History of the Telescope

Before the Telescope

To truly understand what the sky and its lights are all about, human beings eventually realized they couldn't just rely on their eyes. They had to develop tools to help explore what stretched far above them. The telescope was the key invention in the progress to modern astronomy, but it took many thousands of years before anyone succeeded in inventing one. This is somewhat surprising since water's ability to change and sometimes magnify objects beneath the surface must have been noticed as early as caveman times. Glass was discovered around 3500 BCE, and crude lenses were even found in Crete and Asia Minor, dating from 2000 BCE. A number of well-respected writers as early as classic times reported about refraction, reflection, and magnification. But for some reason no one put together all these observations and discoveries in the right way until the early 17th century. Instead, ancient peoples used many other kinds of tools and aids to help them observe and track the stars and planets.

Ancient sites like Stonehenge in Great Britain and the Egyptian pyramids are some of the earliest evidence we have that our ancestors made pretty accurate observations of the sky. We know that because these sites are aligned in special ways. At Stonehenge, for example, one can see the sun rise through one arch at the summer solstice, which is the time when the sun is at its farthest point north in the sky. The sun rises and slips just above a distant stone called the Heel Stone.  Some sort of measuring tools were likely used to arrange the site in such an exact manner, to produce this effect. We may never discover just why this early society aligned their stones this way. It could be to create a "sacred space" for religious ceremonies or magical practices. It is now believed unlikely that Stonehenge was an early observatory, but there's little doubt that the alignment was made on purpose and that the sun was meant to appear where it does at that time.

The pyramids in Egypt are also aligned in a specific way, but in their case it was to point at certain stars. Two shafts which lead down into opposite sides of the Great Pyramid at Giza open on one side to the star Thuban (at the time, the closest star to being the north star) and on the other to the stars in the constellation Orion's belt. But since the pyramid was built to be a tomb, like Stonehenge it is not considered an observatory. The pyramid was really a launching pad for the dead pharaoh's soul. The pharaoh was expected to travel to heaven and continue his destiny there, and the shafts pointed in such a way to symbolically guide him to his new, heavenly home. Actually, even a living person standing in the crypt could not see the alignment of stars, since the shafts were blocked. But the ancient Egyptians obviously had some dependable knowledge of the stars' positions in order to arrange the shafts' alignments in the first place.

Many other ancient sites have been found, in Central and South America, the U.S., and Ireland, which suggest they were built to point at certain stars or the sun or moon. We will probably never know for sure why these structures or patterns were made, but their existence does at least prove that men and women have been interested in the sky's changes for a very long time, and that they kept close track of where heavenly objects appeared.

As time went by, people devised portable aids to star watching that have survived for us to examine. Egyptians invented the merkhet, which was used by two observers facing one another. Each one held a merkhet, and they sat lined up so that one could see the north star just above the other's head. The second person then sat very still as the first one watched and recorded when stars passed by the top of the other's head, by his ear, shoulder, etc. In this way, astronomical positions were recorded so people could predict how the stars moved.

In Alexandria around the 2nd century, Ptolemy and his contemporaries invented more instruments for sky watching. To use a quadrant, one side was aligned with the horizon and the other pointed at the zenith, the spot in the sky directly overhead. Then a movable arm or bar was pointed at a certain star, to measure its distance above the horizon. Quadrants also could point to the sun and tell the time of day. They were used a lot even after the discovery of the telescope, since early telescopes couldn't measure exactly where a star was.

From Alexandria, the center of learning moved to the Middle East, where Arabian astronomers used many fine star-measuring instruments. The astrolabe was either invented or improved at this time; it was used like a quadrant, except one held it by a ring at its top and gravity put it into position to measure the sun or stars. The cross-staff, just a simple cross with a movable bar, was invented by Jewish astronomer Levi ben Gerson (1288-1344). With a cross-staff, mariners could determine the angle between the moon and a star by sliding the bar so the two objects were aligned with the ends of the bar. Then the angle was read off a scale engraved in the middle. Many instruments were constructed to be larger and larger, so that very exact measurements could be made. One quadrant built was 180 feet in size! Tycho Brahe (1546-1601), just before the age of the telescope, had huge instruments at his observatory, Uraniborg, built on an island off Denmark. There astronomers and their assistants could use quadrants and Tycho's improvement of the cross-staff, sextants.

