights observed from a star due to the way they interact with the environment (Celestron, 2018). In Griggs's article, she quoted Hank Green, sharing that the difference between the Hubble Space Telescope (HST) and JWST is that “Hubble stars have four spikes in a cross. JWST stars have six in a snowflake” (Green, 2022). The main imagers used by the JWST are its Near Infrared Camera (NIRCam), Near InfraRed Spectrograph (NIRSpec), and Mid-Infrared Instrument (MIRI) (NASA, 2022). NIRCam focuses on detecting dimmer objects that are in the presence of brighter light sources, which is done by shielding the NIRCam from the brighter light. NIRSpec is a spectrograph, that breaks down information on the image's light into a spectrum. After collecting data on the light emitted from the object, the NIRSpec does an analysis to uncover the properties of the image. Both NIRCam and NIRSpec perform within the 0.6 to 5 micron range. MIRI on the other hand is a combination of a camera and spectrograph, capable of capturing images that are transparent to the naked eye. With these advanced features, the JWST is capable of producing images with more diffraction spikes thus refining the clarity and sharpness of the images. Given the effectiveness of the JWST, when innovating newer models of space telescopes, engineers should look to its hexagonal mirrors, the positions of the strut, and the use of NIRCams.
Reader Response
One of the features that space engineers can study is the usage of hexagonal mirrors. In the JWST, 18 sturdy and lightweight hexagonal segments are attached together to form its primary mirror. The benefit of using hexagonal mirrors is their high filling factor and six-fold symmetry (NASA, n.d. - a). Since hexagonal shapes are symmetrical in nature, they are able to fit perfectly within the frame without any gaps. Removing any gaps would maximize the collecting area of the mirror. The primary mirror is folded to fit within the rocket during launch (Clark, 2022). This allowed NASA to send a primary mirror of 6.5 metres in diameter to space, a significantly larger diameter than that of the HST’s 2.4 metre diameter mirror (NASA, n.d. - b). Furthermore, each hexagonal segment is composed of beryllium and plated in gold, giving the mirror its lustrous and gold appearance (Gohd, 2021). Beryllium has a high durability-to-weight ratio, making it a great choice for the mirror to be both sturdy and light. Hence, I believe hexagonal mirrors are the key to helping space telescopes capture more light, which improves image clarity.
Another change that is noteworthy for fellow engineers to look at in the JWST is the use of 3 supporting struts. Struts are what hold the secondary mirror in place. For the JWST, the struts “are hollow composite tubes” that are made to be durable and light. Each strut measures up to 25 feet with a thickness of 1 millimeter and increases tolerance under extreme conditions in space (Nasa, 2017). HST uses 4 struts, giving the captured image only 4 diffraction spikes which is not as good as JWST. When the JWST unfolds, the struts adopt a tripod-like structure with an angle of 120 degrees between each strut. The struts are aligned in a way where little light is blocked from the mirrors, allowing 8 diffraction spikes to form on the captured image (Siegel, 2022). Undoubtedly, the use of only 3 supporting struts is what ensures that the images are crisp and clear.
A completely new addition to the JWST for engineers to take reference from is the NIRCam. The NIRCam comes equipped with 2 instrument modules, enabling it to simultaneously observe both short and long wavelengths (Arizona.edu, n.d.). What this means is that most temperatures of light from the stars are captured, allowing the image produced to be highly accurate. Moreover, the NIRCam is able to focus the camera on distant and dimmer objects which was not possible on the HST. This is achieved with a coronagraph which obstructs light from the brighter light source (NASA, n.d. - c). Most of the light captured by the camera comes from the smaller and dimmer stars in the background. The NIRCam is used to capture images, but it also doubles as an Optical Telescope Element wavefront sensor, it has imaging correction (NASA, n.d. - c). The camera obtains wavefront sensing data essential for periodic “alignment and phasing of the segments of JWST's primary mirror” (Space Telescope Science Institute, n.d). Unlike its predecessor, the JWST is able to obtain images of greater detail and clarity due to its NIRCam.
However, an obstacle that engineers will not be able to take away from is how hard it can be to repair the JWST. The space telescope is parked 1.5 million kilometers away from earth, which makes it impossible to send astronauts out to correct any mistakes made (Dutch, 2018). When the HST ran into the lens error back in 1993, engineers were still able to send their astronauts to space to correct the lens with additional mirrors. Correcting the HST was only possible as it was still within Earth’s orbit. Were the JWST to run into any issues in space, Nasa’s engineers would be unable to rectify them. This problem can be minimised by conducting risk assessments and ensuring all possible design flaws have been eliminated, but the risk of not being able to repair any errors would still be costly.
To summarise, the JWST is a major leap in development from the HST. It has been further refined to capture clearer and more scientifically accurate images. The primary mirror has been modified to capture more light. The strut position has also been designed to create more diffraction spikes, improving the image quality. Lastly, the NIRCam can filter what light would be needed for specific situations. Thus, to say that the JWST is the gold standard for future the construction of future space telescopes would be accurate.
Reference List
Celestron. (2018, July 3). What is a Diffraction Spike?
Clark, S. (2022, January 7). First wing of Webb telescope’s primary mirror folds into place. https://spaceflightnow.com/2022/01/07/first-wing-of-webb-telescopes-primary-mirror-folds-into-place/
Dutch, S. (2018, January 26) Why can't the James Webb telescope be repaired after it is launched? Quora. https://www.quora.com/Why-cant-the-James-Webb-telescope-be-repaired-after-it-is-launched/answer/Steve-Dutch
Gohd, C. (2021, December 15). NASA's James Webb Space Telescope has a shiny giant mirror made of gold hexagons. Here's why. Space.com. https://www.space.com/nasa-james-webb-space-telescope-mirror-explained
Griggs, M, B. (2022, July 16). Why stars look spiky in images from the James Webb Space Telescope. TheVerge. https://www.theverge.com/23220109/james-webb-space-telescope-stars-diffraction-spike
JWSTNIRCam. (n.d.). NIRCam. http://ircamera.as.arizona.edu/nircam/index.php
Nasa. (2017, Aug 17). NASA's Webb 'Strutting its Stuff' in New 'Behind the Webb' Video. https://www.nasa.gov/feature/goddard/nasas-webb-strutting-its-stuff-in-new-behind-the-webb-video
Nasa. (2022, Sep 6). A Cosmic Tarantula, Caught by NASA’s Webb. https://www.nasa.gov/feature/goddard/2022/a-cosmic-tarantula-caught-by-nasa-s-webb
Nasa. (n.d. - a). Mirrors Webb. https://webb.nasa.gov/content/observatory/ote/mirrors/index.html
Nasa. (n.d. - b). Webb vs Hubble Telescope. https://www.jwst.nasa.gov/content/about/comparisonWebbVsHubble.html#:~:text=Webb%20also%20has%20a%20much,second%20Lagrange%20(L2)%20point
Nasa. (n.d. - c). NIRCam. https://www.jwst.nasa.gov/content/observatory/instruments/nircam.html
Siegel, E. (2022, March 22). Where do James Webb’s unique “spikes” come from? BigThink. https://bigthink.com/starts-with-a-bang/james-webb-spikes/
Space Telescope Science Institute. (n.d.). Science Instrument. https://www.stsci.edu/jwst/instrumentation/instruments