1917

Hooker Telescope

George Ellery Hale (1868–1938)

Back in the early 1900s, scientists had not yet discovered galaxies because they did not yet have a telescope big enough to resolve the stars in distant galaxies. The Hooker telescope, which saw “first light” after being commissioned by American solar astronomer George Ellery Hale in 1917, has a 100-inch (2.5 meter) mirror and was the first telescope big enough to do it. Using the Hooker telescope, Edwin Hubble was able to announce the existence of galaxies in 1929.

The Hooker telescope was the largest telescope in the world for three decades and the second largest for three more. It was an amazing engineering achievement at the time because of the many colossal, precise pieces that had to come together for it to work so flawlessly.

The first piece was the 100-inch mirror, made of a solid glass disk 12 inches thick and weighing 9,000 pounds (4,100 kg). A huge grinding machine was built to grind and polish it to its precise shape. This glass disk was silvered and then mounted in a framework able to hold it steady, keep it from flexing, and swivel and tilt it smoothly and precisely.

The framework and trusses that hold the mirror came from a battleship factory. Combined with the mirror, they weigh 100 tons (91 metric tons). The assembly sits on a mercury bearing to assure smooth operation. The mirror is backed by a network of water-filled pipes to maintain its consistent temperature. And the entire structure moves using a clockwork that keeps the telescope pointing at the same point in the sky for long periods of time as the planet rotates, allowing long-exposure photography of faint celestial objects. A 4,000-pound (1,800 kg) mass pulls on cables as it falls, and turns the whole telescope and dome very slowly in synch with the earth’s rotation.

The engineering and precision of this telescope allowed it to take the photos in the 1920s that for the first time confirmed the existence of galaxies. Amazingly, the same telescope is still used today over one hundred years after the massive glass disk was cast. Future telescopes like Hubble and Keck were developed based on some of the fundamental ideas behind the Hooker telescope.

SEE ALSO The Hubble Space Telescope (1990), Keck Telescope (1993).

Plans for the 100-inch reflecting telescope at Mount Wilson Observatory.