Of these first observatories, built to house large instruments, few survive. While some were just abandoned, like Uraniborg, many in the Islamic world were destroyed when the benefactor or ruler who built it died. Astronomy was at this time tied up with astrology, and so star and planet positions were tracked mainly to forecast the future. But such fortune telling was forbidden by Islamic law. Sky watching was actually a risky pastime in those days. One such sponsor of an observatory was executed and his building and instruments destroyed, all because he was found guilty of, among other things, "communicating with the planet Saturn."

It was this kind of superstitious belief that hampered the progress of astronomy. From the third through the 13th century, the ability of certain kinds of curved glass to magnify objects was variously reported, but such a thing seemed so amazing that it appeared to some to be "black magic." Claims made by a prospective discoverer were either disbelieved or liable to land that person in prison or to be burned at the stake. Not until the Renaissance, when the scientific age began to dawn, were people better prepared for the miracle of the telescope.

Early Telescopes

Phoenicians cooking on sand discovered glass around 3500 BCE, but it took about 5,000 years more for glass to be shaped into a lens for the first telescope. Even cavemen must have noticed how water refracts, or bends, the image of objects below the surface, but this probably seemed specific to liquid and not to sunlight's angle on the water's surface. Crude lenses have been found in Crete and Asia Minor that date from around 2000 BCE, but these weren't clear enough to see through. Euclid wrote about reflection and refraction of light in the 3rd century BCE, and the Roman writer Seneca mentioned how Aristophanes demonstrated that a globe full of water could be used to magnify things. So some of the theory that would be the basis of telescopes was known as early as classic Greece times. With these well-known people publishing such results and theories, why did no one think of inventing a telescope?

Perhaps such an instrument was produced, but its use was so amazing that it appeared to be evil, and its maker was hounded or killed. As ridiculous as that sounds, this is what actually happened to Roger Bacon (c1220-c1292). Bacon wrote about magnification and even applied its use to observing the sky. He wrote that by using lenses, "the Sun, Moon, and Stars may be made to descend hither in appearance . . . which persons unacquainted with such things would refuse to believe." For this he was labeled a magician and imprisoned.

It wasn't until the Renaissance, when so many new and seemingly miraculous discoveries were being made, that people were ready for such a marvelous invention as the telescope. What also made it easier to accept was the general use of spectacles by that time. Spectacles or eyeglasses were only invented sometime in the 13th century. Just who invented them is not known, but a curved glass's ability to magnify its surroundings was probably discovered by mistake. Once fashioned so that both eyes could look through two lenses at once, spectacles became instantly popular, a great benefit to those with aging eyesight or those nearsighted from birth.

A spectacle maker probably assembled the first telescope. Hans Lippershey (c1570-c1619) of Holland is often credited with the invention, but he almost certainly was not the first to make one. Lippershey was, however, the first to make the new device widely known. The story usually told is that Lippershey was handling some lenses in his shop when he happened to look through two differently shaped ones at the same time. He was holding them up toward the light of the open doorway, when he was startled to see a distant church tower seem to jump to the door of his shop. He was amazed to see even the weathervane on the church spire clearly. Lippershey's first idea was to use this curious effect to attract customers. He set up a display with the two lenses, so people who came to his shop could look through them and see how the church appeared to be so close. Lippershey eventually enclosed the two lenses in a tube to make what he called a kijkglas or "look glass."

While that story may be partly or wholly fiction, Lippershey did apply for a patent for his telescope and sent one as a gift to the ruling prince of the Netherlands, Mauritz of Nassau. Lippershey was eventually denied a patent, because, he was told, "too many people have knowledge of this invention." This was indeed true, since toy telescopes were already for sale in Paris, and other spectacle makers were also claiming they invented the telescope. Perhaps Lippershey did invent the telescope independently of others - that would not be an unusual occurrence. Whatever the case, it was the news of Lippershey's invention that reached Galileo Galilei in Italy and intrigued him enough to attempt to make his own telescope.

Galileo's version of Lippershey's "far looker," as he heard it called, also wasn't invented specifically for sky watching. Galileo, like Lippershey, saw an opportunity to make money with this new invention. Both these men realized the enormous military advantage of such a tool. As Lippershey gave a telescope to his prince, Galileo fashioned his own tube and presented it to his ruler, the doge Leonardo Donà of Venice. During the early months of showing off his telescope, Galileo exclusively demonstrated how distant objects on land appeared closer.

