Quicklinks on this Page
Without telescopes, the stars in the sky we see every night would just be twinkling little lights. Hard to imagine what people in pre-telescope times thought these twinkling lights were. For some it must’ve been frightening. For others, it was awe-inspiring.
It began with optics; the lens. Spectacles were being worn in Italy as early as 1300. In the one-thing-leads-to-another theory, no doubt the ability to see better led to the desire to see farther. Three hundred years later, a Dutch spectacle maker named Hans Lippershey, put two lens together and achieved magnification. But he also discovered quite a number of other experimenters made the same discovery when he tried to sell the idea.
Also in the 1600s, Galileo, an instrument maker in Venice, started working on a device that many thought had little use other than creating optical illusions (although they weren’t
called that at the time). In 1610 he published a description of his night sky observations in a small paper called, Starry Messenger (Sidereus Nuncius).
He reported that the moon was not smooth, as many had believed. It was rough and covered with craters. He also proposed the “Milky Way” was composed of millions of stars and Jupiter had four moons. He also overturned the geocentric view of the world system–the universe revolves around the Earth–with the heliocentric view–the solar system revolves around the Sun, a notion proposed around 50 years earlier by Copernicus. The device he invented to make these discoveries came to be known as the telescope.
The telescope was a long thin tube where light passes in a straight line from the aperture (the front objective lens) to the eyepiece at the opposite end of the tube. Galileo’s earlier device was the forerunner of what are now called refractive telescopes, because the objective lens bends, or refracts, light.
NASA’s Great Observatory Program saw a series of space telescopes designed to give the most complete picture of objects across many different wavelengths. Each observatory studies a particular wavelength region in detail.
The telescopes in order of launch were: the Hubble Space Telescope (1990), Compton Gamma Ray Observatory (1991), Chandra X-ray Observatory (1999), and the Spitzer Space Telescope (2003).
The Kepler mission joined the great observatories, launched in 2009, and spent about 4 years looking for Earth-like planets in Earth-like orbits around Sun-like stars. It scanned over 100,000 stars in the constellations Lyra and Cygnus in hopes of finding a few dozen planetary transits, in which the star’s light dims slightly as a planet passes across its disk. Instead it found thousands of exosolar planets, more than any scientist’s wildest dream! Just recently put to rest after the gyroscopes finally gave out, scientists have years of data to examine with more hope than ever that earth-like planets exist in the galaxy and beyond.
NASA also has launched many smaller observatories through its Explorer program. These missions have probed the “afterglow” of the Big Bang (COBE and WMAP), the ultraviolet light from other galaxies (GALEX and EUVE), and the violent explosions known as gamma-ray bursts (SWIFT).
Sometimes several of the observatories are used to look at the same object. Astronomers can analyze an object thoroughly by studying it in many different kinds of light. An object will look different in X-ray, visible, and infrared light.
Recent experiments with color explored the way a prism refracts white light into a array of colors. A circular prism separating colors of visible light is known as chromatic aberration, but the process limits the effectiveness of existing telescopes. A new telescope design using a parabolic mirror to collect light and concentrate the image before it was presented to the eyepiece. This resulted in the Reflective Telescope.
Reflective Telescopes are constructed with giant mirrors–or lenses–and collect more light than can be seen by the human eye in order to see objects that are too faint and far away.
Solar Telescopes, designed to see the Sun, have the opposite problem: the target emits too much light. Because of the sun’s brightness, astronomers must filter out much of the light to study it. Solar telescopes are ordinary reflecting telescopes with some important changes.
Because the Sun is so bright, solar telescopes don’t need huge mirrors that capture as much light as possible. The mirrors only have to be large enough to provide good resolution. Instead of light-gathering power, solar telescopes are built to have high magnification. Magnification depends on focal length. The longer the focal length, the higher the magnification, so solar telescopes are usually built to be quite long.
Since the telescopes are so long, the air in the tube becomes a problem. As the temperature of the air changes, the air moves. This causes the telescope to create blurry images. Originally, scientists tried to keep the air inside the telescope at a steady temperature by painting solar telescopes white to reduce heating. White surfaces reflect more light and absorb less heat. Today the air is simply pumped out of the solar telescopes’ tubes, creating a vacuum.
