The JWST’s Infrared Detectors Observing The Universe’s Past
With the James Webb Space Telescope finally on the way to L2, we can look forward to the invaluable information it provides. However, before sending any information at all it must go through a long and complex deployment process. If successful it will look into the past of the universe and discover incredible things. A lot of people know Webb is an infrared telescope, but not exactly how it works.
The James Webb Space Telescope is an infrared telescope that uses a combination of complex components including infrared detectors. These are very important parts of the JWST thanks to their unique architecture, design, and purpose. All of which combine into a single component ensuring Webb can see deep into the universe. Here I will go more in-depth into their purpose and how exactly they work.
With so many different intricate and fascinating parts of Webb, it can be hard to learn about all of them. One of the big differences between space telescopes such as the JWST and the Hubble is not only the size but type of telescope. Infrared increases Webb’s capabilities significantly, helping make it stand out from past telescopes we have sent into space.
JWST Infrared Background
The James Webb Space Telescope is an infrared telescope. This means it uses infrared light to detect distant celestial bodies. This type of light is one of several types of radiation present in the electromagnetic spectrum. Taking a closer look at the features of Webb, the mirrors collect light from the sky and direct it to the science instruments. These instruments filter the light and spectroscopically disperse it before finally focusing on the detectors. An example could be an MIRI detector which is housed in a brick-like unit called a focal plane module. It has a pixel array of 1024 by 1024 arsenic doped silicon pixels. Each of the instruments on Webb has its own detectors.
These detectors are where photons are not only absorbed but also converted into electronic voltages that NASA can measure. Due to the conditions of Webb and its goal of looking deep into the past of the universe, it needs extremely sensitive detectors. These help it record the feeble light from far-away galaxies, planets, and stars. In addition, there are large arrays of these detectors ensuring Webb can efficiently survey the sky. In the process of creating Webb, a lot of work has been done to improve and innovate past designs for infrared detectors. This has extended the state of the art for infrared detectors and made them much better overall. Some of the improvements include producing arrays that are lower noise, larger format, and longer-lasting than their predecessors.
Infrared Detector Features
Different Types – The first important aspect of these detectors and their impact on Webb are the different types. The JWST uses two different types of detectors including mercury-cadmium-telluride detectors for near-infrared, and arsenic doped silicon detectors for mid-infrared. Both of which were made in California and made for the James Webb Space Telescope. Each of these detectors has about 4 million pixels and the mid-infrared detectors have about 1 million each. One of the most unique parts about the detectors I mentioned is the material they are made of.
The mercury-cadmium-telluride can tune the material to sense longer or shorter wavelength light by varying the ratio of mercury to cadmium. Specifically, the JWST takes advantage of this by using two compositions of the material. This includes one with proportionally less mercury for 0.6 to 2.5 microns and another with more for 0.6 to 5 microns. This provides many different benefits such as the possibility of tailoring each detector for peak performance over the specific wavelength it will be used. Altogether there is a large number of these different detectors each contained within a different instrument on Webb. The increased options and information available thanks to using different detectors will have a direct impact on the future information provided by Webb.
Architecture – Another important feature of the infrared detectors is the build or architecture behind this piece of technology. NASA mentions that “All of Webb’s detectors have the same basic sandwich-like architecture.” They describe this sandwich as having three different parts. The first layer is a thin semiconductor absorber layer. The second layer uses indium interconnects to join each pixel in the absorber layer to the readout. Lastly, you have the 3rd layer with a silicon readout integrated circuit or ROIC to read out millions of pixels using a manageable number of outputs. The absorber layer and silicon ROIC are fabricated separately rather than together.
This fabrication difference allows for each part of the process to be carefully tuned to the materials that are used. Taking a closer look at Indium, this is a soft metal that deforms under moderate pressure to form on cold weld per pixel between the detector layer and the ROI. When designing and building these detectors NASA needed to increase the mechanical strength. To do this the detectors vendors flow a low viscosity epoxy resin between the indium bonds during the latter stages of hybridization. On the exterior in some cases, the detectors are positioned within a black baffle that admits light onto the four detectors while blocking it from hitting any surfaces that might reflect, such as the edges of a detector.
Design – The last feature I want to mention and highlight is the design. Specifically how exactly do these detectors work to gather and direct the infrared light. Starting with the near and mid detectors, the first step in the detection process involves an incident photon being absorbed by the semiconductor yielding mobile electron-hole pairs. From here they move under the influence of built-in and applied electric fields until they find their way to where they can finally be collected. In addition, it is possible to read the pixels in a Webb detector more than once prior to resetting them. This provides a lot of different benefits for the detectors and Webb as a whole.
For example, NASA mentions “it is possible to average multiple non-destructive reads together to reduce the read noise compared to doing only one read.” Another advantage to reading the pixels more than once is that when using multiple samples of the same pixel, you can see the jumps in the signal level that are a tell tale sign that a cosmic ray has disturbed a pixel. After finding out that a cosmic ray has disturbed the pixel, it’s then possible through ground-based processing to apply a correction to recover much of the scientific value of the affected pixel. All of this works together within each detector helping gather and direct infrared light to the larger number of science instruments scattered across Webb.
Conclusion
Right now the JWST is headed to L2 with just under a month to go. From here it will continue to deploy and have multiple engine burns ensuring it enters the correct orbit and stays there. If everything goes according to plan Webb will start gathering incredible information about the universe’s past and much more. This would not be possible without the infrared detectors used throughout the telescope. These detectors are unique and very important thanks to their architecture, different types, and design. All of which combine into a very important piece of technology that has a direct impact on what the telescope can see. We will have to hope for Webb’s success through the deployment process and see the impact it has on the space industry.
