Summary/ Reader Response Draft 4

 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 (JWST). These distortions are determined by the telescope being used, 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 Near-Infrared Camera (NIRCam) uses near infrared lights to capture the image while the Mid-infrared instrument (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 less distortion, which produces images that are less clear (Griggs, 2022). 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 (B. D. NASA, 2018). This will aid in capturing the surroundings accurately. The James Webb Space Telescope (JWST) has more improved features as compared to the Hubble Space Telescope (HST) due to its final system tests, and the gyroscope. 

One big difference between the JWST and HST is in the final system tests.The HST 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, coupled with the fact 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 places a large focus on incremental testing, together with independent checks at each stage of assembly, to reduce the amount of uncertainty left for the system test at the end while keeping costs low. A final system test is also performed, and it utilizes the JWST active optics.. Although this last test will guarantee that JWST can be “aligned on-orbit”, it will also ensure that the test is accurate and sufficiently redundant to identify any problems early and minimize any potential losses. (Lee D, 1990).

The HST's gyroscopes are mechanical devices reliant on bearings for operation, and as such, they encounter issues common to such designs. An alternative gyroscope technique has been adapted by the JWST. The "Hemispherical Resonator Gyroscope" (HRG) detects the inertial rate by vibrating a quartz hemisphere at its resonant frequency (jwst NASA). The hemisphere is designed to resonate in a vacuum, and the interaction of the hemisphere with various sensing electrodes on the HRG housing allows for the detection of the hemisphere's rate of motion. As a consequence, a package without flexible leads and bearings is incredibly dependable. Any housing leaks would actually enhance performance once the gyroscope is in space since the HRG's interior working environment is a vacuum. According to Robert B, the HRG eliminates the bearing wear-out failure mode, leaving only random failure and the electronics' radiation susceptibility, which are shared by all such devices and may be reduced by screening and shielding.  “Stress analysis 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.” (Robert B, 2021).

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 (Ethan S, 2021). 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. Servicing plays a big part in all mechanical designs. Servicing is ideal as it will allow for the designs to be maintained or even improved over time. However, with the JWST’s location, it makes servicing for the JWST not worth the increased complexity and mass that it would introduce. It 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 (Ethan S, 2021). As mentioned earlier, the JWST does not require maintenance and its fuel has been tested to be able to sustain for 1 to 2 decades (Elizabeth H, 2022). Having its expected service life be already predetermined, could also be an opportunity to replace it with a successor that is potentially much better in terms of design and functionality. Overall, the JWST 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.

 

References:  

B. D. (2018). James Webb Space Telescope. NASA. https://www.nasa.gov/mission_pages/webb/observatory/index.html

E. H. (2022). James Webb Space Telescope should have fuel for about 20 years of science. Space. https://www.space.com/james-webb-space-telescope-fuel-20-years

E. S. (2021). Why NASA’s James Webb Space Telescope Will Never Live As Long As Hubble. Forbes. https://www.forbes.com/sites/startswithabang/2021/07/20/why-nasas-james-webb-space-telescope-will-never-live-as-long-as-hubble/?sh=6ad4dec7386d

GRIGGS, M. B. (2022). Why stars look spiky in images from the James Webb Space Telescope. The Verge. https://www.theverge.com/23220109/james-webb-space-telescope-stars-diffraction-spike

Lee, D. (1990). Simple Test Would Have Found Flaw in Hubble Telescope : Space: NASA chief says the relatively inexpensive check was never performed. It may have been a victim of cost overruns. LA Times. https://www.latimes.com/archives/la-xpm-1990-07-10-mn-115-story.html

R. B. (2021). The James Webb Space Telescope Reliability Lessons. Medium. https://flyingbarron.medium.com/gyros-and-gimbals-oh-my-the-james-webb-space-telescope-9741480266a

(n.d.). JAMES WEBB SPACE TELESCOPE GODDARD SPACE FLIGHT CENTER. jwst.NASA. https://jwst.nasa.gov/content/forScientists/faqScientists.html#:~:text=and%20testing%20approach.-,Will%20astronauts%20be%20able%20to%20service%20Webb%20like%20they%20did,so%20they%20might%20be%20remedied.