James Webb Telescope: Why Mirrors Over Lenses

Have you heard that most previous space telescopes used lenses instead of mirrors? It's a common misconception. In this article, we'll explore the reasons why the James Webb Space Telescope (JWST) utilizes mirrors, and not lenses, for its optical system.

Why Mirrors Prevail

Despite the popular belief, very few space telescopes (on record) use lenses. Reflecting telescopes, which use mirrors, are more favored over refracting telescopes (which use lenses) for several critical reasons.

Reflecting telescopes like the JWST are significantly lighter, allowing for easier deployment and maneuvering in the harsh environment of space. Additionally, mirrors do not suffer from chromatic aberration, which causes different colors of light to be focused at different points, resulting in a distorted image. Mirrors uniformly focus all wavelengths of light at a single point, ensuring clearer and more accurate images.

Comparing Lenses and Mirrors

Lenses have their own set of challenges. One of the main issues is their limited size. The largest refracting telescope in the world is the Yerkes Observatory at 1.02 meters, far smaller than the 6.5 meters of the JWST's primary mirror. This limitation becomes a serious drawback when trying to capture light from far away celestial objects.

Another issue with lenses is their susceptibility to distortion. Due to their weight, large lenses need to be supported from the outside, causing them to sag and potentially distort the image. This problem is significantly less pronounced with mirrors, which are not structurally subject to the same limitations.

The JWST's Optical System

The JWST's optical system is an efficient design that leverages the power of mirrors. The primary mirror is composed of an array of 18 smaller mirror segments that are meticulously aligned to focus incoming light. This design ensures optimal light collection and precise imaging.

Here's a simplified overview of how the JWST's optical system works:

The primary mirror collects light from celestial objects. The light is then directed to the secondary mirror, which further focuses the light. The secondary mirror sends the focused light down the central shaft, where it bounces off the tertiary mirror and the fine steering mirror. The light then enters the Integrated Science Instrument Module (ISIM), which houses the scientific instruments. Each instrument has its own set of pick-off mirrors to direct the light into the instruments where the data is processed further.

This complex yet elegant system allows the JWST to capture and analyze the faintest and furthest light from the universe, providing crucial insights into the cosmos.

Capture of Infrared Light

A key distinguishing feature of the JWST is its ability to capture infrared light, which cannot be perceived by the human eye or even a normal optical telescope. Infrared light is crucial for studying the early universe and distant galaxies, as it allows us to observe regions that are obscured by cosmic dust.

The JWST's four scientific instruments are specifically designed to capture this faint infrared light. This capability enables the telescope to peer through cosmic dust, study very cold and distant objects, and provide a clearer picture of the universe's development over time.

The signals captured by the JWST are transmitted back to Earth and then relayed to the Webb Science and Operation Center at the Space Telescope Science Institute in Baltimore, USA, where the data is analyzed and interpreted.

Conclusion

The future of space telescopes lies in the use of mirrors over lenses. The James Webb Space Telescope's innovative design and reliance on mirrors ensure that it can capture and analyze light from the most distant and intriguing corners of the universe. Understanding why mirrors are preferred over lenses is crucial to appreciating the groundbreaking work of the JWST and the invaluable insights it provides into the cosmos.