40-foot telescope

Lore & Background

The telescope was constructed by Sir William Herschel with the assistance of his sister Caroline Herschel, with components made in Clay Hall near Windsor. The 40-foot tube was made of iron, and the telescope was mounted on a fully rotatable alt-azimuth mount. It was paid for by King George III, who granted £4,000 for its construction and £200 a year for maintenance, though the telescope remained Herschel's property. During construction, while the tube lay on the ground, the King and the Archbishop of Canterbury visited; the King commented, 'Come, my Lord Bishop, I will show you the way to Heaven!'

Reader's Guide

The 40-foot telescope was the largest in the world for 50 years, until surpassed in 1845 by Lord Rosse's 72-inch reflecting telescope. Despite its size, it did not prove a substantial improvement over smaller telescopes due to problems with the mirrors and its unwieldy nature; the weather was rarely suitable, and most objects observed were also visible in Herschel's smaller telescopes. Its one achievement was the possible discovery of Enceladus and Mimas, though this is uncertain as Herschel used other telescopes simultaneously. The telescope was a local tourist attraction, visited by the rich and famous on their way to Windsor Castle, and was featured on Ordnance Survey maps. It was dismantled in 1840 by Herschel's son John due to safety concerns; the original mirror now resides in the Science Museum, London, and a 10-foot section of the tube is in the Herschel Collection of the National Maritime Museum, Royal Observatory, Greenwich.

Did You Know?

• Herschel eliminated the diagonal mirror of a standard Newtonian reflector, tilting the primary mirror so he could view the image from an observing cage—a design now called a Herschelian telescope.
• The first observation was on 19 February 1787, when Herschel crawled into the incomplete tube and used a hand-held eyepiece to view the Orion Nebula.
• The telescope's frame was dismantled in 1839–1840 by John Herschel, who feared it might collapse due to rot and endanger his young children.

Engineering a Giant

The Great Forty-Foot telescope was built between 1785 and 1789 at Observatory House in Slough, with components forged at Clay Hall near Windsor. King George III bankrolled the project with £4,000 for construction plus £200 annually for upkeep, though the instrument remained Herschel's personal property. His sister Caroline assisted throughout the build. The 12-meter iron tube sat on a fully rotatable alt-azimuth mount. The optical heart was a 48-inch speculum mirror—an alloy of copper, tin, and arsenic—cast at a London foundry. The first casting in October 1785 proved too thin at the center, and over a year of grinding and polishing couldn't save it. A second, thicker mirror was attempted in early 1788; the first try cracked during cooling, but a recast three weeks later succeeded. Herschel kept polishing it until August 1789. Rather than using a standard Newtonian diagonal mirror, he tilted the primary and stood in a cage at the front to view the image directly, a configuration now called a Herschelian design.

Gazing at the Heavens

The first light came on 19 February 1787, when Herschel crawled into the unfinished tube and hand-held an eyepiece toward the Orion Nebula, noting its extreme brightness and resolving several small stars within it. The telescope's most celebrated claim is the possible discovery of Saturn's sixth and seventh moons, Enceladus and Mimas, though Herschel was simultaneously working with smaller instruments, so attribution remains uncertain. His description of Sirius entering the field of view—comparing its brilliance to the rising sun—captures the visceral impact of the instrument. Beyond pure science, the telescope carried social significance: Caroline Herschel received a £50 annual pension as her brother's assistant, making her the first woman in England to be paid for astronomical work. Yet the giant proved unwieldy in practice. Weather rarely cooperated, the speculum mirrors tarnished quickly and demanded constant re-polishing, and most targets were equally visible through his smaller scopes. The last recorded observation was in 1815.

A Landmark of the Age

For half a century, no telescope on Earth surpassed the Forty-Foot in size, and its 48-inch mirrors held the world record until 1845. The instrument became a local landmark, appearing on Ordnance Survey maps and drawing curious visitors—wealthy travelers and dignitaries passing through Slough on their way to Windsor Castle. During construction, when the tube still lay on the ground, King George III and the Archbishop of Canterbury peered into its open mouth; the King quipped to the Bishop, 'Come, my Lord Bishop, I will show you the way to Heaven!' The telescope's cultural footprint extended into heraldry: it was emblazoned on the Herschel family coat of arms, depicted on a green mount with the astronomical symbol of Uranus, Georgium Sidus, shining above. The name itself reflects an era when telescopes were identified by tube length rather than mirror diameter, a convention that has since fallen out of use.

Dismantling and What Remains

By the late 1830s, the wooden frame had begun to rot, and John Herschel, returning from his observations in South Africa, feared a collapse that could endanger his young children. In early December 1839, the family gathered for a small ceremony to mark the dismantling, and after the work was done, they sang a commemorative song inside the tube itself. The iron tube fared better than the wooden structure, and a 10-foot section of it, along with the original mirror, survives to this day. The instrument had served its purpose: from 1789 to 1840 it stood on the Slough grounds, but its practical astronomical yield had been modest compared to Herschel's smaller, more nimble reflectors. The decision to take it apart was less a judgment on its scientific worth than a practical safety measure, yet it marked the end of a fifty-year reign as the world's largest telescope and the close of an era in which a single family's backyard housed the most powerful eye on the sky.