Because it’s so necessary to control the air inside the telescope and the important instruments are large and bulky, solar telescopes are designed not to move. They stay in one place, while a moveable mirror located at the end of the telescope, called a tracking mirror, follows the Sun and reflects its light into the tube. To minimize the effects of heating, these mirrors are mounted high above the ground.
Astronomers have studied the Sun for a long time. Galileo, among others, had examined sunspots. Other early astronomers investigated the outer area of the Sun, called the corona, which was only visible during solar eclipses.
Before the telescope, other instruments were used to study the Sun. The spectroscope, a device invented in 1815 by the German optician Joseph von Fraunhofer, spread sunlight into colors and helps astronomers figure out what elements stars contain. Scientists used a spectrum of the Sun to discover the element helium, named after the Greek word for Sun, “helio.”
In the 1890s, when the American astronomer George Ellery Hale combined the technology of spectroscopy and photography and came up with a new and better way to study the Sun. Hale called his device the “spectroheliograph.”
The spectroheliograph allowed astronomers to choose a certain type of light to analyze. For example, they could take a picture of the Sun using only the kind of light produced by calcium atoms. Some types of light make it easier to see details such as sunspots and solar prominences.
In 1930, the French astronomer Bernard Lyot came up with another device that helped scientists study both the Sun and objects nearby. The coronagraph uses a disk to block much of the light from the Sun, revealing features that would otherwise be erased by the bright glare. Close observations of the Sun’s corona, certain comets, and other details and objects are made possible by the coronagraph. Coronagraphs also allow scientists to study features like solar flares and the Sun’s magnetic field.
Today, more technologically advanced versions of the spectroheliograph and coronagraph are used to study the Sun. The McMath-Pierce Solar Telescope on Kitt Peak in Arizona is the world’s largest solar telescope. The Solar and Heliospheric Observatory project is a solar telescope in space that studies the Sun’s interior and corona, and solar wind, in ultraviolet and X-rays as well as visible light. Astronomers also use a technique called helioseismology, a kind of spectroscopy that studies sound waves in the Sun, to examine the Sun down to its core.
Basic telescope terms:
- Concave – lens or mirror that causes light to spread out.
- Convex – lens or mirror that causes light to come together to a focal point.
- Field of view – area of the sky that can be seen through the telescope with a given eyepiece.
- Focal length – distance required by a lens or mirror to bring the light to a focus.
- Focal point or focus – point at which light from a lens or mirror comes together.
- Magnification (power) – telescope’s focal length divided by the eyepiece’s focal length.
- Resolution – how close two objects can be and yet still be detected as separate objects, usually measured in arc-seconds (this is important for revealing fine details of an object, and is related to the telescope’s aperture).
Telescopes come in all shapes and sizes, from a little plastic tube bought at a toy store for $2, to the Hubble Space Telescope weighing several tons. Amateur telescopes fit somewhere in between. Even though they are not nearly as powerful as the Hubble, they can do some incredible things. For example, a small 6-inch (15 centimeter) scope can read the writing on a dime from 150 feet (55 meters) away.
Most telescopes come in two forms: the refractor and reflector telescope. The refractor telescope uses glass lenses. The reflector telescope uses mirrors instead of the lenses. They both try to accomplish the same thing but in different ways.
Telescopes are metaphorically giant eyes. The reason our eyes can’t see the printing on a dime 150 feet away is because they are simply too small. The eyes, obviously, have limits. A bigger eye would collect more light from an object and create a brighter image.
The objective lens (in refractors) or primary mirror (in reflectors) collects light from a distant object and brings that light, or image, to a point or focus. An eyepiece lens takes the bright light from the focus of the objective lens or primary mirror and “spreads it out” (magnifies it) to take up a large portion of the retina. This is the same principle that a magnifying glass (lens) uses. A magnifying glass takes a small image on a sheet of paper, for instance, and spreads it out over the retina of the eye so that it looks big.
When the objective lens or primary mirror is combined with the eyepiece, it makes a telescope. The basic idea is to collect light to form a bright image inside the telescope, then magnifying that image. Therefore, the simplest telescope design is a big lens that gathers the light and directs it to a focal point with a small lens used to bring the image to a person’s eye.
A telescope’s ability to collect light is directly related to the diameter of the lens or mirror (the aperture) used to gather light. Generally, the larger the aperture, the more light the telescope collects and brings to focus, and the brighter the final image. The telescope’s magnification, its ability to enlarge an image, depends on the combination of lenses used. The eyepiece performs the magnification. Magnification can be achieved by almost any telescope using different eyepieces.
