Themes > Science > Astronomy > Equipment and Devices > Telescope > History of Telescope > How Telescopes Improved


Of course, those first telescopes weren't perfect. While distant objects did appear closer, they weren't very clear. It took several hundreds of years and a lot of experimentation to get really sharp images through telescopes. And people were always trying to see farther and farther into space, so the telescope was constantly being improved and sometimes even reinvented.
The kind of telescope first constructed by Lippershey and Galileo was a refracting telescope, which worked by allowing light to pass through two lenses, one convex or curving outward and the other concave or curving inward. As light from the sun, moon, or stars travels through these two lenses to the eye of the observer, it becomes refracted or bent by the lenses. However, when light is bent through a glass in that way, the different colors that make up light bend at different angles. Just like when light passes through a prism and makes rainbow spots on the wall, or when light passes through a rain shower and makes a rainbow, light through a refracting telescope lens breaks an image into several different colored images. All these images bunch close together, but they do not align perfectly one on top of the other to produce a unified image. When this happens, the image seems to have a fuzzy appearance. The yellow image is brightest, for example, but its blue and red counterparts are there too, except slightly to one side or the other, just like the separations of a rainbow. So the brightest image looks as if it has a colored haze around it. This "color aberration" or color problem was a real annoyance to the first telescope owners, and this problem, along with a related problem called "spherical aberration," were both due to the curve of the convex lenses and remained unsolved for many decades. [Diagram of Galileo/Lippershey refractor]
But before the color and spherical problems were even identified, other refinements in telescopes took place. Johannes Kepler (1571-1630) was the first to make significant improvements to the telescope. Instead of a concave and convex lens combination, he proposed two convex lenses, which would increase the astronomer's field of vision. Kepler got this idea by studying the structure of the human eye. With this arrangement, the resulting image in the telescope would be upside-down, but that was a small price to pay for a better view. Another lens could flip the image right-side up, but the final image would not be as bright. Modern refractors still produce an upside-down image, but astronomers who use them to take pictures just publish the photos upside down. [Diagram of Kepler refractor]
Kepler may not have actually made a telescope like he suggested. He was very nearsighted and probably lacked the practical skill to construct one. Instead he investigated and wrote down theories on optics, the study of light and its changes, and dioptics, the study of how different lenses worked. Kepler was the first to understand the role that light plays in vision. In Kepler's time, people believed that when you saw something, a beam of light came out of your eye and in some way touched the object seen. Kepler insisted that the reverse happens - light bounces off the object and enters the eye, producing an image inside the eye.
Kepler's work influenced many others in the new scientific age. William Gascoigne, an amateur astronomer in England, was using a Kepler-style telescope when part of a spider's web found its way inside the telescope. One small web line happened to fall right at the focus point, so both the thin line and the image Gascoigne was viewing were magnified together. Gascoigne realized that he could more accurately point the telescope using the line as a guide, and he went on to invent the telescopic sight by purposefully placing wires at the focus point. This helped astronomers make more accurate observations and measurements of objects in space, using the thin wires as a reference point. [Telescope with wires at the focus]
Isaac Newton (1642-1727) also studied Kepler's work and constructed a telescope along the lines he suggested. Newton also tried to solve the telescope's color problem. He was the first to realize that white light was a combination of all the colors, rather than the absence of all colors, as was generally believed. But Newton eventually decided there was no way to prevent the breaking up of the different colored images once light passed through a refracting lens. He was later proved wrong, but unfortunately Newton's conclusion discouraged others to try. It was 50 years before someone did at last find a way to remove the color blurs in refracting telescopes.
While he couldn't fix refractors, Newton did have a solution to the color problem - he created an entirely new kind of telescope. He just used a mirror instead of a curved lens for the object glass which collects the final image. A mirror wouldn't refract or bend the light, so there would be no color fringes around the image. In fact, a parabolic-shaped mirror was later found to solve the spherical problem, discovered to be a separate problem. Newton's first reflecting telescope was a great advance in clearer viewing. [Diagram of Newton reflector with side eyepiece]
But reflectors also had problems at the very beginning. Back then, people didn't know how to make mirrors that wouldn't tarnish. Newton's mirror was made of bell-metal, copper, tin, and a little arsenic for whitening. Such a combination got dull quickly and had to be resurfaced, usually a very expensive and time-consuming process. It wasn't until two centuries later that Léon Foucault (1819-1868) discovered how to layer silver on glass, a process used until layering aluminum on glass was developed.
