The past 250 years of optical telescopes have seen revolutionary discoveries and technology that the telescope's inventor, a seventeenth century spectacle-maker by the name of Hans Lippershey, maybe wouldn't have believed possible.
When we look back through the annals of telescope history, we find that a significant turning point came, coincidentally, just five years after the United States' Declaration of Independence was christened.
It was back in England, in 1781. William Herschel had just made what was possibly the greatest astronomical discovery the world had seen up to that point: a new planet, Uranus. The fact that Herschel had found a seventh planet from the sun was revolutionary in itself. All the other planets, from Mercury to Saturn, had been known since antiquity, obvious in the night sky to the naked eye.
Uranus, on the other hand, isn't really visible without optical aid, and its discovery illustrated the power of the telescope to dramatically widen our vistas. Moreover, Herschel found the new planet using a 6.2-inch (157-millimeter) reflecting telescope that he had constructed himself. He was looking through it from the back garden of his home in the Somerset city of Bath.
Herschel was a prolific builder of telescopes, polishing and shaping their speculum mirrors. The 6.2-inch telescope was a midget compared to some of his other beasts, including the famous discovery machine that was the 20-foot, or -meter, in focal length telescope with its 18-inch (457-mm) aperture, and the less successful 40-foot (12-meter) telescope.
Herschel proved that telescopes could do serious science. "As a self-taught astronomer, William Herschel transformed the reflecting telescope from what had generally been thought of as a scientific toy into a serious scientific tool," British science historian Robert Smith of the University of Alberta in Canada told Space.com. "At the root of all Herschel's efforts is his telescope building, because he had to build these big telescopes himself."
Refractors and reflectors
Telescopes come in two main forms: the reflector and the refractor.
Reflectors use mirrors to reflect light to a focal point where the eyepiece is located; refractors use lenses to focus light. In the 18th and 19th centuries, British reflectors like Herschel's were the dominant telescope category, as exemplified by those constructed by the likes of Liverpool's William Lassell and Ireland's Third Earl of Rosse, William Parsons. However, across the English Channel in mainland Europe, refractors, which at the time were optically higher quality, were dominant instead.
"You have to distinguish what's going on in Britain with what's going on elsewhere," said Smith.
Lassell built reflectors with apertures of 24 and 48 inches (61 and 122 centimeters). And the great Leviathan of Parsonstown in Ireland is Lord Rosse's own behemoth, still standing today as a UNESCO World Heritage Site. While the likes of Lassell and Lord Rosse were wealthy, self-taught "grand amateur" scientists, in Europe refracting telescopes were used by academics at universities to make precise measurements of the cosmos, from the orbits of double stars to the distance to stars using parallax.
"On the European continent, refractors were used by professional astronomers whose focus was on precision, whereas for Lassell and Rosse the focus was on collecting more light to see fainter objects," said Smith.
In comes the U.S.
Building large refracting telescopes had its challenges and led to a period referred to as the "Telescope Race," where prestige was equally a motivation alongside science. The "race" was won by the 40-inch (one-meter) refractor at Yerkes Observatory in Chicago in 1897, which cost $500,000 at the time (about $20 million in today's money) and was provided by financier Charles Yerkes who made his money developing Chicago's public transport system, often resorting to bribery in order to win franchises.
Businessmen building telescopes was nothing new, of course: William Lassell made fortunes from brewing, but he also used his own telescopes. What was different now was that men wealthy beyond anyone's dreams saw observatories as vanity projects rather than as scientific instruments to use themselves. The introduction of substantial sums of American money provided by wealthy businessmen was a sea change in both the building of telescopes and the fortunes of astronomy in the U.S.
"I have argued that in the 1880s, America was a bit of an astronomical backwater compared to Europe, but by the 1920s the United States had become the leading nation, certainly in terms of observational astronomy," said Smith.
It wasn't just Yerkes. In California, wealthy landowner James Lick founded the Lick Observatory on Mount Hamilton after being dissuaded from his original plan to build a giant pyramid as a monument to himself in downtown San Francisco. Percival Lowell, obsessed with his delusions about canals on Mars, founded the Lowell Observatory in Arizona in 1894. And on Mount Wilson in California, something big was stirring in part thanks to benefactor John Hooker.
Reflectors come of age
Size has always been a driving force, a challenge and a problem for telescope builders. Even today, the Yerkes refractor is still the champion refractor, only rivaled by the one-meter Swedish Solar Telescope on La Palma, but even this is stepped down from 42 inches to 39 inches (107 cm to 99 cm). The issue is twofold: the thicker the lens, the more the incoming light is attenuated and hence the greater the light loss, so you lose any benefits a large lens might provide. Plus, the heavier a lens is, the more it will sag and deform, destroying its ability to focus properly.
This glass ceiling, if you'll pardon the pun, was circumvented by reflecting telescopes following a vital development in the mid-1800s. Herschel and his peers had been using mirrors made from speculum, which is a reflective and slightly toxic (thanks to the small quantities of arsenic added to the copper and tin mix) metal that is prone to tarnishing easily. Herschel had to polish his telescope mirrors frequently to keep them spick and span.