Hans Lippershey, living in Holland, is credited with inventing the refractor in 1608. It was first used by the military. Galileo was the first to use it in astronomy. Both Lippershey’s and Galileo’s designs used a combination of convex and concave lenses. Around 1611, Kepler improved the design to have two convex lenses, which made the image upside-down. Kepler’s design is still the major design of refractors today, with a few later improvements in the lenses and in the glass used to make the lenses.
Refractors have a long tube, made of metal, plastic, or wood, a glass combination lens at the front end (objective lens), and a second glass combination lens (eyepiece). The tube holds the lenses in place at the correct distance from one another. The tube also helps to keeps out dust, moisture and light that would interfere with forming a good image. The objective lens gathers the light, and bends or refracts it to a focus near the back of the tube. The eyepiece brings the image to the eye, and magnifies the image. Eyepieces have much shorter focal lengths than objective lenses.
Achromatic refractors use lenses that are not extensively corrected to prevent chromatic aberration, which is a rainbow halo that sometimes appears around images seen through a refractor. Instead, they usually have “coated” lenses to reduce this problem. Apochromatic refractors use either multiple-lens designs or lenses made of other types of glass (such as fluorite) to prevent chromatic aberration. Apochromatic refractors are much more expensive than achromatic refractors.
Refractors have good resolution, high enough to see details in planets and binary stars. However, it is difficult to make large objective lenses (greater than 4 inches or 10 centimeters) for refractors. Refractors are relatively expensive. Because the aperture is limited, a refractor is less useful for observing faint, deep-sky objects, like galaxies and nebulae, than other types of telescopes.
Isaac Newton developed the reflector telescope around 1680, in response to the chromatic aberration (rainbow halo) problem that plagued refractors during his time. Instead of using a lens to gather light, Newton used a curved, metal mirror (primary mirror) to collect the light and reflect it to a focus. Mirrors do not have the chromatic aberration problems that lenses do. Newton placed the primary mirror in the back of the tube.
Because the mirror reflected light back into the tube, he had to use a small, flat mirror (secondary mirror) in the focal path of the primary mirror to deflect the image out through the side of the tube, to the eyepiece; the reason being his head would get in the way of incoming light. Because the secondary mirror is so small, it does not block the image gathered by the primary mirror.
The Newtonian reflector remains one of the most popular telescope designs in use today.
Rich-field (or wide-field) reflectors are a type of Newtonian reflector with short focal ratios and low magnification. The focal ratio, or f/number, is the focal length divided by the aperture, and relates to the brightness of the image. They offer wider fields of view than longer focal ratio telescopes, and provide bright, panoramic views of comets and deep-sky objects like nebulae, galaxies and star clusters.
Dobsonian telescopes are a type of Newtonian reflector with a simple tube and alt-azimuth mounting. They are relatively inexpensive because they are made of plastic, fiberglass or plywood. Dobsonians can have large apertures (6 to 17 inches, 15 to 43 centimeters). Because of their large apertures and low price, Dobsonians are well-suited to observing deep-sky objects.
Reflector telescopes have problems. Spherical aberration is when light reflected from the mirror’s edge gets focused to a slightly different point than light reflected from the center. Astigmatism is when the mirror is not ground symmetrically about its center.
Consequently, images of stars focus to crosses rather than to points. Coma is when stars near the edge of the field look elongated, like comets, while those in the center are sharp points of light. All reflector telescopes experience some loss of light. The secondary mirror obstructs some of the light coming into the telescope and the reflective coating for a mirror returns up to 90 percent of incoming light.
Compound or catadioptric telescopes are hybrid telescopes that have a mix of refractor and reflector elements in the design. The first compound telescope was made by German astronomer Bernhard Schmidt in 1930. The Schmidt telescope had a primary mirror at the back of the telescope, and a glass corrector plate in the front of the telescope to remove spherical aberration. The telescope was used primarily for photography, because it had no secondary mirror or eyepieces. Photographic film is placed at the prime focus of the primary mirror. Today, the Schmidt-Cassegrain design, which was invented in the 1960s, is the most popular type of telescope. It uses a secondary mirror that bounces light through a hole in the primary mirror to an eyepiece.
