Unveiling the Past With the James Webb Space Telescope
“The space telescope is a kind of grand intellectual adventure for all of us, which will cast light, not just on the cosmos, but also, on ourselves.”
– Carl Sagan
The James Webb Space Telescope, launched on Christmas day of 2021, continues to prove itself as a revolutionary instrument aiding our ancient quest to discover more about our collective past and the origins of life as we know it. It is named after James E. Webb, who was the administrator of NASA from 1961 to 1968 during the Mercury, Gemini, and Apollo programs.
Led by the U.S space agency National Aeronautics and Space Administration (NASA), in collaboration with the European Space Agency (ESA) and the Canadian Space Agency (CSA), it took over 3 decades for the Webb Space Telescope to become a reality, not to mention the 10 billion dollars that went into its building. It is the largest known optical telescope in space and designed to be capable of seeing 40% of the sky from any position. It requires a period of six months to see the whole sky.
Launched on an Ariane 5 rocket from Kourou, French Guiana, it arrived at the Sun–Earth L2 Lagrange point in January 2022. The first images it took were released to the public on July 11, 2022. It is currently deployed in a solar orbit near the Sun–Earth L2 Lagrange point, about 1.5 million kilometers (930,000 mi) from Earth with a sun shield for protection.
Primarily designed to conduct infrared astronomy, JWST weighs half of the Hubble Space Telescope which studies the universe at near-infrared, optical and ultraviolet wavelengths. It’s fair to question why JWST was designed to make observations in the infrared section of the electromagnetic spectrum.
This is because JWST was engineered with the goal of observing some of the very first stars and galaxies.
What do the very first stars have to do with infrared at all?
Envision this — it’s 13.7 billion years ago and the Big Bang has just occurred. The universe resembles a hot soup of subatomic particles — electrons, neutrons and protons. Gradually (a few hundred million years after the Big Bang) stars and galaxies form, ending the cosmic dark ages.
Now think about the light emitted by these stars and galaxies ~13.5 billion years ago — light that is captured by our telescopes. And now take some time to realize that we’re seeing these celestial bodies as they were when the light first left them 13.5 billion years ago.
Get how JWST is seeing into the past?
We’re not done yet. Why infrared? Einstein’s theory of general relativity tells us that the expansion of the universe means the space between objects stretches, causing objects (galaxies) to move away from each other.
Because of the massive distances light has to travel, along with the rate of the expansion of the universe, by the time this emitted light reaches us, its color has shifted to red (it’s wavelength getting longer and less energetic). This process is called redshift, as it is the color with the longest wavelength. This happens because wavelengths of light stretch with the expansion of space.
In short, redshift refers to light that is emitted at a certain wavelength and is then shifted to longer, and hence redder, wavelengths as it travels. This cosmological redshift is due to the universe expanding.
When we attempt to view these distant objects at at visible wavelengths of light, they are very dim (or invisible) because their light is reaching us as infrared light. However, since JWST observes the universe in mid and near-infrared, it will be able to pick up these terrific tales of our past!
Seeing in infrared is immensely beneficial to the Webb Space Telescope — it can not only see ancient galaxies that have been redshifted, but also has the ability to see through the dust clouds of star forming regions. This allows the telescope to see the birth of new solar systems — a phenomenon previously withheld from us due to dust clouds which block visible light.
Now, we know what JWST does. But how exactly does it work?
The James Webb Space Telescope is meticulously designed, incredibly complex, and an extremely beautiful example of engineering at its finest.
The James Webb Space Telescope has 18 hexagonal mirror segments made of gold-plated beryllium, which combined create a 6.5-meter-diameter (21 ft) mirror — the primary mirror. This gives JWST a light-collecting area of a 25.4-m².
It uses 132 small motors called actuators to aid positioning and occasional adjustment of optics. Each of the 18 primary mirror segments is controlled by 6 positional actuators. These actuators can position the mirror with 10-nanometer accuracy. That’s 10 raised to the power of negative 8!
JWST’s optical design is a three-mirror anastigmat, which makes use of curved secondary and tertiary mirrors to deliver images that are free from optical aberrations over a wide field.
As JWST observes in a lower frequency range, from long-wavelength visible light (red) through mid-infrared (0.6–28.3 μm), it must be kept extremely cold, below 50 K (−223 °C). This is to ensure that the infrared light emitted by the telescope itself does not interfere with the collected light.
