On 25 December 2021, the James Webb Telescope (JWT) launched. This last month up to the launch had a couple of delays due to weather and an incident for which they had to ensure there was no damage. At the time that I am writing this, the JWT has not yet been brought up to full operation. But thus far, things have been going well. The JWT is often thought of as the successor to the Hubble telescope. Some call it a replacement, but its capabilities are not identical to that of Hubble. It was designed based on some of the findings of Hubble. I’ve got some readers whose living memory does not go back as far as the Hubble telescope. Let’s take a brief walk-through history.
Edwin Hubble (the person, not the telescope) is most well-known for his astronomical observations and discoveries. Some of his discovers included that there were galaxies beyond the Milky Way, found methods to gauge cosmic distances, and discovered that the further aware from earth that an observed galaxy is, the faster that it is moving away from other galaxies (this is known as “Hubble’s Law”). Edwin Hubble performed many of his observations using what was then the world’s largest telescope, named after James D. Hooker. Naming large telescopes after people was a bit off a tradition.
Space telescopes were proposed in the early 1920s. As is the case with many high investment scientific endeavors, Hubble’s planning was a joint venture that crossed international borders. The USA’s NASA and the European Space Agency both made contributions to Hubble. The project was started in the 1970s with plans to launch in 1983. There were delays that prevented this. But it finally launched in 1990. Much to the disappointment of many, after launch it was discovered that the Hubble’s main mirror was incorrectly manufactured; the telescope was taking distorted images. It was possible to use software to make some corrections in the image, but servicing was needed to correct the problem. Hubble, being positioned in low earth orbit, was accessible to astronauts by way of the space shuttle. A few years after its launch in 1993 a servicing mission corrected the optical problems. Through several other missions Hubble was maintained and upgraded until 2009. The telescope had been used for over 30 years. The telescope is still partially operational now. Some of the gyroscopes have failed as has one of the high-resolution cameras. But some other cameras and instruments are still operational. A near-Infared telescope is functional but remains offline for the time being. It is expected to be able to maintain functionality until 2040.
While Hubble was operating in its earlier years, plans for its successor had begone. Planning for the James Web Telescope began about 1996. The year prior, in 1995, was the Hubble Deep Field photograph. The Hubble telescope was aimed at a dark patch of sky and took a long exposure photograph. For 10 days the telescope collected whatever bits of light that it could. The result was an image that was full of galaxies! Around 10,000 galaxies were observed through the deep field imaging. Visible, infrared, and ultraviolet wavelengths were used in the imaging.
Earlier I mentioned Edwin Hubble’s discovery of how galaxies further aware are recessing from earth at a faster rate than ones that are closer. The faster the galaxy is moving away, the more red-shifted the light from it is. Red shifting is a form of the doppler effect observed on light. Just as the pitch of a sound will be higher in pitch if it is moving toward and observer and lower in pitch when it is moving away, visible light shifts to become red if the source is moving away from an observer and blue if it is moving closer. Part of the purpose of the JWT is to make observations of astronomical bodies much more distant than the Hubble could. Since these bodies will be more red shifted, the JWT was designed to be sensitive to light that is red shifted. While both the Hubble Telescope and JWT have infrared capabilities, the JWT is designed to see light that is much more red. Because of this goal, the JWT has some rather unusual elements of design and constraints.
Objects radiate their heat out as electromagnetic waves. For objects that are hot enough, we are able to see this radiation as light; a hot piece of metal may glow red or orange. Objects with no glow in visible light may still give off light in the Infared spectrum. Such objects include the earth and the moon, which reflect infrared from the sun and emits heat.
The Hubble was positioned in low earth orbit, about 570km above earth. The moon is about 385,000 km from earth. To avoid the glow of the earth and moon, the JWT is much further aware at 1,500,000 km. The Hubble was in orbit around the earth, but the JWT isn’t really in orbit. It is in a Lagrange point. Objects positioned in a Lagrange point tend to stay in position with very little active adjustments needed.
The telescope is still exposed to the sun, which would potentially heat the telescope up and cause the telescope to have its own glow that would interfere with imaging. To prevent the sun from being a problem, the telescope has a multilayered shield on the portion that is facing the sun. The shield is designed to reflect light away and to dissipate heat before it reaches the imaging elements of the telescope. Another unique element of the telescope is the exposed reflector. The reflector is composed of several hexagon-shaped mirrors coated in gold. Gold reflects infrared light very well. Using hexagon segments for the mirror simplifies manufacturing and allows the elements to be more easily folded; the telescope was launched in a fairing with the mirror folded and the sunshield sandwiched over the mirror.
The JWT’s field of vision is much wider than that of Hubble. It collects about 15 times more light than the Hubble and has a wider field of view. The telescope’s look stands out in that there is no tube wrapped around the optical elements. Optical tubes on terrestrial telescopes protect the elements from debris and stray light. Because of the telescope’s sun shield and its position, it won’t be exposed to stray light from the sun. I’ve not been able to find references on any concern for the mirror being exposed to debris in space (despite being a hard vacuum, it isn’t without debris) but unlike on earth, there are not concerns with it collecting dust. With these differences in design and capabilities and design, what are the plans on how this telescope will be used?
While I’m not a fan of this description, I often see its purposed summarized as “looking back in time.” Despite my dislike of this description, it isn’t inaccurate. Light takes time to travel. If you look toward the moon, the light reflected from the moon took 3 seconds to travel to your eyes. You are seeing how the moon looked three seconds ago. For the sun, it’s eight minutes ago. These bodies to change dramatically enough for the delay to make a significant difference. But as we look at bodies that are further away, the time it takes to travel becomes more significant. From Mars to earth is about 22 minutes. Jupiter to earth is about 48 minutes. It takes a few hours for light to travel between Pluto and earth. For other galaxies, light takes years. While light-years is a unit of distance, it also tells you how long it takes for light to travel from a body. The JWT’s light collection capabilities make it capable of seeing light far enough aware to collect information on the earlier universe. The Hubble telescope was able to collect information on the universe from about ~13.4 billion years ago while the James Webb Telescope is expected to collect data from about 13.7 billion years ago. That 300,000,000 difference
As of yet, the James Webb Telescope hasn’t taken its first image. This is about 4 days after launch. It has deployed the sun shield. It will take about another 25 days for the telescope to reach its intended position. Before then, the mirror segments must be unfolded into place. If you are waiting to see images from the JWT, it will be a while. There’s calibration and preparation needed. Other than test images, we might not start seeing full images for another six months.
If you want to keep track of where the telescope is and its status, NASA has a site available showing the tracking data.
Developments on the James Webb Telescope will be slow to come at first, but it should be interesting.