Telescope makers struggled with lenses for 200 years, then finally were able to make large refractors by the end of the 19th century. Then as the 19th century came to a close, they reached the limit of refractor size. It took several years for them to learn to make large reflectors.
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Galileo did not invent the telescope, nor was he the first to turn one to the heavens. The history of telescopes began in 1608 when Jacob Metius in the Netherlands developed the ‘spyglass,’ which had an objective lens at one end and an eyepiece lens at the other. The objective lens was about 15 mm in diameter. The eyepiece was a double concave lens. Metius’ spyglass had a magnification of only about 3x. Englishman Thomas Harriott bought one of Metius' spyglasses in July of 1609, and pointed it at the moon. Over the next few years he made many drawings of our satellite, which show many of the features we are now so familiar with.
Several months after Harriott first viewed the moon, Galileo took Metius’ concept and built his own telescope. It was 38 inches long. He gradually improved it with a larger objective lens, which gave him more light gathering power, and a longer tube, eventually five to six feet, which gave him more magnification—eventually about 30x, and he mounted it on a stand which steadied it to make viewing more stable.This is called a refractor telescope because the objective bends the light rays to a focal point.A telescope's power is determined by the ratio between the objective lens’ focal length (fl) in millimeters or centimeters and the eyepiece fl in millimeters or centimeters. Just so both are in the same measurement. For example, if the objective's fl is 1000 mm and the eyepiece’s fl is 25 mm, the magnification is 40x.
Galileo also made drawings of the moon, but his flash of genius was to turn his telescopes to the wanderers in the sky—the planets. He discovered Jupiter's four largest moons, and Saturn's rings.
After Galileo, Johannes Kepler proposed making the objective thinner and the eyepiece convex. Theoretically, this should give the telescope a wider field of view and remove some of the fuzziness Galileo’s instruments suffered from. Kepler never built his instrument, but others did.
The Kepler design, however, suffered from a basic flaw. The flatter objective bent the incoming light rays less, so its fl was much greater. That meant the eyepiece had to be much further away. Some of these early refractors were over 100 feet long. Their length made them unwieldy and they did not see much use. Most discoveries during this period were made with refractors with fls of 30 to 40 feet.
And all these early telescopes suffered from another problem. Because white light is made up of the colors of the rainbow—different frequencies of light—the different colors are bent by lenses at different angles. So the fl of each color is at a different point. This results in chromatic aberration, a rainbow of colors around the image. A similar phenomenon accounted for the fuzziness of the images in Galileo’s and all early refractors—spherical aberration.
By the early 1700s, astronomers discovered that different kinds of glass bent light more than others, and that different shaped lenses did as well. In 1729, Chester Hall, and English amateur astronomer, ground a concave lens from flint glass—used to make cut-glass decorations—and a convex lens from crown glass—used for windows. He combined the two. There was no chromatic aberration.
Another Englishman, John Dollard, went one step further and actually cemented the two lenses together. Spherical aberration was history.
Yet, one problem remained for refractors. So far, lens makers could not make a useable objective larger than four inches. Bubbles appeared in larger pieces of glass that made them useless.
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The Great Refractors
Then the history of telescopes continues in the early 1800s when French artisan Pierre Guinand and German optician Joseph von Fraunhofer developed a technique for making larger lenses without flaws. The era of the large refractor was born.
The first was at Harvard. It boasted a 15 inch objective, the largest lens made to that date.
Up to that time, astronomy had been done the way many still think of it—a lone astronomer peering through an eyepiece to see some astronomical event never before seen. The Harvard telescope changed that forever.
The telescope was mounted on a clockwork driven mount so it could automatically follow the motions of objects in the heavens. In 1850, Director William Cranch directed his instrument at the moon. Instead of peering through an eyepiece, he exposed a photographic plate. He took the first photo of the moon through a telescope. A few months later, he did the same with the star Vega. From that time forward, astronomy has been done photographically.
The next great refractor was the 26-inch, built at the Naval observatory in Washington, D.C. in 1873. At 40 feet long, it would be the largest telescope in the world for 10 years. Unfortunately, no usable photos of the instrument are available.
Then by 1895, astronomer George Hale pulled together the funding and resources to build a larger one. The 40-inch, 60 foot long Yerkes refractor at the University of Chicago.
This would be the last large refractor ever built. The 40-inch objective was so heavy it sagged under its own weight. It had to be rotated periodically to maintain its symmetry.
Astronomers had to learn how to grind large mirrors for reflectors if they were to build larger telescopes.
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Newton developed a practical reflector, a milestone in the history of telescopes, but it was not until the 20th century that telescope makers learned to make mirrors large enough to peer far into the cosmos. Once they did, George Hale pushed the art to build bigger and bigger instruments first on Mt. Wilson, then on Mt. Palomar where he began building the 200--inch, a telescope that eventually would bear his name.History of reflectors, telescopes
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The Birth of Reflectors
Reflecting telescopes do not use a lens to gather the light from the heavens. Instead, they use a mirror to do so, and reflect the image back to the eyepiece.
In 1616, just seven years after Galileo built his first refractor, Niccolo Zucchi attempted to build a reflecting telescope using a metal reflector. He found it impractical. In 1663 James Gregory described the design of a reflector, but it was ten years later that Robert Hooke actually built it. It was small and the astronomer’s head blocked much of the incoming light.