To protect its instruments from radiation and to maintain the required temperature, it has a five-layered sun shield, each layer as thin as a human hair which blocks light and heat from the Sun, Earth, and Moon, while its position near the L2 keeps all three bodies on the same side of the spacecraft at all times.
Each layer of the shield is made of Kapton E film and coated with aluminum on both sides, with a layer of doped silicon on the Sun-facing side of the two hottest layers facilitating the reflection of the Sun’s heat back into space.
Now let’s look at the instruments that help JWST do what it does!
Housing the four main instruments that will detect light from distant stars, galaxies, and exoplanets is the Integrated Science Instrument Module or ISIM, the so-called “heart” of the James Webb Space Telescope. It is one of three major elements that comprise the James Webb Space Telescope Observatory flight system, the others being the Optical Telescope Element (OTE) and the Spacecraft Element (Spacecraft Bus and Sunshield).
Optical Telescope Element
Often considered to be the eye of the JWST, it is a sub-section of the telescope. It consists of its main mirror, secondary mirrors, and the framework and controls to support the mirrors. The OTE is what takes in light and sends it to various instruments in Webb’s Integrated Science Instrument Module.
Spacecraft Element
The spacecraft bus is a primary support component of the James Webb Space Telescope. It contains many structural parts, as well as a multitude of computing, communication, electric power, and propulsion parts.
The solar panel is in green and the light purple panels are radiators. Source: Wikipedia
According to NASA’s website for the James Webb Space Telescope, “To find the first galaxies, Webb will make ultra-deep near-infrared surveys of the Universe, and follow up with low-resolution spectroscopy and mid-infrared photometry (the measurement of the intensity of an astronomical object’s electromagnetic radiation).”
The ISIM includes the following instruments:
- Near-Infrared Camera, or NIRCam:
This serves as the observatory’s wavefront sensor. It is required for wavefront sensing and control activities and is used to align and focus the main mirror segments. It is an infrared imager which has spectral coverage ranging from the edge of the visible (0.6 μm) through to the near-infrared (5 μm). - Mid-Infrared Instrument, or MIRI:
It measures the mid-to-long-infrared wavelength range from 5 to 27 μm. It contains both a mid-infrared camera and an imaging spectrometer. The temperature of the MIRI must not exceed 6 K (−267 °C; −449 °F). To ensure this, there is a helium gas mechanical cooler on the warm side of the environmental shield to provides cooling.
- Near-Infrared Spectrograph, or NIRSpec:
It performs spectroscopy over the same wavelength range. Its design provides three observing modes: a low-resolution mode using a prism, an R~1000 multi-object mode, and an R~2700 integral field unit or long-slit spectroscopy mode. - Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph (FGS/NIRISS):
This is used to stabilize the line-of-sight of the observatory during science observations. Measurements by the FGS are used to control the overall orientation of the spacecraft and to drive the fine steering mirror for image stabilization.
Now that we’ve got an idea of what JWST is made up of, let’s finally get into how we’re able to see what JWST can. We know the telescope is engineered to detect infrared — the telescope makes the observations and captures images.
But this begs the question — how can it convey things the human eye simply can’t see?
The images you see doesn’t portray the object as JWST captured it. They appear the way they do after processing and data extraction. Recall how JWST captures images in infrared. These objects are only visible to us in infrared because they either aren’t very bright, or because dust is blocking the visible light.
In the case of JWST, these objects are only detectable in infrared because they’re really, really far away and their light gets redshifted. This is where JWST’s infrared cameras — NIRCam and MIRI come in! They collect several “brightness images” in grayscale. Data from the telescope that is downloaded to Earth is refined and pre-processed into a usable form before it is passed on to the image processors.
Before being processed, the image comes as a binary file with metadata and an image in black and white. A process called stretching is carried out in which the darkest part of the image which contains the most information is brightened. When this is done, it is made sure that none of the already bright areas are affected.
JWST captures images with a variety of filters from different instruments, each representing different wavelengths of light. This so-called stretching process is applied to each one. Each filter is assigned a color. The filter capturing the longest wavelength is red; the filter capturing the shortest wavelength was blue, with the other colors in between. The processors select a combination of unique filters to create a color image, which is how they can merge data from multiple instruments to form one comprehensive picture.
How are these filters chosen? The processors look for filters that can emphasize different details or give hints of the composition of the object.
And the final processed image is what we see! This is how JWST is unveiling the past — by capturing images of stars and galaxies as they were millions or billions of years ago.
