An Insight into the James Webb Telescope

Important Updates!

24th January: the James Webb reaches its destination; the L2 Lagrange point!

19th January: the James Webb successfully deploys all 18 hexagonal mirrors to unfold the large primary mirror

7th January: The James Webb begins to unfold its hexagonal mirrors to deploy the primary mirror.

5th January: Deployment of the secondary mirror begins! Watch it live HERE!

4th January: Tensioning and separating of layers of the shield completed!

3rd January: Tensioning and separating of the layers of the shield begin

31st December: The James Webb successfully unfurls the sun-shield!(keep in mind there are more than 50 intricate steps involved in the furling of the shield)

29th December: sunshield deployment begins! (A very nerve-racking and perilous procedure, considering the vital importance and flimsy nature of the sun-shield)

25th December: the James Webb Space Telescope launched successfully at 12:20 UTC(5:30 IST)

Sometime in the last couple of months of 2021, the long awaited Jamess Webb Space Telescope will launch into space to succeed the Hubble Space Telescope to uncover many secrets deep in space and back in time.

The JWST’s primary mirror is made of 18 pre-folded hexagonal mirrors which would fold out into a large mirror of a diameter of 6.5 meters in space. This means a mirror almost 4 times the area than that of the Hubble. Apart from the size of the mirror, the main difference of this telescope from any other space observatory is the spectrum of light it would observe. Unlike the hubble, which detected light with wavelengths randing from 0.1 to 1 μm (near ultraviolet, visible, and near infrared spectrum) the JWST would detect lower wavelengths in the range of 0.6 to 28.3 μm(visible to mid-infrared).

The L2 Lagrange Point:

One interesting feature of the telescope that always stirs up thought or conversation is its location at the L2 lagrange point. What is a lagrange point? Why is it placed that particular lagrange point? Why is it placed so far away from the earth?

A lagrange point is a point in space where the gravitational force of two large masses(the Sun and Earth) equals the centripital force required to move a small object(telescope) with them. This means any object placed in these points would stay in the same position relative to the large bodies - like parking a satellite.

In the Eath-sun system, there are five lagrange points named L1 to L5. The L1 to L3 points are unstable equilibria, meaning a small perturbance in the position would cause a force in the same direction, and the L4 and L5 are stable equilibria.

The JWST would occupy the L2 position, which is 1.5 million kilometers from the Earth. This position is ideal because of its stability, thus reducing fuel consumption greatly, and its position behind the sun and earth which allows for capturing of solar power.

We can intuitively understand the reason for the point’s stability - normally, an object placed further away than the earth would have a larger time period, ie. lag behind the earth(T = 2πr/v = 2π√(rm)/√F => an increase in radius would have to be compensated for by an increase in force). The earth’s gravitational force, however, compensates for this ‘lag’ and the telescope would move along with the earth.

The significance of the observed frequency range:

The JWST would observe light in a lower frequency range(long wavelength visible to mid infrared) than the Hubble. This change in range would hope to achieve the following:

• Ability to observe redshift objects

  • The universe is expanding. As stars and galaxies move away from us, their apparent wavelength becomes larger. This is kinda like how a siren’s sound changes pitch as you move across it in a moving car. The doppler effect for light is a little more complicated than that for sound, but its easy to understand why observing lower frequency would prove beneficial.

• cold objects such as debris disks and planets emit most strongly in the infrared region, so they wouldn’t be picked up as well with a high-frequency telescope

• This range of frequency has never been studied by ground-based telescopes because of various problems caused by the earth’s atmosphere, so the james webb could give a lot of insight hitherto difficult to obtain.

  • This is due to a phenomenon known as atmospheric absorption. The atmosphere is actually not completely transparent - different molecules in the atmosphere absorb different frequency waves. For example, O2 and O3 absorb almost all wavelengths shorter than 300 nanometers. Water (H2O) absorbs many wavelengths above 700 nm. This variation in the electromagnetic waves going past the atmosphere makes for certain ‘pockets’ of visibility, which can be clearly understood from this graph:

As you can see, the infrared region is not FULLY blocked by the atmosphere, but this does complicate analysis. (Radio waves are able to pass through completely. Do the scientists communicate with space shuttles and satellites with radio waves?)

The importance and implications of the temperature difference:

One of the major differences between the Hubble and the James Webb is their temperature. The latter wasn’t placed at the L2 for easy maneuvarability and reduced fuel consumption alone, for another important reason for the placement was the telescopes occlusion from the Sun, which, along with its heat shield, would keep the temperature very low - a mere 50K(compared to 288K of the Hubble)

Why does the James Webb have to be kept at such low temperature? The answer lies in black-body radiation. Every body emits electromagnetic waves - the wavelength of which depends on the bodies temperature. At normal temperatures in our environment, bodies radiate mostly invisible radiation(infrared). Once bodies like metals go to higher temperatures, however, they begin to emit visible radiation too. That is why steel at 1200C is ‘red hot’ and at higher temperatures it goes to ‘yellow hot’ too.

The graphical representation of the wavelengths of radiation emitted by a body at a certain temperature is called its black body curve.

Eg. black body curve of an ideal black body. Notice how the intensity increases with increase in temperature

Similary, the body James Webb Telescope radiates EM waves too. However, considering the fact that the telescope’s job is to detect such radiations of distant stars and galaxies for us to see, its own radiation would screw up the readings. This problem is escalated since the James Webb captures the lower wavelength range, which is often radiated by bodies even in cool temperatures(compared to higher wavelenth range which is only emitted at very high temperatures). Since the intensity of radiation of any body decreases with decrease in temperature, immense efforts were taken to keep the telescope as cold as possible.

To give you an idea of how much a difference this makes, here is a graph I got from a blackbody curve graphing simulator, which gives a curve for any temperature and emissivity(effectiveness in emitting energy as thermal radiation and varies between 0.0 and 1.0)

I went on to graph the curve for 288K and 50K(you should know what these two temperatures represent by now), and was surprised by the results. Treating both bodies as ideal black bodies, the disparity in intensity was enormous.

Do you see it? Look again - the barely visible thin blue line at the bottom

(The telescopes cannot actually be treated as ideal bodies as different materials have different curves, with some even having abrupt changes in slopes. This serves as a good rough estimate however, and the difference in radiation is more importantly highlighted.)

Since this graph doesn’t effectively express the difference in intensity, I graphed the values with the y-axis in log scale in excel. Graphing in this manner allows for the effective comparison of two values that differ greatly in magnitude, as is the case here.

I saved the file in a csv format and proceeded to graph these values in MS Excel with the y-axis is log scale.

The limits of the x-axis correspond the wavlenth/frequency range measured by the James webb Telescope(long wavelength visible to mid-infrared). (keep in mind that the x-axis here is Frequency, so UV would be in the far end and infrared would be in the near end of the y-axis)

As you can see, the radiation given from the body of the Hubble(orange) and that of the James Webb(blue) has a huge difference in the region of interest. The closest the James Webb comes to the Hubble is by a factor of 1029 at around the mid-infrared region! Around 4*10^13 onwards, the radiation from the JWST continues to plummet greatly so I didn’t bother adding those values to the graph.

Hopefully this gave you an understanding of the implication of the temperature difference of the telescopes.

Thus comes the end of the blog on the upcoming James Webb Space Telescope. Hopefully this gave you insight into the important details and its advantages over previously built telescopes. Please comment any new ideas or observations! Thank you!