It was inevitable, though, that one night the moon would catch his attention, and Galileo would think of pointing his telescope at its bright disk. From that moment, Galileo went on to discover new "lands" in the sky above. The craggy features of the moon surprised him, the increased number of stars he saw amazed him, but his most astounding discovery was when he realized four moons were orbiting Jupiter! Galileo published this incredible find, and the news rocked the world, because at that time people believed that everything in the sky orbited the Earth.

Galileo went on to see many more wonders with what was now called a telescope. The telescope gave such a drastically different view of the sky that some people's most cherished beliefs were threatened. Galileo's reports on things like sunspots, for example, got him into trouble with the traditionalists of his time. No one before could really see what was in the sky, not clearly, and so much of what was believed about the universe was based on philosophy and religious ideas. Galileo started the trend to actually observe and measure objects in the sky, for he showed that the worlds above seemed to obey the same physical laws known to work on Earth. For example, he calculated the height of mountains on the moon after observing their shadows move on the lunar surface. This was a revolutionary idea, to think that an ordinary person could know something about a distant world just by writing down what is seen and then applying basic mathematics to the problem.

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.

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.

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.

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.

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.

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.

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. 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.

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 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.

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. Chester Moor Hall (1703-1771) happened on this combination effect, although it wasn't until John Dollard 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.

Observatories: Housing the Great Telescopes

In the 19th and early 20th centuries, advances in mirror making, glass production, and lens grinding led to the construction of increasingly larger and larger telescopes, which in turn led to the building of permanent structures to hold these huge instruments. At first these observatories were built near big cities or where it was convenient for astronomers or their sponsors to visit them. But eventually people realized that where you put your telescope is almost as important as how big you can make it. Locations were chosen where the weather was best for observing, usually a place with a dry and somewhat warm climate. For the biggest observatories, mountain tops became the spot of choice, where the atmosphere was thinnest and least bothered by wind movement, making the view the clearest of all.

At the Dorpat Observatory in Estonia, the Dorpat Equatorial, a 14-foot long telescope with a 9 1/2 inch lens, was for many years the largest refracting telescope in the world. It was called an equatorial because it was mounted in such a way that a clock device could move the telescope at the same rate as the Earth turned. Its movement went opposite the direction of the Earth, so an astronomer could lock onto a star or planet and examine it continuously without having to manually readjust the telescope's position. With the Dorpat, Wilhelm von Struve (1793-1864), the observatory director, did a complete survey of the northern hemisphere. He examined 120,000 stars. Two thousand of them were double stars, of which only 700 were previously known. Struve also directed the Pulkovo Observatory near Leningrad, where a refractor with a 15-inch wide lens was eventually installed.

William Parsons (1800-1867), third Earl of Rosse, constructed at his castle in Ireland what was eventually known as the Leviathan of Parsonstown, a 72-inch mirror attached to a 56-foot tube. He then built a supporting structure around it. The problem of providing a stable foundation was solved by placing the telescope on a platform of 27 cast-iron plates on a base of tree trunks. The whole assembly then rested on a ball and socket set into solid rock. When the telescope was mounted, fortress-like walls, fashioned to match the castle, were raised to shield the telescope on two sides. Lord Rosse employed most of the people in his district to make this grand achievement possible.

In time, many other large telescopes were constructed all over the world, both reflectors and refractors, sporting wider and wider lenses, or mirrors with diameters of astounding size. Large 48-inch reflectors were mounted in Malta and Australia, for example, and in California, the Lick Observatory assembled a 36-inch refractor that helped Edward Barnard discover Amalthea, the first new moon of Jupiter found since Galileo's time. In Wisconsin, the Yerkes Observatory sponsored a 40-inch refractor, although the weather at Lake Geneva was at times so cold that they had to close the observatory dome for fear of damaging the instruments. One astronomer's nose froze to the telescope's metal side as he observed, and he tore off a chunk of skin while pulling away!

George Ellery Hale (1868-1938), who helped raise money for the Yerkes 40-inch refractor, was also responsible for even larger telescopes in California: the 60-inch and 100-inch Mt. Wilson reflectors and the 200-inch Mt. Palomar reflector, called the Hale Telescope in his honor. The 100-inch Hooker telescope at Mount Wilson was the largest in the world for 30 years. Through it, spiral cloud-like nebulae were at last identified as being "island universes" or what we now call galaxies, just like our own Milky Way galaxy.

