From Hubble to the Webb Space Telescope

by S. Prasad Ganti

The first pictures from the James Webb telescope were released recently. The prize comes after decades of hard work by thousands of scientists and engineers and billions of dollars of cost overruns. In the end it was worth it. These are only the first pictures and there is a lot of science left to be done with this latest science-engineering marvel.  The story of the telescope starting from its inception to the first pictures has been captured eloquently in the recent TV program titled “The Ultimate Space Telescope” in the Nova series on the PBS. The link below can be used to watch the one hour video in its entirety. 

https://www.pbs.org/wgbh/nova/video/ultimate-space-telescope/

Webb’s predecessor is the Hubble space telescope. It is still going strong watching the skies and reporting back what it sees. While the new vanguard picks up the mantle. The time used on such expensive telescopes is really valuable and the watching projects need to be prioritized appropriately. In 1995, scientists took a gamble using Hubble. They decided to focus the telescope on a dark patch of the sky where there seems to be nothing visible either to the naked eye or the other powerful ground based telescopes. They were surprised to find thousands of galaxies from the early history of our universe showing up in this deep field image. They followed it up with an ultra deep field image in 2004. The image given below courtesy NASA, required 800 exposures taken over the course of 400 Hubble orbits around the Earth. Obviously, such complex images are assembled from thousands of individual pictures and colors are assigned based on the frequencies of the signals received. 

To enable us to look further out into the Universe and look back into the time when our Universe was in its infancy, there was a need for a more powerful telescope. The quest for the new Webb telescope started. After going through long travails of design challenges and increasing costs, it was launched on Christmas day in 2021. And everything happened in a text book fashion thereafter. The unfolding of the heat shield and the complex mirror in space and its travel of a million miles into space to L2 Lagrangian point where the gravity of the Sun almost cancels out the gravity due to Earth. And the telescope gets a free ride to watch the Universe.   

To watch the early Universe, the observations need to be done in the infrared region. Because the ancient light is red shifted due to the expansion of the Universe. So much red shifted that it has passed into the invisible infrared region. Hubble had some infrared capabilities but Webb is exclusively built for infrared observations. As a result, while Hubble can observe 500 million years after the Universe was formed, Webb pushes back the timeline to the 200 million year old Universe. These timelines are very early compared to its present age of about 13 billion years.

To meet the challenges of observing in the infrared region, there should be no other source of heat around the telescope or even from within itself. It has to be cooled to very very low temperatures, close to absolute zero. A giant heat shield was built to prevent the heat from the Sun, the Moon and the Earth reaching the telescope. The whole telescope is kind of a large freezer. 

The mirror is about an order of magnitude larger than that of Hubble. It is made of specially processed  material Beryllium whose expansion and contraction due to temperature differences is very minimal. It is very conducting with heat flowing uniformly across the complete surface. It is very stiff to encounter the harshness of the space. It is coated with a very thin film of gold to get the maximum reflectivity. Yet it is about a tenth as heavy as Hubble’s mirror. Unlike Hubble’s, the mirror was constructed from 18 hexagonal segments. Each of them could be moved very precisely using the attached motors. Very minute movements were required to align all of them to get a perfect picture. The movement of a millimeter took almost a day. Anything quicker would generate more heat and jeopardize the telescope. It was literally watching the paint dry. That is why it took several months to do the alignment. The engineering specifications are clearly mind boggling.       

Electronics contained within are also state of the art. Having fancy acronyms, basically they are the infrared cameras and spectrographs (which analyze the spectrum of the infrared radiation received). The first pictures from the Webb telescope have been released recently. They are spectacular. More like Van Gogh paintings. My favorite is an equivalent of the Hubble extreme deep field and is shown below. Courtesy of NASA and titled “SMACS 0723 galaxy cluster”, it represents a tiny sliver of the sky, yet contains thousands of galaxies. 

Located at a vast distance where no human has ever gone, the telescope needs to be abandoned if it cannot be fixed. There cannot be any servicing missions of any sort. So far things are good. Let us hope for the best for the life of Webb. Such scientific observations triggered by engineering marvels inspire us. Coming in the wake of the dirty side of the human mind which results in divisive politics and territorial wars. Not long ago, Hubble was the ultimate space telescope. Now it is Webb. What would the future space telescope look like?

This entry was posted in Mid-summer 2022, Sidereal Times and tagged , , . Bookmark the permalink.

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