Summary/ Reader Response Draft 1
The article “Why stars look spiky in images from the James Webb Space Telescope” by Griggs (2022) discussed the causes of distortion in the images produced by the James Webb Space Telescope. These distortions are determined by the telescope being used, and its design and hardware. The JWST is a reflecting telescope that uses light from the universe to form images using mirrors and imaging instruments. The shape of the mirrors affects how lights can diffract around the mirror’s edge. The JWST consists of primary and secondary mirrors, where the primary hexagonal mirrors contribute to the six diffraction spikes in the produced image, and the secondary mirrors are held by three struts that are 25 feet away from the primary mirrors. These struts also contribute to the distortion due to how light diffracts off the struts. There are mainly two instruments that produce images from the telescope. The NIRCam uses near infrared lights to capture the image while MIRI uses mid infrared lights. In near-infrared light, the diffraction spikes are prominent as lights at these wavelengths are bright, which enables the stars in images to be more distinct. Images produced by mid infrared lights have lesser distortion, which produces images that are less clear. (Webb Space Telescope, 2022). The James Webb Space Telescope (JWST) is an improvement as compared to the Hubble Space Telescope (HST).
Some of the instruments being used in the JWST are the NIRCam and MIRI as mentioned earlier, along with the Near-Infrared Spectrograph(NIRSpec) and Near-Infrared Imager and Slitless Spectrograph(NIRSS), all provide different spectroscopic capabilities in different wavelengths and bandwidths. This will aid in capturing the surroundings accurately.
One big difference between the JWST and HST is in the final system tests.The Hubble Space Telescope ultimately did not have a final system test because it was not cost effective considering that it is the most expensive test of a large space telescope, and that it was a complicated process. While HST requires regular maintenance, the JWST was designed for it to be able to function without servicing. The test plan emphasizes incremental testing, accompanied by independent checks at each level of assembly to minimize the uncertainties left for the final system test, while still being affordable. The plan does include a final system test, and this test makes use of the JWST active optics. This final test will assure that Webb can be aligned on-orbit, making the test cost effective yet retaining adequate redundancy and accuracy to detect any problems.
The gyroscopes on HST are mechanical devices dependent on bearings for their function, and they face problems typical of such designs. Webb has adopted a different gyroscope technology. The "Hemispherical Resonator Gyroscope" (HRG) uses a quartz hemisphere vibrating at its resonant frequency to sense the inertial rate. The hemisphere is made to resonate in a vacuum, and the hemisphere's rate of motion is sensed by the interaction between the hemisphere and separate sensing electrodes on the HRG housing. The result is an extremely reliable package with no flexible leads and no bearings. The internal HRG operating environment is a vacuum, thus once the gyroscope is in space any housing leaks would actually improve performance. The HRG eliminates the bearing wear-out failure mode, leaving only random failure and radiation susceptibility of the electronics (which all such devices share, and which can be mitigated by screening and shielding). Stress analyses of HRGs show this design has a "mean time before failure" of 10 million hours. As of June 2011, this type of device had accumulated more than 18 million hours of continuous operation in space on more than 125 spacecraft without a single failure.
However, with its superior features, the JWST also brings limitations. One factor that will not be able to compare to the HST is in its longevity. The HST has been marking more than 3 decades in operation, while the JWST is said to not be able to even last a decade. This is due to its location. With HST, it is located in low earth orbit, making it more accessible as compared to JWST, where it is orbiting the sun. This makes servicing for the JWST not worth the increased complexity and mass that it would introduce. Servicing will also not be cost effective considering that it is so far away from earth.
That being said, even though the JWST may not be able to compare to the HST in its longevity, it fundamentally still has an advantage in its design. It allows for it to be far superior in terms of temperature, resolution, light-gathering power and wavelength range. As mentioned earlier, the JWST was designed in such a way that it does not require maintenance as well. This could also be an opportunity to further improve its design. Overall, the James Webb Space Telescope with all its features will surpass the ability of the HST during its lifespan, which is an improvement as compared to the Hubble Space Telescope.
Daisy Dobrijevic, Elizabeth Howell (Sep 23, 2022)
Mary Beth Griggs (Jul 16, 2022)
https://www.theverge.com/23220109/james-webb-space-telescope-stars-diffraction-spike
NASA
https://www.nasa.gov/mission_pages/webb/observatory/index.html
Ethan Siegel (Jul 20, 2021)
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