The 200-inch Hale at Palomar, completed in 1949, then took over as the world's largest telescope. Weighing more than the Statue of Liberty, the huge instrument had the most perfect mirror ever polished. It revealed such wonders as the first quasars, star-like radio sources moving at incredible speeds at the edge of the visible universe. Astronomer Allan Sandage (1926-    ), the first to spot a quasar and Palomar's most frequent and productive observer, showed the universe to be 10 times larger in size using evidence obtained with the Hale reflector. The telescope seemed to reveal the farthest parts of the universe. Strangely enough, when it was being built, some people feared it would allow mortals a view of heaven itself! Even in modern times, the awesome power of telescopes could still frighten people as it did in the 13th century.

Built next to the 200-inch Hale the year before is the Big Schmidt, now called the Oschin Telescope, a 48-inch telescope-camera that is both a reflector and a refractor in one. This combination was invented by Bernhard Schmidt (1879-1935). Its advantage over the much larger Hale Telescope is its wider field of vision. Using the Schmidt, astronomers made a complete photographic map of the sky, something it would have taken the 200-inch Hale reflector millions of individual photographs to do.

After the observatories on Palomar were built, 30 years passed before the construction of larger ground-based telescopes, although a number of telescopes between the 100- to 200-inch range were constructed. In 1996, the two 10-meter (400-inch) Keck telescopes were completed atop Mauna Kea in Hawaii. The twin Keck reflectors, when used together, have the resolving power of a single telescope 90 meters or 295 feet in diameter. With its very thin atmosphere, the area around the extinct volcano of Mauna Kea is almost perfect for sky watching; a total of nine huge telescopes, with two more under construction, are taking advantage of these ideal conditions.

Efforts to create even larger Earth-based telescopes continue. The invention of the rotating furnace allowed Roger Angel to cast an 8-meter or 330-inch mirror for the Large Binocular Telescope to be built in Arizona. The final telescope will have the equivalent light-gathering power of a single 11.8-meter or 464-inch instrument, with the image sharpness of a 22.8-meter or 897-inch mirror, making it the largest telescope anywhere.

But despite the great achievements of large ground-based telescopes, there are others who focused on improving our view of outer space by getting rid of the atmosphere's influence altogether. As early as the 1940's, a telescope was proposed that would be completely above the atmosphere. The plan was not to use an even higher mountain, but to place the telescope in outer space itself, positioned in orbit around the Earth and operated by remote control. Today the Hubble Space Telescope is the realization of that dream, although it took many other smaller efforts and projects to make that dream a reality.

The Telescope Joins with Other Inventions

The original telescope as designed by Galileo and others helped people see farther and deeper into space. That was a great advantage, and its original discovery was one of the great accomplishments of our civilization. But the first telescopes only showed what is visible. As time went on, scientists and astronomers started to realize that the human eye limited what could be discovered through the telescope.

First of all, one person looking through a telescope had to depend on his or her memory and skill in recording what was observed. But once photography was invented and astronomers thought of attaching photographic plates to telescopes, taking pictures instead of just looking through telescopes proved far more practical. Not only could several astronomers instead of just one look at the same patch of sky captured at the same instant of time, but with careful preservation, astronomers for years to come can view the same picture. A permanent record of how the sun, moon, and planets look over long periods of time can be assembled. A photographic survey of all the sky was now possible, too, which would be much more accurate than any drawings or other records which astronomers of past centuries made.

Also, by leaving a film shutter open longer, fainter stars appear in the film, even ones invisible to the ordinary human eye. Observations with today's largest telescopes are now almost exclusively done by camera-like devices, since photography has been replaced by machines that electrically detect light to produce an even better, sharper image than a photograph. This recording of the telescope's images is a great advantage and allows teams of astronomers to see the same area of sky without having to schedule hard-to-obtain viewing time on a large telescope. In fact, astronomers can live far from any observatory and yet conduct their research, using images taken at the 200-inch Hale Telescope or even the orbiting Hubble Space Telescope. The invention and development of photography and electronic imaging is one of the most important advances in astronomy since the invention of the telescope itself.

The other big limitation discovered about the human eye is that visible energy is only a small part of what is radiated outwards from stars, our sun, and actually all matter in general. There's a wide range of information to be gathered by investigating what lies outside the visible spectrum. The trick is inventing the right "receiver" to detect that invisible world.

