A Mirror of the Past – Herschel and the James Webb Space Telescope

William Herschel


For the longest time, curious minds have been drawn upwards to contemplate the light of the stars. The vastness of space, populated with innumerable glowing pinpricks, has an extraordinary power to draw you in – a sense shared by astronomers the world over, including Bath’s most renowned stargazer, William Herschel.

Friedrich Wilhelm Herschel first arrived in England in 1757, not as an astronomer, but as a young musician fleeing Hanover during the Seven Years’ War. During his early years in England he performed as a soloist, composed music (including 24 symphonies in his lifetime), and in 1766 was installed as the organist of the Octagon Chapel in Bath.

As Herschel circulated in English society, he began to encounter amateur astronomers who piqued his interest in the heavens. Not one to do things half-heartedly, and dissatisfied with the quality of hired telescopes, Herschel set to the task of assembling his own reflecting telescope with which to pursue his interest. This kind of telescope, which uses a curved mirror to focus light, has some advantages over the lens-based instruments used by other amateur astronomers of the time, but the technology was still in its infancy.  

In the late 18th Century, the only material appropriate for a telescope mirror was speculum, a temperamental alloy made from copper and tin. Because speculum is very fragile, casting a large, symmetric surface from it is challenging, and therefore telescopes were restricted to very small mirrors – most not exceeding 3-inches in diameter. Speculum also tarnishes very easily, requiring frequent re-polishing. With each successive polish, the astronomer runs the risk of deforming their carefully engineered focus, creating blurriness and rendering the telescope inoperable.

Unperturbed and full of ambition, Herschel estimated that he would need a mirror at least 6-inches in diameter to compensate for the inadequacies of his rental scopes. Devoting himself to this cause, he converted his home from a music studio into an astronomers workshop, in which he cast, ground, and polished ever larger speculum mirrors. After much practice and several iterations, Herschel eventually produced a functioning reflecting telescope, featuring a 6-inch mirror with a 7-foot focal length. According to writer and science historian Richard Holmes, upon taking the telescope out into the street to gaze at the stars, ‘it was immediately apparent that Herschel had created an instrument of unparalleled light-gathering power and clarity’.

Working from the garden of his house at 19 New King Street (now the Herschel Museum of Astronomy), Herschel would nightly set up his new telescope to stare into the heavens. It was on such a night, on the 13th of march 1781, that he first observed Uranus – which at that time had been incorrectly designated as a far-off star in the constellation of Taurus. With his improved optics, Herschel clearly saw the planet appearing as a disk rather than a pointlike star, and at first assumed it to be a comet or nebula. By making a series of observations at different magnifications, He was able to conclude that the object did not behave like a star, and indeed seemed to have changed position over the course of several days. 

Herschel then reported the sighting of this ‘comet’ to the Astronomer Royal, Nevil Maskelyne. Little did he know it, but this correspondence marked the start of the first planetary canonisation since our five nearest solar siblings were observed by the ancient Babylonians, causing a revolution in our understanding of the Solar System. 

It is remarkable that, despite the passage of nearly a quarter-millennium since Herschel gazed at the stars through his home-made telescope, the fundamental challenges of telescope design remain largely unchanged. When pared down to its essential components, the James Webb Space Telescope (JWST), an ambitious multi-decade project set to launch on December 18th, operates along the same principles as Herschel’s 7-foot cylinder. It’s functionality is, too, largely dictated by the quality of its primary mirror, and, like Herschel in his workshop, engineers have laboured over the design for years to ensure that it is fit for purpose.

JWST is the successor to the Hubble Space Telescope (HST), our previous eye in the sky and the go-to device for observing extremely distant newborn galaxies. The Hubble Deep Field, one of HST’s most significant achievements, is an image of thousands of these newborn galaxies, fully formed shortly after the Big Bang. Of course, if such galaxies formed this early, it begs the question of how, and when did it happen? Unfortunately, Hubble is only designed to see visible light, but when objects are as old and far away as the dawn of the universe, the light they emit starts to shift into the infra-red portion of the spectrum, making them invisible to Hubble. The primary objective of JWST is to detect these extremely faint infra-red emissions in order to discover how the first stars and galaxies formed.

To measure the light from these distant galaxies, JWST scientists and engineers calculated that a primary mirror 6.5 metres across would be required. Although we have come some way since Herschel’s 6-inch speculum mirror, 6.5-metres is still an ambitious undertaking. Such a huge reflector is difficult to make even for use on the ground, and a mirror of this size has certainly never been launched into space before. The Hubble Space Telescope’s 2.4 metre mirror, for example, would simply be too heavy to launch into orbit if it were scaled up to match this new design. 

The JWST team therefore had to come up with innovative ways to construct the mirror so that it was light enough – only one-tenth the mass of Hubble’s mirror per unit area – yet strong enough to support its own weight. These unique requirements, much like in Herschel’s day, have brought the candidate materials down to just one possible match; in this case Beryllium. Beryllium is a light metal, very strong for its weight, that holds its shape well across a wide range of temperatures. As it was with Herschel’s 6-inch mirror, this 6.5-metre behemoth was cast, ground, and polished to exacting standards – ensuring that the surface would focus the light of far off galaxies perfectly onto a detector.

Much like Herschel’s revolutionary reflecting telescope, JWST represents a leap forward in imaging capacity, a curtain being lifted, granting us a new view of the universe. In Herschel’s day this resulted in a re-imagining of our place in the Solar System. Perhaps JWST will completely change how we think about the early universe, revealing a key step in our great cosmic journey that has been waiting, hidden amongst the stars since the birth of the Universe.

Don’t miss our upcoming talk on The New Generation of Hubble Telescope. Click here to get tickets.


  1. https://www.sciencehistory.org/distillations/a-giant-of-astronomy
  2. The Age of Wonder: How the Romantic Generation Discovered the Beauty and Terror of Science, Robert Holmes (2008)
  3. https://jwst.nasa.gov/