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.
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.