Most of the energy in the world cannot be seen, so regular telescopes cannot detect it. For example, the heat which radiates off a hot stove is energy, but it is not visible. You can feel it, but you can't see it because the eye isn't made to detect it. Sound is energy, but it also cannot be seen. Our eardrums are built to detect sound. But all these different kinds of energy, visible light, heat, and sound, have one thing in common - they move outward in waves from their source. One way they are different is in how short or long their wavelengths are. Sound has longer wavelengths than heat, and heat has longer wavelengths than light. Another way this is often described is by measuring an energy source's "frequency." When something has a high frequency, it means that the crests or tops of the waves of energy are arriving quickly, or frequently. Sound waves from a radio transmitter have very long wavelengths, so a complete wave arrives at our ears a lot less frequently than the shorter or higher-frequency waves of energy coming to us from the sun.

Interestingly enough, the color-fringe problem of refracting telescopes was what led astronomers to find that energy has a much wider spectrum than just what we can see. As astronomers and other scientists focused on how to remove color blurs from refracting lenses, they experimented with the way white light breaks up into the colors of the rainbow. On one side of the rainbow spectrum is red, and William Herschel in 1800 identified the energy that has slightly longer wavelengths than red, called infrared. At the other side of the rainbow of visible light is violet, and William Huggins in 1875 was the first to detect the energy that has slightly shorter wavelengths than violet, called ultraviolet. Both infrared and ultraviolet are invisible to the human eye, but scientists found they could build special instruments which could detect those wavelengths. There are other shorter and longer wavelength energies with familiar names: the high-frequency ones above ultraviolet are X rays, gamma rays, and cosmic waves; and low-frequency energy sources, below red and infrared, are microwaves, TV broadcast waves, short-wave radio, long-wave radio, and the energy sent through powerlines as a source of household electricity.

Instruments to detect these other wavelengths were then attached to telescopes to see what could be discovered. The earliest spectrum experiments took place when Joseph Fraunhofer (1787-1826), a glass and lens maker, constructed what would later be called a spectroscope as he searched for a solution to the color problem of refractors. His spectroscope was a combination of a lens, a prism, and a small telescope, all positioned in front of a slit in a window shade. Light came through the slit, traveled through the lens and into the prism, and the separated colors were then picked up through the small telescope. To his surprise, he not only got bands of color, but also dark lines running at different intervals among the colors.

Fraunhofer had no idea that the spectrum he produced was really a coded map of the chemical composition of our sun. The information in those lines, eventually called Fraunhofer's lines, would later tell scientists what elements our sun is made of. Fraunhofer did notice that the moon and planets, when shining their light through his device, also produced the same arrangement of colors and dark lines. This is because they reflect the same light as the sun. But once he viewed other stars with his spectroscope, the dark lines in the colors changed position. Fraunhofer made drawings of each star's resulting spectrum colors and lines, but he didn't know why each star had a unique look to its spectrum.

It wasn't until 33 years later that Gustav Robert Kirchhoff (1824-1887), while studying luminous gases, was looking at some vaporized sodium through a spectroscope and noticed two bright lines in the same position as Fraunhofer's dark lines. By further experimenting with sunlight, different elements, and Fraunhofer's spectrum drawings, Kirchhoff decoded the sun's spectrum. The dark lines and their position indicated that the sun contained such familiar elements as sodium, magnesium, iron, calcium, copper, and zinc. Astronomers now had the key to what elements made up the farthest stars - they could find out just by using a spectroscope attached to the familiar telescope.

Another important event associated with a different use of the telescope was the discovery that we can "hear" the universe. Karl Jansky (1905-1950), an engineer working for Bell Telephone Laboratories, discovered the first extra-terrestrial radio signals coming from the center of our galaxy. He had been charged with tracking down an annoying static which was interfering with new transatlantic phone lines, and he decided to build a large movable antenna in a field in New Jersey to find the source of the noise. What he discovered was a radio source which sounded like hissing, moving across the sky in the same direction as the sun. At first he thought it was coming from the sun, but in time Jansky realized it moved ahead of the sun, coming about four minutes earlier each day. Eventually the sound was louder at midnight when the sun was nowhere around.

Jansky knew enough about astronomy to know that there was a four minute difference between a solar day and a sidereal day. A solar day is 24 hours long, the time it takes for the Earth to fully rotate from, for example, one noon when the sun is directly overhead, to the next noon, when the sun appears to return there. But a sidereal day is only 23 hours and 56 minutes long, because that's the time it takes Earth to rotate so a particular star returns to the same place in the sky. The sun appears to move a little each day, and it's that small movement that adds four minutes to a full day when measured relative to the sun.