Then in the 1850s scientists Léon Foucault and Carl August von Steinheil figured out a way to add a thin layer of silver to glass, creating mirrors that were much more reflective than speculum, that weighed less and which didn't tarnish. This paved the way for building bigger and better reflectors, fulfilling the promise of the work started by William Herschel.
One key figure in the resurgence of the reflecting telescope around the turn of the 20th century was the astronomical optician George Ritchey, most famous today for being one half of the duo who invented the Ritchey–Chrétien telescope design that is popular with amateur astronomers, particularly planetary imagers, and which is also employed on dozens of professional telescopes including the largest ground-based telescopes currently in operation.
Ritchey had built a 0.6-meter (24-inch) reflector at Yerkes before he was hired by George Ellery Hale, who was the director of Mount Wilson Observatory. On Mount Wilson Ritchey designed a 1.5-meter (60-inch) reflector in 1908 and then what would be the largest telescope in the world at that time, the 2.5-meter (100-inch) Hooker Telescope, made possible thanks to a generous financial gift from John Hooker. The Hooker Telescope saw its first light in 1917.
"If I had to pick one key optician around about 1900, I would pick George Ritchey," said Smith. "Though Ritchey's innovative approach sometimes became too innovative for Hale. Maybe 'firing' is too strong a term, but Ritchey left Mount Wilson because he just didn't get on with Hale."
A revolution in cosmology
Despite the parting of ways, Ritchey's legacy was secure because the Hooker Telescope transformed astronomy and cosmology, thanks to the work of Mount Wilson staff astronomer Edwin Hubble and his colleague and assistant, Milton Humason. Thanks to the sheer resolving power of the Hooker Telescope, Hubble was able to resolve the mysterious spiral nebulae as galaxies in their own right, proving that galaxies other than our Milky Way exist.
He was able to confirm their distance by using the Hooker Telescope to identify individual stars in those galaxies, stars we call Cepheid variables, which have a particular relationship between their period of variation and their peak brightness. The work of Henrietta Swan Leavitt tells us that the longer a Cepheid's period of variability, the brighter they become, and knowing how luminous they should be, Hubble could compare that to how bright they appeared in the night sky through the Hooker Telescope and then deduce how far away they and their galaxies must be. Hubble and Humason later measured the redshifts of these galaxies, finding they are almost all moving away from us and that the cosmos is expanding.
The Hooker Telescope was superseded by the Hale Telescope, named after the Mount Wilson director and which has a 5.1-meter (200-inch) mirror and came into operation on Mount Palomar in 1949. The Hale Telescope remained the largest optical telescope in the world until 1975 and the Soviet Union's six-meter BTA-6 telescope, and this wasn't beaten until 1993 and the construction of the first of the twin 10-meter telescopes of the Keck Observatory on Mauna Kea in Hawaii. At this size, mirrors have to be composed of multiple individual segments rather than being cast as one solid mirror because gravity would otherwise cause the mirror to deform.
During the second half of the twentieth century, astronomers began to identify the best locations for giant optical telescopes, away from the smog and light pollution of cities whose urban sprawl was growing so great that not even Californian mountaintops could escape them. Today, the best telescopes in the world cluster atop Mauna Kea, on numerous peaks in Chile's Atacama Desert, and in the Canary Islands.
How the Hubble Space Telescope democratized astronomy
For the most exceptional views, you can't beat space itself. In orbit above our obscuring atmosphere, or stationed 1 million miles (1.6 million kilometers) away at the L2 Lagrange point, space telescopes enjoy unprecedentedly clear views of the cosmos. The most recent space telescopes include the James Webb Space Telescope and, launching in September, the Nancy Grace Roman Space Telescope, but the most famous of all is of course the Hubble Space Telescope.
Launched in 1990, Hubble continues against all the odds to push the frontiers of science.
"Its output is staggering," said Smith. It has made over 1.7 million individual observations in the past 36 years, and 23,000 research papers based on those observations have been published by a total of nearly 29,000 astronomers.
"This is the democratization of astronomy," said Smith, and it completes a transformation in astronomy over 250 years in the making. Back when the United States was being founded, astronomers were generally lone wolves, self-taught and self-motivated like William Herschel. Then, throughout the nineteenth century astronomy increasingly became the purview of wealthy men, and even moving into the twentieth century, astronomical discovery was confined to a small group.
"For example, in the early 20th century, the only people who had access to the Mount Wilson telescopes were the staff at the Mount Wilson Observatory, and if you were not on staff then you just couldn't use the largest telescopes in the world, so as an observational astronomer you would be at a disadvantage," said Smith.
While this began to change in the latter half of the twentieth century, with organizations such as the European Southern Observatory and the National Optical Astronomy Observatory (now NOIRLab) giving access to telescopes to a wider swathe of people, the Hubble Space Telescope has really spearheaded this democratization.
"People all over the world can use Hubble," said Smith. "It opens up so many possibilities for astronomical research, with thousands of people using it."
From well-heeled enthusiasts to businessmen looking for a legacy, astronomical research has now truly become accessible to people worldwide thanks to the largest telescopes in space and on Earth. Perhaps that is the ultimate achievement of the past 250 years of telescopes.
