Cosmology: Looking into the Past

James Webb Space Telescope (JWST)

"…the laws of physics, carefully constructed after thousands of years of experimentation, are nothing but the laws of harmony one can write down for strings and membranes. The laws of chemistry are the melodies that one can play on these strings. the universe is a symphony of strings. And the “Mind of God,” which Einstein wrote eloquently about, is cosmic music resonating throughout hyperspace."

- Michio Kaku

The biggest news of 2021 which gravitated not only the experts but has been the talk of the town ever since its launch is none other than the James Webb Space Telescope. The 8.8 billion dollars telescope with an estimated operating cost of 1 billion dollars was launched on December 25, 2021. The telescope deemed to be the successor of Hubble and its counterpart, has delivered images that have left the world in awe and put our understanding of Physics into question. So, let’s dive into it.

James Webb Space Telescope

Webb is a spectacular example of engineering and physics. We have long discussed how light gets red-shifted as it travels longer and longer distances in the Universe. Visible light emitted from these far-away bodies gets red-shifted into the infrared region by the time it reaches Earth, becoming invisible. Lucky for us JWST has been designed to work primarily in the infrared region.

Red-shift due to the expansion of the Universe

The telescope is broadly divided into its parts, namely, Optical Telescope Element (OTE), Integrated Science Instrument Module (ISIM), a sun shield and a Spacecraft Bus. We would look into each of these one by one.

Parts of JWST

A. The Optical Telescope Element (OTE) consists of the mirrors and the backplane. It is the eye of the observatory. It gathers the light coming from space and provides it to instruments placed in ISIM. The OTE consists of JWST’s segmented honeycomb-shaped mirror. It is the largest mirror ever flown in space. It consists of 18 hexagonal segments with each segment about 1.32 meters across. Each segment is made out of lightweight beryllium and coated with a thin layer of gold, making it more sensitive to infrared light. The hexagonal shape of the mirror helps in folding the mirror on Earth and then unfolding it in space. While in space, the focus of the mirror is adjusted on the secondary mirror with an accuracy of 1/10000th the thickness of a human hair! The order of various mirrors is the primary mirror, secondary mirror, fine steering mirror and infrared detector.

Optical Telescope Element

B. The light is collected on the secondary mirror. The detector converts these photons into their supposed electric voltages which are then processed to yield the spectacular pictures we have been getting. The second mirror consists of the Integrated Science Instrument Module (ISIM), which further contains instruments such as a Near-Infrared Camera (NIRCAM), Near-Infrared Spectrograph (NIRSPEC), Fine Guidance Sensor/ Near Infrared Imager and Slit-less Spectrograph (FGS/NIRISS), and Mid-Infrared Instrument (MIRI). We would discuss them briefly here:

Integrated Science Instrument Module (ISIM)

1. The Near Infrared Camera (NIRCAM) is Webb's primary imager that covers the infrared wavelength range of 0.6 to 5 microns. Equipped with ten sensitive detectors it detects short wavelength channels (0.6 - 2.3 microns) and long wavelength channels (2.4 - 5 microns).

NIRCAM detects light from the earliest stars and galaxies in the process of formation, the population of stars in nearby galaxies, as well as young stars in the Milky Way, and Kuiper Belt objects. NIRCAM is equipped with coronagraphs. They filter bright light and help in detecting fainter sources of light like the ones coming from exoplanets. With the coronagraphs, astronomers hope to detect planets orbiting nearby stars.

A basic Coronagraph

While NIRCAM is excellent when it comes to taking pictures, it doesn’t give us any idea about the physical properties of the body.

2. This problem is solved by the Near Infrared Spectrograph (NIRSPEC). It operates over a wavelength range of 0.6 to 5 microns. A spectrograph is used to disperse light from an object into its spectrum. Different elements have their own characteristic spectra. Analyzing the spectrum of an object can tell us about its physical properties, including temperature, mass, and chemical composition. It reveals a plethora of information about the body being observed.

Spectroscopy: Emission and Absorption Spectra of various elements

The most significant drawback of using spectrographs is, the mirror must stare at them for hundreds of hours in order to collect enough light to form a spectrum. To the rescue comes JWST’s very own micro shutter system made of 250 thousand shutters. It controls how light enters the NIRSPEC. It has been developed by Goddard scientists. It allows us to observe hundreds of objects at a time saving a lot of time and resources.

3. FGS (Fine guidance sensor) – Different parts of the Universe can be brightly illuminated. To capture the relevant light, the telescope has to constantly be directed at different targets. This is achieved by a fine guidance sensor (FGS). It allows Webb to point precisely so that it can obtain high-quality images. FGS is a "guider," which helps point the telescope. Canadian scientists developed the near-infrared imager and slitless spectrograph (NIRISS). It is used to investigate exoplanets, detect first light and find out more about the physical characteristics of the observed body. FGS/NIRISS has a wavelength range of 0.8 to 5.0 microns. It is a specialized instrument with three main modes, each of which addresses a separate wavelength range.

FGS and NIRISS

Farther the source of light, the more red-shifted its wave and longer its wavelength. The Mid-Infrared Instrument (MIRI) is equipped with a camera and a spectrograph. It works with longer wavelength infrared light, in the mid-infrared region of the electromagnetic spectrum. Longer wavelengths can penetrate thicker dust clouds. MIRI covers the wavelength range of 5 to 28 microns. Its sensitive detectors allow it to see the red-shifted light of distant galaxies, newly forming stars, and faintly visible comets as well as objects in the Kuiper Belt. Due to its objective of working with longer wavelength infrared lights, it is important to be careful that it doesn’t start registering its own heat. The temperatures have to be kept below 6.7K. A special cryocooler that uses helium is used to keep it cool.

Near, Mid, and Far Infrared photography

C. Webb also has a Sun Shield with dimensions being 21 m long and 14 m across. It is made of five layers. Each is made from a special film called Kapton, a material that can absorb high temperatures. Additionally, there are layers of aluminium and the first two layers also have doped silicon. The sun shield protects it from the heat of the Earth, Moon and Sun.

Sun Shield

D. The Spacecraft Bus provides the support functions for the operation of the Observatory. The bus houses the six major subsystems needed to operate the spacecraft: the Electrical Power Subsystem, the Attitude Control Subsystem, the Communication Subsystem, the Command and Data Handling Subsystem, the Propulsion Subsystem, and the Thermal Control Subsystem.

Spacecraft Bus and other parts of JWST

E. Other elements include:

The momentum flap balances the solar pressure on the sun shield, like a trim flap in sailing. It's not adjustable in orbit, but it is while it's on the ground.

The Earth-pointing antenna sends science data back to Earth and receives commands from NASA's Deep Space Network.

The solar array is always facing the sun to convert sunlight to electricity to power the Observatory.

The star trackers are small telescopes that use star patterns to target the observatory.

I would love to talk more but this article is turning out to be longer than I expected. I guess we would need another one to answer the remaining questions. Until then, let's enjoy some beautiful images taken by the Webb.