Soon Jansky realized the strange hissing was coming from a fixed point in the sky, somewhere among the stars beyond our solar system. After a few more years of investigating this phenomena, he published what he found. At first people thought Jansky had discovered some kind of intelligent signal from an extra-terrestrial civilization, but it was soon understood that many bodies in outer space give off strong radio signals. What Jansky had found were radio waves coming from the center of our Milky Way galaxy. Since then, many other stars, some not even visible, have been found giving off radio signals. The planet Jupiter, for instance, is a strong source of radio noise. Because of Jansky's work, an entire new field of exploration opened up.

Radio telescopes were built along the same lines as regular telescopes, but with a much larger dish to catch the much longer, weaker radio waves. Like the mirrors of reflecting telescopes, early pioneers in radio astronomy like Grote Reber (1911-    ) made their collecting device concave, or bowl shaped. Carefully polished mirrors weren't necessary, since radio reception doesn't need the fine resolution that visual images need. So Reber made his antenna dish out of 45 wedge-shaped pieces of sheet iron. Radio astronomers early on realized the need for radio telescopes to be as large as possible, to catch what would be much weaker sound wavelengths than the shorter, stronger visual wavebands.

Just as those who developed regular telescopes pushed for larger and larger lenses and mirrors, the radio astronomers thought up ways that they could get better reception of the radio waves coming from outer space by making stronger and larger radio dishes. One of the largest single radio receivers in the world is at Arecibo, Puerto Rico, where a 1,000-foot radio telescope is installed into a natural depression in the ground. The large dish cannot be moved, but positioned above the dish are instruments which help funnel a particular source in the sky into the dish for collection of its radio energy.  By placing the huge dish into the ground, the Arecibo telescope avoids the problems which eventually plague the movable radio antennas. There are limits to how big a movable radio dish can get before its own weight will make it impossible to maneuver or maintain.

A key advance in making more sensitive radio telescopes was the discovery that two or more radio antennas can be linked together in an array. The first large set of connected antennas, called the Very Large Array, was completed in 1981 on a plain in New Mexico. The VLA is made up of 28 linked antennas (27 working ones plus a spare), each with an 82-foot antenna dish. Each dish is mounted on a pedestal which can point the antenna upward in all directions, and each pedestal is on a kind of railroad track so that all the antenna dishes can be moved into different configurations, all spread out for some observations and all bunched up for others.

The largest array of radio telescopes today is an array of ten 82-foot radio antennas spanning the entire U.S., from the Virgin Islands, across the mainland states, and out to Hawaii. Called the Very Long Baseline Array, these radio telescopes are connected by the Internet and synchronized by super-accurate atomic clocks. Each antenna weighs 240 tons and is nearly as tall as a ten-story building when pointed straight up.

Shorter wavelengths like the ultraviolet, X-ray, gamma ray, and cosmic ray energy sources are prevented from fully reaching Earth by our protective atmosphere. To detect these high-frequency energy sources, plans had to be made to put specially constructed telescopes where they could work without the atmosphere's interference. Only above Earth's cloud layers would an X-ray telescope or ultraviolet spectrometer work at its best. The radio telescopes can detect radio waves during the day or in cloudy weather, but these other instruments needed space-age technology to get above the limitations of Earth's atmosphere. With rocket telescopes, satellites, and orbiting telescopes, we could finally begin to explore the universe's many other, invisible faces, as well as see farther out into the visible world than we ever could from Earth.

Telescopes above the Atmosphere

Once the technical problems with telescopes were solved and instruments were invented to help detect energy beyond what we could physically see, only one big hindrance remained for astronomers - the Earth's atmosphere. Even on the tops of mountains, the atmosphere prevents the hugest telescopes from getting a really sharp picture of distant objects.

But how do we get telescopes into space, and how would they operate? Rockets were one way found to get such devices above the atmosphere, but rockets could only stay up for a few minutes before falling back down. They also couldn't point a telescope in a specific direction, so only simple detectors could be used that would just scan the general area. Airplanes and big weather balloons were tried, but neither one escaped the atmosphere altogether, and they, too, couldn't stay up for a very long time. Balloons also couldn't point the equipment in a specific, steady direction. What was needed was advanced equipment with complex controls that could make a flying telescope point steadily at a chosen target. This would require an expensive satellite or spacecraft, but such projects only get financial support if scientists can show that a lot can be learned with such a telescope. To get satellites and spacecraft observatories funded, smaller projects using those rockets, planes, and balloons had to come first, to justify spending the money on putting remote-control telescopes in space.