Then in 1668, Sir Issac Newton had one of his many flashes of genius. If he put another mirror in the path of the light reflected from the mirror, he could redirect the image to a comfortable viewing position. He did, and the Newtonian Reflector, the basis of all astronomical telescopes since, was born.
Newton’s reflector was however primitive. The mirror, made of metal, was only two inches in diameter. And it was spherical, which meant the light rays did not come to a single focal point.
Newton continued to work to improve his design, and eventually built a six inch reflector. But he never found a way to make anything but a spherical mirror. That was left to Scotsman James Short. He found a way to make a parabolic mirror. This focused all the light to a single focal point. The shortcomings of Newton’s reflector were overcome. Telescope makers made some large mirrors. Sir William Parsons built a 72-inch mirror, but it, like all metal mirrors of the period, tended to tarnish quickly. The huge instrument was also so unwieldy it was all but impossible to use. Most telescopes of the period were 19 inch to four feet in diameter.
Then, about 1850 German chemist Justus von Liebig developed a method for coating glass with a thin coating of silver. German astronomer Carl von Steinheil and French doctor Leon Foucault applied this technique to telescope mirrors. The glass mirrors were half the weight of metal mirrors, did not tarnish as quickly, and were easily resilvered when they did.
Still, telescope makers could not make satisfactory mirrors large enough to be completely useful until Andrew Common, an English amateur in Yorkshire, England persevered and produced a 36-inch silvered glass mirror towards the end of the 19th century. Common’s instrument was eventually donated to the Lick Observatory, where it began the discovery of galaxies.
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The Era of the Giant Reflectors
Once Common’s technique became known, telescope makers began working to construct larger mirrors. They also set out to improve on Newton’s reflector design.
One improvement was the Cassegrain. In this design, Newton’s angled secondary mirror is replaced with a hyperbolic secondary that directs the light back through a small hole in the primary. The Cassegrain design puts the focal point at a more easily accessible point than it would be in a standard Newtonian. It also makes a more compact instrument for a given fl. The design actually goes back to Newton's time, but there is some disagreement on who 'Cassegrain' was. Most believe it was Laurent Cassegrain, a Frenchman. It was however not until the 20th century that mirror grinding became sufficiently developed to make the design viable.
The major improvement came around 1910 when George Ritchey and Henri Chretien designed a Cassegrain reflector with two hyperbolic mirrors. Hyperbolic mirrors eliminate coma and spherical aberration. Many of today’s major instruments, including the Hubble, are Ritchey-Chretien telescopes. Significantly missing from the list are the 100-inch Mt Wilson scope and Mt. Palomar’s 200-inch, for reasons we will discuss later.
Those two giants were preceded by a 60-inch mirror at Mt. Wilson made by Ritchey in 1908, before he and Chretien developed their new design. George Hale, now Director at Mt. Wilson, however wanted an even larger telescope. Ritchey gave him one—a 100-inch mirror that at the time was the largest telescope in the world.
Ritchey began his design of the instrument before he and Chretien developed their Cassegrain design, so the giant has its focal point high up in its cage. Fortunately, few astronomers have a fear of heights.
But even as Edwin Hubble was turning astronomical thought on its ear with the 100-inch, George Hale was dreaming of a mirror twice as large—a 200-inch telescope that would dwarf anything ever made. He set Ritchey to its design.
Ritchey wanted to use his and Chretien’s new design, but Hale balked. The two came to a parting of the ways, so the big telescope would be a Newtonian/Cassegrain with a Coude focus point for spectrographic analysis. Coude is French for ‘at an angle.’ At the prime focus, the instrument would have a fl of 3.3, giving it a very wide angle view. At the Cassegrain focus its fl would be 30. It would be a highly flexible astronomical instrument.
A 200-inch mirror turned out to be a problem. Glass was unstable with temperature changes, so Hale’s team tried quartz. That did not work as the quartz formed bubbles too readily.
But a new type of glass had just been introduced by Corning Glass Works. It was called Pyrex. It was perfect for the task. And to save weight, the team developed a new approach to casting the blank. Rather than casting a thick cylinder of Pyrex, they created a mold that formed a thin blank with a ribbed structure on its back. It was very light but very strong.
Later, after WWII, they would make one more advance in mirror construction. They would use aluminum for the reflective coating, not silver. Aluminum does not tarnish.
Unfortunately, George Hale never saw the completion of his giant mirror. He died in 1938. Upon completion in 1948, it was named for him, and today is known as the Hale Telescope.
And in a twist of fate, Edwin Hubble, who had been slated to use the giant to continue his churning of astronomical theory, suffered a heart attack just before the telescope was completed. He had to retire.
The Hale remained the world’s largest telescope until 1975 when the Soviet Union built its BTA-6 236 inch reflector. But the Soviet giant never functioned properly, so the Hale remained the world’s largest viable instrument until the millennium neared. Then the entire concept of telescope design changed. A single giant mirror would be replaced by myriad smaller mirrors, each movable, in instruments such as the Keck, the Large Binocular Telescope and the Magellan.
These 21st century telescopes are described in related articles.