If the curvatures of the lens surfaces are just right, all parallel rays of light say, from a star are bent, or refracted , in such a way that they converge toward a point, called the focus of the lens.
At the focus, an image of the light source appears. In the case of parallel light rays, the distance from the lens to the location where the light rays focus, or image, behind the lens is called the focal length of the lens. As you look at Figure 3 you may ask why two rays of light from the same star would be parallel to each other.
But remember that the stars and other astronomical objects are all extremely far away. By the time the few rays of light pointed toward us actually arrive at Earth, they are, for all practical purposes, parallel to each other.
Put another way, any rays that were not parallel to the ones pointed at Earth are now heading in some very different direction in the universe. To view the image formed by the lens in a telescope, we use an additional lens called an eyepiece. The eyepiece focuses the image at a distance that is either directly viewable by a human or at a convenient place for a detector. Using different eyepieces, we can change the magnification or size of the image and also redirect the light to a more accessible location.
Stars look like points of light, and magnifying them makes little difference, but the image of a planet or a galaxy, which has structure, can often benefit from being magnified. Many people, when thinking of a telescope, picture a long tube with a large glass lens at one end. This design, which uses a lens as its main optical element to form an image, as we have been discussing, is known as a refractor Figure 4 and a telescope based on this design is called a refracting telescope.
However, there is a limit to the size of a refracting telescope. The largest one ever built was a inch refractor built for the Paris Exposition, and it was dismantled after the Exposition.
Currently, the largest refracting telescope is the inch refractor at Yerkes Observatory in Wisconsin. Figure 4: Refracting and Reflecting Telescopes. Light enters a refracting telescope through a lens at the upper end, which focuses the light near the bottom of the telescope.
An eyepiece then magnifies the image so that it can be viewed by the eye, or a detector like a photographic plate can be placed at the focus. The upper end of a reflecting telescope is open, and the light passes through to the mirror located at the bottom of the telescope.
The mirror then focuses the light at the top end, where it can be detected. Alternatively, as in this sketch, a second mirror may reflect the light to a position outside the telescope structure, where an observer can have easier access to it.
One problem with a refracting telescope is that the light must pass through the lens of a refractor. That means the glass must be perfect all the way through, and it has proven very difficult to make large pieces of glass without flaws and bubbles in them.
Also, optical properties of transparent materials change a little bit with the wavelengths or colors of light, so there is some additional distortion, known as chromatic aberration. Each wavelength focuses at a slightly different spot, causing the image to appear blurry. In addition, since the light must pass through the lens, the lens can only be supported around its edges just like the frames of our eyeglasses.
The force of gravity will cause a large lens to sag and distort the path of the light rays as they pass through it. Finally, because the light passes through it, both sides of the lens must be manufactured to precisely the right shape in order to produce a sharp image. A different type of telescope uses a concave primary mirror as its main optical element. The mirror is curved like the inner surface of a sphere, and it reflects light in order to form an image Figure 4.
Telescope mirrors are coated with a shiny metal, usually silver, aluminum, or, occasionally, gold, to make them highly reflective. If the mirror has the correct shape, all parallel rays are reflected back to the same point, the focus of the mirror. Thus, images are produced by a mirror exactly as they are by a lens.
Figure 5. Focus Arrangements for Reflecting Telescopes: Reflecting telescopes have different options for where the light is brought to a focus. With prime focus, light is detected where it comes to a focus after reflecting from the primary mirror.
With Newtonian focus, light is reflected by a small secondary mirror off to one side, where it can be detected see also [link]. Most large professional telescopes have a Cassegrain focus in which light is reflected by the secondary mirror down through a hole in the primary mirror to an observing station below the telescope.
Telescopes designed with mirrors avoid the problems of refracting telescopes. Because the light is reflected from the front surface only, flaws and bubbles within the glass do not affect the path of the light. In a telescope designed with mirrors, only the front surface has to be manufactured to a precise shape, and the mirror can be supported from the back. For these reasons, most astronomical telescopes today both amateur and professional use a mirror rather than a lens to form an image; this type of telescope is called a reflecting telescope.
The first successful reflecting telescope was built by Isaac Newton in In a reflecting telescope, the concave mirror is placed at the bottom of a tube or open framework. To construct the telescope, Newton placed a curved mirror at the bottom of the tube.
That mirror sent reflected light forward to a second, smaller, flat mirror. This second mirror was angled to deflect the light rays to the eyepiece. With this design, Newton decreased the length of the telescope and eliminated the problem of light refracting, since the light didn't pass through lenses. The Hubble Space Telescope is a reflecting telescope. Even though Newton invented the reflecting telescope to study what he knew about light, by astronomers were using it to study the Universe.
They found that the bigger the mirror, the more light it reflected. These scientists began to build huge telescopes again, with larger and larger mirrors that reflected more and more light.
Now it was the size of the primary mirror, not the distance between lenses that described how powerful a telescope was. In , Sir William Herschel became interested in astronomy. He combined different amounts of copper, tin, and other metals to find a mixture that improved reflection by 60 percent. The first telescope he made was seven feet long and had a 6-inch diameter mirror.
It magnified what he looked at by 40 times. Herschel could clearly see Saturn's rings with his new telescope. But if a little is good, then a lot is better, Herschel thought. Knowing that a telescope's most important quality was its ability to gather light, he built a telescope with a mirror diameter of nine inches and a length of 10 feet.
Then he built one that was 20 feet long with a mirror diameter of 18 inches. In his mind there was no limit to the improvements he could make and the objects he would be able to see. By Herschel had designed a foot-long telescope. But the mirrors needed for this telescope were more than the local foundry could handle.
Herschel set up the equipment in the basement of his home and prepared to pour the disks himself. The first mirror he cast cracked, and the second attempt broke the mold, spreading the liquid metal over the floor.
But that didn't stop him. Herschel's greatest design was a completed foot telescope with a mirror diameter of 48 inches. William Parsons , the third Earl of Rosse, was an avid amateur astronomer in Ireland. His dream was to build the world's largest telescope with a mirror six feet in diameter. Each improvement in the telescope grew from the function and workings of the current optical instrument.
Which improvement do you think was the most important, and why? The reflecting telescopes made in the 18th and 19th centuries had different problems than refracting telescopes. While the mirrors made of the tin and copper alloy were easier to make than grinding lenses, these metal mirrors tended to discolor, which meant they had to be polished quite often.
The astronomers also ran into a physics problem. Because the mirrors and the telescopes that held them were so very large, scientists found the huge instruments difficult to move.
Changing the field of view to a new area of the sky was slow and time-consuming. By the mids, astronomers decided that it wasn't the telescopes but the atmosphere that caused distant objects to still look hazy. They realized that as light passes through the air, it bends in unpredictable ways.
With the unaided eye, this is what makes stars appear to twinkle. But when you use a telescope, this bending light just makes images look blurry, kind of how things appear when you look up from under water.
It was this problem of not being able to clearly focus distant images that led to the development of the Hubble Space Telescope. To be a successful astronomer, you had to be awake when the stars were out.
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