The first rocket carrying an instrument to investigate ultraviolet radiation was launched in 1949. Ultraviolet was a good choice to begin such studies, since it was already known to have a big influence on human health. A small dose of ultraviolet builds up vitamin D, but large amounts cause sunburn and skin cancer. Earth's atmosphere keeps most of the harmful ultraviolet radiation from affecting us, and that's why a detector in space could be useful, to measure the full amount that the sun and stars radiate.

The ultraviolet detector was carried in the nose cone of a German V-2 rocket being fired to test its propulsion system. It was really just along for the ride, installed in the place where bombs had once been. When scientists were given this opportunity to put an experiment in the rocket, an ultraviolet photometer was chosen specifically because the photometer could read the sun's spectrum without being carefully pointed. Once launched, the rocket was above the cloud layers only for a few minutes, but the photometer was able to detect not only the sun's ultraviolet radiation, but solar X rays as well. As experiments continued, some X rays were detected from sources other than the sun. Scientists became eager to find out more about these mysterious cosmic X rays.

To determine if there were enough X rays coming from outside our solar system to study in the first place, a detector which would specifically target the X-ray wavelength was built into a rocket. The first two rockets launched with this instrument ran into technical difficulties. The first rocket engine failed, and while the second rocket launched successfully, the door to the instrument area got stuck, and so the only thing detected was the inside of the rocket chamber. The third try on June 18, 1962, was finally a success. Once above 80 kilometers, the rocket doors opened and the instruments found very strong X rays coming from the southern sky. The five-minute flight had discovered a cosmic X-ray source hundreds of times brighter than anyone thought existed.

After a few more rocket surveys, an X-ray detector was sent up on one of the Orbiting Solar Observatories. The more experiments done with X rays, the more promising were the discoveries. Invisible X-ray stars were found. Some were also strong radio sources. Scientists now pushed to get an X-ray telescope in space. The X-ray Explorer, nicknamed "Uhuru," was launched from Kenya in 1970. Its success led to the launch of the Einstein X-ray Observatory in 1978, which operated for two and a half years. Observations were made of 5,000 objects ranging from comets in our solar system to quasars billions of light-years away. One key achievement was the discovery of a uniform glow of X rays throughout the sky, probably coming from far outside our galaxy. If you compare it to how our eyes see the night sky as black with bright points of light scattered here and there, when viewed in the X-ray wavelength, there is no black sky at all. This bright X-ray background could mean that very hot gas exists between galaxies, or perhaps it is produced by millions of distant X-ray sources, like quasars, which are star-like radio sources. More study and observation will not only clear up such mysteries, but will likely reveal more amazing things about X-ray energy sources.

Gamma rays, which are produced by the decay of radioactive material, were first found in space by sending detectors up in balloons 20 miles above the Earth's surface. Enough interesting waves in that spectrum were discovered to argue for a small gamma-ray telescope to be included on board the second Small Astronomy Satellite (SAS-2) in 1972. SAS-2 made a gamma-ray map of the entire sky. Gamma rays are associated with neutron stars, which are stars once bigger than our sun that have exploded and collapsed into very dense material, so dense that a piece of a neutron star the size of a grape would weigh about a billion tons. Then in 1979 the first spacecraft flown to detect gamma rays from outer space was the third High Energy Astronomical Observatory. HEAO-3 had lots of difficulties at first, because many false gamma ray readings hit the detector. Eventually scientists sorted out the false readings and learned that gamma rays and their radioactive sources are probably coming from novae, which are partial explosions of stars, a process which happens 1,000 times more frequent than supernovae, the complete explosion of a star.

Infrared is another wavelength in which scientists wanted to map the sky. Infrared energy is like heat, and every living thing, and even nonliving things which retain heat, emit an infrared glow, though humans can't see it. Certain snakes have an infrared detector so they can catch mice and small animals at night by sensing the prey's body heat. At one point, some scientists were anxious to view Mars in the infrared, thinking that we could sooner determine if life existed there if infrared pictures showed large concentrations of infrared energy.

