All the light we cannot see

How do we colourise photos of space?

Hubble and Webb have been blessing our feeds with beautiful and stellar images of the Universe. Anyone who looks at them cannot deny the awe and wonder it fills you with. But, do they actually look like this? More precisely, if we were to observe this Universe with the naked eye would we actually see the bodies as their images show them to be? Are they as colourful as is shown or are we somehow manipulating them to see what we understand?

All the colourful pictures that we see on our devices use three colours as their primary colours, i.e.; red, green and blue. These colours in various proportions make other colours. For example, in a hexadecimal representation of colours, we have six digits. The first two digits represent a red value, the next two are the green value, and the last two are the blue value.

Some common hex codes and their corresponding colour

The human eye consists of six million photoreceptor cells in the retina, called the cones. There are three types of cones or receptors, the short-S (blue), medium-M (green), and long-L wavelength (red) sensitive cones. These cones are present in different quantities. The short wavelength cones make for about 10% of the cones. It responds most to blue-wavelength light peaking at 420 nm. The medium wavelength cones respond most to light of yellow to green, and peak at about 530 nm. The long wavelength or red-sensing cones make for about 60% of the cones. It responds most to the longer wavelengths peaking at about 560 nm. An important thing to be noted is the peaks aren’t the same for all the individuals. It might lie in the range of 420-440 nm, 534-545 nm, and, 564-580 nm respectively for different individuals.

Responsivity vs Wavelength (nm)

This is the guiding principle used in colouring black-and-white images. We use a process called broadband filtering which is essentially taking pictures of objects with different filters and then recombining them to get the desired image. For example in the picture given below, we have taken a black-and-white photograph of the flowers. The same black-and-white photography is done using red, green, and blue filters. The relative absence and presence of a particular colour in the given photographs predict the actual colour present. This calculation is done on a pixel-by-pixel basis and we obtain a colourful image.

Broadband filtering at play

The images taken by Hubble and Webb are in black and white. The main purpose of these telescopes is to measure the brightness of light reflecting off  of objects in space which is clearest in black and white. These images are then digitally coloured. We take images filtering various wavelength ranges and then recombine the image to get the picture.

Scientists also use colours to map out how different gases interact to form galaxies and nebulae. A process called narrowband filtering is used to capture specific wavelengths of light. Hubble can record very narrow bands of light coming from individual elements like oxygen, carbon, and hydrogen. We can then use colours to track their position in an image.

Narrowband filtering of Hydrogen, oxygen, and Sulphur

The most common application of narrowband filtering is studying the formation of stars and galaxies. The filters isolate light from hydrogen, sulfur, and oxygen, the three key building blocks of stars. This is not a true colour image. It is more of a colourised map. The characteristic wavelengths of hydrogen, sulphur, and oxygen are 656.2 nm, 672 nm, and 495.9 nm respectively. Hydrogen and sulphur are naturally seen in red light, and oxygen is seen in blue. These correspond to the colours red, red, and cyan. To get a better image the wavelengths are adjusted and assigned their places as red, green, and blue according to their chromatic order. Sulphur is denoted by red colour, hydrogen takes green and oxygen is shown by blue.

Pillars of Creation in True and false colours

What about infrared light? We do know the primary wavelengths Webb is working in are in the infrared region. How do we colourise invisible light?

In infrared light too, a similar process is followed. We assign different colours to different elements. Take their images through broadband and narrow band filtering and then recombine them to show the stunning images we are presented with.

Infrared photography of Helix Nebulae

Have we been duped? Are colours even real? Are we artificially colouring the Universe to make it look more beautiful than it actually is?

Well, yes and no. It’s true that if we are presented with actual images of the galaxies taken, they may appear bland and boring. But, the biggest fact we aren’t paying heed to is how limited we as humans are in our perception of light. We see a small part of the electromagnetic spectrum and call it visible light. If we could see the complete spectrum imagine how colourful we would find the Universe to be! We could see temperatures, the all-surrounding microwave background radiation, and telephones communicating in various radio waves. The very thought about it makes me trippy. What about you? So, we have been duped ever so slightly to understand the Universe in a colour language we know. In reality, the Universe is much more colourful than we can ever comprehend.

I hope you liked this article. Drop a comment and tell me what you thought about it. See you soon.

Auf Wiedersehen!

Looking into the Past (Part 2)

  

James Webb Space Telescope

Welcome back, readers. If this is the first article you have opened, I’d highly recommend reading the first part of this article. While the former was about the engineering elements of JWST, this article is mostly focused on the physics surrounding JWST.

While discussing the parts of JWST, we talked about the Sun Shield. For the sun shield to successfully protect us from the heat of the Earth, Moon and Sun it should be placed 1.5 million kilometres from Earth. The telescope is placed at a point in space called Lagrange Point 2. It is one of the five Lagrange points.

The five Lagrange Points

Lagrange points are locations in space where both the Earth and the Sun exert a gravitational pull in the same direction. An object at this point has two gravitational forces pulling on it to make it move in a circle. This not only allows it to orbit the sun with a higher velocity, but it also keeps it at a fixed point relative to our planet. The JWST orbits the sun instead of the Earth. We want the JWST to be both further from the sun and complete a solar orbit in the same amount of time as the Earth? To make it easier to control, the telescope would also have to remain in the same position relative to the Earth. Here, the Lagrange point comes into the picture.

