From the dawn of consciousness, we humans have pondered the sky, wondering, “What is out there?” For thousands of years our ancestors were limited to what they could see with their eyes: The Sun, Moon, five planets, comets and 10,000 stars. In 1609 everything changed when Galileo first turned a telescope on the sky. His telescope had a lens with a diameter of 37mm and a magnification of 20-30 times, similar to large binoculars today.
But it was enough that, for the first time, a human could see mountains on the moon, spots on the Sun, the four largest moons of Jupiter, and vastly more stars than anyone had ever imagined.
The era of the telescope had begun. Since then, we astronomers have sought ever larger, more powerful telescopes to see further into the depths of space and to resolve more detail.
To see further we must capture more light, so we now make telescopes with mirrors that can be made much larger than the lenses in binoculars and small telescopes. We characterise telescopes by their “light gathering power” which depends on the area of the aperture. As an example, when your eyes are dark-adapted at night your pupil has a diameter of 5mm. A modern large telescope may have a mirror with a diameter of 5m collecting a million times more light than your naked eye.
The resolution of a telescope – its ability to see detail – also depends directly on the diameter of the aperture. So, astronomers always want bigger telescopes to see fainter, further and in more detail.
On 2 November 1917 the world’s then biggest telescope, the 2.5-m diameter “Hooker” telescope, captured its first light at Mount Wilson Observatory, which sits on a 1740-m peak in the San Gabriel Mountains above Los Angeles in California. The English poet Alfred Noyes was present that night, and he used the observatory as a setting for the opening of his ‘Watchers of the Sky’, an exploration of scientific discovery in poetry.
With the Hooker Telescope Edwin Hubble discovered galaxies beyond the Milky Way. He and Milton Humason discovered that the Universe is expanding, leading to the Big Bang theory. And Fritz Zwicky found the first evidence for dark matter.
By 1928 the founding director of the Mt Wilson Observatory, George Ellery Hale, was planning a bigger telescope, a 5-m behemoth to go on Palomar Mountain near San Diego to get away from the bright lights of Los Angeles. But casting a perfect mirror blank of glass that size with no imperfections had never been done before.
Hale contracted with George McCauley of Corning Glass works, in New York State, to pour the giant mirror blank using Corning’s special Pyrex glass, which has a low expansion with changing temperatures. The first attempt in 1934 failed, because part of the mould broke, but McCauley decided to allow the glass to cool (anneal) for 1 year to see what it looked like.
The 17-ton failed mirror blank was not satisfactory to use for the telescope, so in 1951 it was hoisted into place vertically in a room that was built around it at the world-famous Corning Glass Museum. I was delighted to visit that museum in March 2025 and see the famous blank for myself, along with other glass treasures going back 4000 years.
There was huge public interest as a new, perfect mirror blank was made by Corning, cooled for a year, and shipped to California in 1936. The radio commentator Lowell Thomas reported at the time of the pouring of the glass that the event was “the greatest item of interest to the civilized world in 15 years …”.
It took 11 more years to grind away 4.5 tons of glass to give the mirror its perfect shape. The Hale 5-m telescope became the largest telescope in the world at its first light on 19 January 1949, two months and two days after I saw my own first light in San Diego in 1948.
The Palomar telescope reigned supreme for 26 years. As a child in San Diego growing up almost in the shadow of Palomar Mountain, I dreamed of being a scientist and using this great telescope to study the Universe.
I never did get to do that, but I do work with much larger telescopes now: the 8-m Very Large Telescope (VLT) in Chile, the 8-m Subaru Telescope in Hawaii, and the 10-m Southern African Large Telescope (SALT) at Sutherland here in South Africa – a dream come true.
Now in Chile the European Extremely Large Telescope (ELT) is under construction at a cost of €1.5 billion! It will have 798 individual 1.4-m mirrors aligned to only a few billionths of a metre, giving the telescope a diameter of 39m. This gigantic telescope will be one of the most sophisticated and complex machines ever built. The ELT is a telescope way beyond the imagination of Galileo or George Ellery Hale. We astronomers eagerly await its completion.
- Donald Kurtz is Extraordinary Professor at North-West University in Mahikeng. He has an A-1 rating from the South African National Research Foundation, its highest rating. He also holds appointments in the UK of Emeritus Professor at the University of Central Lancashire and Visiting Professor of Astrophysics at the University of Lincoln. He was previously Professor of Astronomy at the University of Cape Town, where he worked for 24 years.
- This article was first published in Talk of the Town, April 24, 2025. The newspaper serving the communities of Ndlambe and the Sunshine Coast, with a weekly wrap of Makhanda news, is available at stores from early on Thursdays.