The first airborne infrared survey was done in a plane. The success of this and other experiments with infrared detectors eventually led to an internationally sponsored Infrared Astronomical Satellite carrying an infrared telescope, launched in 1983. This device ran mainly by computer and made almost four complete surveys of infrared energy in outer space. Since there is a lot of gas, dust, and general space debris like burned-out rocket stages orbiting Earth, many of the readings had to be thrown out as just interference. But these thrown-out readings were also kept, since a mistaken infrared source could be an unknown distant 10th planet or a dark star companion to our sun. Scientists will be able to refer to this infrared map in the future if some interesting object is discovered at a later time.

While these specialized projects uncovered many interesting things about the universe, other astronomers insisted that we also had to have a large all-purpose telescope in orbit, one with a range of sensitivity from the infrared through the visual spectrum and into the ultraviolet. Many years of planning, development, and battles over funds finally produced the Hubble Space Telescope, named after Edwin Hubble (1889-1953), the astronomer who discovered the redshift in numerous galaxies, proving the universe was expanding. Launched in 1990, the Hubble was flown to outer space in the Space Shuttle, which limited the size of its mirror and overall structure, since it had to fit into the shuttle's cargo bay. But a 2.4-meter or 96-inch mirror in outer space still promised to get sharp images of distant star systems and clouds of gases 10 times better than possible from Earth.

The Hubble telescope is arranged so that all instruments are installed behind the main mirror, and a hole in that mirror faces a smaller mirror which reflects images back into the instrument area for recording and analysis. A wide-field camera and a faint-object camera are on board, as well as devices for analyzing the color spectrum of very distant objects. The Hubble is not studying the sun or moon, because the light from these bodies is too bright and would damage the telescope. In fact, the Hubble is not usually pointed at any object which lies within 50 degrees of the sun or 15 degrees of the sunlit moon or Earth. The telescope completes an orbit every 95 minutes and holds steady by locking onto guide stars. Flying about 600 kilometers above the Earth, it is expected to operate for 15 years, with the Space Shuttle visiting it every three years to service it and install any updated equipment it might need.

Soon after the Hubble Space Telescope was launched, a problem was discovered with its main mirror. The mirror had a very slight flattening at its edge, so slight that it was hardly detectable. But this tiny flaw produced images which just weren't the sharp quality which was expected of a space telescope. Since replacing the mirror in space would have been extremely difficult and expensive, Hubble engineers decided to trick the mirror into working properly. They built duplicates of some of the equipment that worked with the mirror and made those devices with an opposing flaw, to "correct" the defect of the slightly warped mirror. In this way, the images the Hubble produced would come out right. It was like the color problem of early telescopes all over again. Just as flint and crown glass lenses made images bend in complementary ways to produce one perfect image, the Hubble's mirror and altered equipment together create a correct image. Sometimes two wrongs can make a right!

Plans are already underway for a bigger and better orbiting telescope, presently called the Next Generation Space Telescope. Early ideas for this next space telescope included possibly installing it on the moon, which would allow it a stable foundation instead of needing complex control systems to point it steadily in space. But although the moon has no atmosphere to interfere with such a telescope, there are limitations to any telescope which stands anywhere on firm ground. The telescope would be restricted to pointing only to the half of the sky it is facing, and the sun and sunlit Earth would have to be constantly avoided. The current plans are instead to put an 8-meter or 314-inch reflecting infrared telescope in deep space around the year 2007. A far-Earth orbit is planned to help keep the equipment at a colder temperature and to eliminate the problems of having to avoid a close sunlit Earth and moon so much of the time.

While the Hubble can detect the near infrared, which is closest to the visual wavelengths, the Next Generation Space Telescope will cover longer wavelengths as well so it can study the first stars and galaxies that formed after the universe cooled. This is possible to see when looking with an infrared telescope, since the process of star formation is thought to be very violent, releasing energies hundreds of billions of times more than our sun. Even though these events happened so long ago, they still exist visibly for us, since the light we see from these distant stars was radiated billions of years ago.

Understandably, once telescopes got very powerful and could see to the visible edges of the universe, the planets in our own solar system sometimes got neglected in favor of the farthest stars, nebulae, and mysterious quasars. But during the last 30 years, we no longer had to peer at planets like Jupiter and Saturn using just a mere 200-inch mirror. With robotic spacecraft, we can now travel to the planets and take our pictures close up!

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