Why Infrared?

A question that keeps appearing, again and again, is why we have chosen infrared as our desirable wavelength for detection?

Infrared waves have a longer wavelength than visible light. It means it can easily penetrate dust clouds and we can observe more far-off objects. Moreover, the light of galaxies that are billions of light years away from us travels to us through an ever-expanding space. This stretches the wavelength of visible light into the infrared region. Near-infrared light reveals the formation of galaxies and due to its longer wavelength, it can pass through the dust layers that enclose the newborn stars. Mid-infrared light peers through the cold, dusty regions where stars form, and reveals how massive stars and black holes shape their surroundings.

Southern Ring Nebulae in Near and Mid-Infrared Regions

Moreover, various types of celestial objects – including the planets of the solar system, stars, nebulae, and galaxies give off energy at wavelengths in the infrared region of the electromagnetic spectrum.

A follow-up question that comes to mind is if we are so adamant about looking at the Universe’s past. Why not take it to extremes and study light in microwave regions? As far as we know the Cosmic Background Radiation is in the Microwave region. Won’t it enable us to look further into the past, right to the origins of the Universe?

Well, the answer is yes and no. This means it is a little complicated…

Infrared and microwave imaging both have their own advantages and are useful for different purposes in astronomy. Infrared imaging is preferred over microwave imaging in many cases because it provides higher spatial resolution and can reveal more details about the characteristics of astronomical bodies. Infrared radiation has shorter wavelengths than microwaves, which means it can be used to study smaller features in the Universe. In addition, infrared radiation is absorbed and emitted by many astronomical objects, which makes it useful for studying the temperatures and compositions of these objects. Spatial resolution is inversely proportional to the observing wavelength. The higher the wavelength, the lesser would be its resolution and vice-versa.

Different Spatial Resolutions

Microwave imaging is preferred in some cases because it can penetrate through clouds of gas and dust that may obscure infrared radiation. Microwaves are also less affected by atmospheric turbulence, which can distort images taken at other wavelengths. This makes them useful for studying objects behind dense clouds of gas and dust, such as the centres of the galaxies. Microwave imaging is also useful for studying the cosmic microwave background (CMB), which is the leftover radiation from the Big Bang.

Cosmic Microwave Background

In summary, both infrared and microwave imaging have their own advantages and are used for different purposes in astronomy. Infrared imaging is preferred due to its higher spatial resolution and ability to reveal details about astronomical objects, while microwave imaging is useful for studying objects behind dense clouds of gas and dust and for studying cosmic microwave background radiation.

The Sun in different wavelengths

The next order of business is asking what makes Webb different from Hubble. In what ways is Webb an upgrade or a downgrade in comparison to Hubble?

Both JWST and Hubble are reflecting telescopes that conceptually work the same. Light reflects off a large primary mirror onto a secondary mirror, which sends it back through a hole in the primary mirror and into science instruments for analysis.

The basic difference between the two telescopes is the wavelengths they work at. Hubble, which is in Earth's orbit, is optimized for visible and ultraviolet wavelengths of light. 

Karina Nebulae as seen from Hubble and Webb

Webb essentially orbits the sun and is situated at Lagrange point 2 which is 1.5 Million Kilometres away from Earth as opposed to Hubble which is 535 Kilometers. This makes maintenance work on Hubble easier than it is for Webb.

Distance of Hubble and Webb from Earth

Hubble’s single mirror is 2.4 meters (7.9 feet) wide, whereas JWST’s segmented honeycomb-shaped mirror is 6.6 meters (21.7 feet) across. JWST has the largest mirror ever flown in space. Tiny actuators shape each mirror to provide a single, sharp image for the telescope’s science instruments to digest. Even though Webb’s mirror is a lot bigger in size and a hundred times more powerful, it is still 113 Kilograms lighter than Hubble’s mirror.

Hubble vs JWST Primary Mirror

The two telescopes also have very different cooling requirements. Hubble does not have as sophisticated cooling needs as JWST.

How has Webb changed and challenged our understanding of Physics?

A picture that made big news two months back was captured by Webb. It showed multiple galaxies that were formed way before our current understanding of Physics would permit. These galaxies grew way too large way too soon after the big bang. News spread everywhere. Webb has broken the Big Bang theory. So, do these pictures put our current understanding of the Big Bang and the Standard Model into question?

The oldest galaxies captured by James Webb

As fascinating as it might be, the answer is no. Recently, researchers took a closer look at the data and concluded that the distant galaxies discovered by Webb are in fact in perfect compatibility with our modern understanding of Cosmology.

As things go this might not even be the final answer and astronomers may find galaxies at very large distances with very large masses that puts our understanding of Physics into question.

But, we must always remember that in science, it’s always important to keep an open mind. For now, we can keep the exaggerated claims to rest.

I hope you liked this article and enjoyed reading it as much as I enjoyed writing it. Goodbye for now.

Auf Wiedersehen!

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.

The image that broke the internet! Perfectly visible Gravitational Lensing

Pillars of Creation

Images that put our understanding of the Universe into question

One of the first few pictures by JWST. Look at this beauty!

Comparing Hubble and JWST

This article fills me with glee, excitement, and hopeful anticipation about the future. What were your reactions? Comment below.

See you soon.

Bis Bald.