Newton constructed his reflecting telescope with another smaller mirror facing the main mirror. The smaller mirror angled the image to the side of the telescope, where Newton put the eyepiece, the hole through which to view the image. A Frenchman named N. Cassegrain in 1672 also proposed a reflecting telescope, but instead of a small mirror angling the image to the side, Cassegrain's main mirror had a hole in the center, and the smaller mirror reflected the image back through that hole to an eyepiece behind the main mirror. Newton ridiculed this arrangement; perhaps he was defensive because some thought Cassegrain invented his reflecting telescope before Newton did. Newton was so well respected an authority at the time that Cassegrain didn't challenge this attack and perhaps even felt his idea wasn't a good one. It's interesting to note, however, that one of the greatest telescopes of our time, the Hubble Space Telescope, is designed as Cassegrain suggested. [Diagram of Cassegrain reflector]
Refracting telescopes were still used even after reflectors were invented, for a number of reasons. Many more artisans were making lenses than were making the right kind of mirrors for telescopes. Refractors were easier to get, and they revealed a larger area of the sky. People continued trying to improve refractors. In the process, they discovered that magnification increased with the length of the telescope's tube. To get a brighter image, however, the size of the lens called the object glass had to be larger in diameter. A wider lens could also be made with less curvature, which would reduce the annoying color and spherical problems that still plagued refractors. So the ideal telescope that everyone sought after was one that had the longest possible tube and widest available object glass. The problem with getting such a telescope was that really long telescopes needed scaffolding or long masts and cranes to hold them up. Some shook when a breeze came along, and others collapsed altogether. They were hard to maneuver into position. Some astronomers eventually learned that their problems outweighed their benefits. Advances in firmly securing and maneuvering telescopes had to occur before very large telescopes would be practical. [long telescope on a mast]
Despite their awkwardness, longer telescopes with larger lenses helped make more and more discoveries. Saturn's ring was identified by Christiaan Huygens (1629-1695) who, with his brother Constantine, constructed telescopes that were 12, 23, and even 123 feet long. Christiaan Huygens also developed an aerial telescope, an eyepiece joined by a taut thread to the main telescope, that was itself perched on a tall pole. [Aerial telescope. image 14, p. 143 Hoskin] Many of Saturn's moons were discovered by Jean Dominique Cassini (1625-1712), who used telescopes as long as 17, 34, 100, and 135 feet. It seemed every time Cassini made a longer telescope, he discovered another moon! Cassini also saw that Saturn really had two rings. He mounted one of his long telescopes on a water tower that he had the Paris Observatory move to the observatory grounds at great expense. [Paris Observatory with water tower supporting telescope]
Another astronomer, Adrien Auzout, constructed telescopes that were 300 and 600 feet long. He eventually planned to build a telescope as long as 1,000 feet. He hoped it would allow him to see animals on the moon! As time passed, the reports of more moons around planets, moon shadows on Jupiter, and double stars thrilled people, who then wanted the latest improved telescope so they themselves could see such wonders. One rich man, Nicholas De Peirese, had over 40 telescopes. There seemed no end to what a bigger and wider telescope would show.
William Herschel (1738-1822), a musician, became interested in reflecting telescopes after he used one that was just two feet long. He had some refractors but liked the clearer image the reflector gave him. Herschel wanted to see how much better a five- or six-foot reflector would work, but found that no one made the larger mirrors for them. Herschel decided to try and make his own mirrors. He was able to obtain some mirror-making equipment from a man who was giving up the hobby. His first reflector was seven feet long, then he made one 10 feet long. On March 13, 1781, using just the seven-foot telescope, he discovered a new planet, later called Uranus. He thought that what he had found was just a comet, since no one had any clue that there were undiscovered planets. After that success, he built a 20-foot reflector and finally arranged the construction of an enormous 40-foot reflector. [Herschel's 20-foot reflector]
Herschel saw farther into space than anyone had before. He found that many stars were not just simple points of light, but actually quite different from each other when viewed with a powerful telescope. Some were double stars, and others seemed cloudy, which he called "nebula." He was the first to suggest that nebulae might be other galaxies like our own Milky Way. Herschel often examined over 400 stars in a night. This was the first time that stars were examined for themselves, and not just as reference points for observing moons and planets. But his favorite object was Saturn, and he discovered several new moons orbiting the ringed planet.
Herschel continued to make the mirrors for all his telescopes and even made and sold smaller telescopes to help pay his expenses. Herschel polished his mirrors himself, sometimes for 16 hours straight. Often he refused to stop for meals, so his sister Caroline, who herself became a noted astronomer, would feed him as he worked. While his 40-foot telescope was a marvel of its age, the scaffolding he used on his larger telescopes was rickety and dangerous. On more than one occasion he narrowly escaped structures collapsing on him. In the end, Herschel decided that using the 40-foot telescope was often more trouble than it was worth. It took too long to prepare, he had to hire people to help him uncover and maneuver it, and with the limited clear evenings available for observing in England, he found he used his smaller telescopes more often. [Herschel's 40-footer]
Meanwhile, the color problems of refractors was at last solved by putting one flint glass concave lens up against a crown glass convex lens. The different types of glass broke up light at somewhat opposite angles, so all the colors blended together perfectly. [Diagram of flint glass and crown glass different refracting of colors] Chester Moor Hall (1703-1771) happened on this combination effect, although it wasn't until John Dollond [correction made 2001-10-29] manufactured many telescopes with this correction that the solution became widely known. Another great improvement was contributed by Pierre Louis Guinand, who developed a way of stirring the glass when it was forming, making it more defect free. This made possible the creation of larger and larger glass lenses, which up to that time always had bubbles or flaws in them when they were made with a very large diameter.


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