The second type of compound telescope was invented by Russian astronomer, D. Maksutov, although a Dutch astronomer, A. Bouwers, came up with a similar design in 1941, before Maksutov. The Maksutov telescope is similar to the Schmidt design, but uses a more spherical corrector lens. The Maksutov-Cassegrain design is similar to the Schmidt Cassegrain design.
Telescope Mounts are another important feature of telescopes. The alt-azimuth is a type of telescope mount, similar to a camera tripod, that uses a vertical (altitude) and a horizontal (azimuth) axis to locate an object. An equatorial mount uses two axes (right ascension, or polar, and declination) aligned with the poles to track the motion of an object across the sky.
The telescope mount keep the telescope steady, points the telescope at whatever object is being viewed, and adjusts the telescope for the movement of the stars caused by the Earth’s rotation. Hands need to be free to focus, change eyepieces, and other activities.
The alt-azimuth mount has two axes of rotation, a horizontal axis and a vertical axis. To point the telescope at an object, the mount is rotated along the horizon (azimuth axis) to the object’s horizontal position. Then, it tilts the telescope, along the altitude axis, to the object’s vertical position. This type of mount is simple to use, and is most common in inexpensive telescopes.
There are two variations of the alt-azimuth mount. The ball and socket is used in inexpensive rich-field telescopes. It has a ball shaped end that can rotate freely in the socket mount. The rocker box is a low center-of-gravity box mount, usually made of plywood, with a horizontal circular base (azimuth axis) and Teflon bearings for the altitude axis. This mount is usually used on Dobsonian telescopes. It provides good support for a heavy telescope, as well as smooth, frictionless motion.
Although the alt-azimuth mount is simple and easy to use, it does not properly track the motion of the stars. In trying to follow the motion of a star, the mount produces a “zigzag” motion, instead of a smooth arc across the sky. This makes this type of mount useless for taking photographs of the stars.
The equatorial mount also has two perpendicular axes of rotation: right ascension and declination. However, instead of being oriented up and down, it is tilted at the same angle as the Earth’s axis of rotation. The equatorial mount also comes in two variations. The German equatorial mount is shaped like a “T.” The long axis of the “T” is aligned with the Earth’s pole. The Fork mount is a two-pronged fork that sits on a wedge that is aligned with the Earth’s pole. The base of the fork is one axis of rotation and the prongs are the other.
When properly aligned with the Earth’s poles, equatorial mounts can allow the telescope to follow the smooth, arc-like motion of a star across the sky. They can also be equipped with “setting circles,” which allow easy location of a star by its celestial coordinates (right ascension, declination). Motorized drives allow a computer (laptop, desktop or PDA) to continuously drive the telescope to track a star. Equatorial mounts are used for astrophotography.
An eyepiece is the second lens in a refractor, or the only lens in a reflector. Eyepieces come in many optical designs, and consist of one or more lenses in combination, functioning almost like mini-telescopes. The purposes of the eyepiece are to produce and allow changing the telescope’s magnification, produce a sharp image, provide comfortable eye relief (the distance between the eye and the eyepiece when the image is in focus), and determine the telescope’s field of view.
Field of view is “apparent,” or, how much of the sky, in degrees, is seen edge-to-edge through the eyepiece alone (specified on the eyepiece). “True or real” is how much of the sky can be seen when that eyepiece is placed in the telescope (true field = apparent field/magnification).
There are many types of eyepiece designs: Huygens, Ramsden, Orthoscopic, Kellner and RKE, Erfle, Plossl, Nagler, and Barlow (used in combination with another eyepiece to increase magnification 2 to 3 times). All eyepieces have problems and are designed to fit specific telescopes.
Eyepieces with illuminated reticules are used exclusively for astrophotography. They aid in guiding the telescope to track an object during a film exposure, which can take anywhere from 10 minutes to an hour.
Finders are devices used to help aim the telescope at its target, similar to the sights on a rifle. Finders come in three basic types. Peep sights are notches or circles that allow alignment with the target. Reflex sights use a mirror box that shows the sky and illuminates the target with a red LED diode spot, similar to a laser sight on a gun. A telescope sight is a small, low magnification (5x to 10x) telescope mounted on the side with a cross hair reticule, like a telescopic sight on a rifle.
Filters are pieces of glass or plastic placed in the barrel of an eyepiece to restrict the wavelengths of light that come through in the image. Filters are used to enhance the viewing of faint sky objects in light-polluted skies, enhance the contrast of fine features and details on the moon and planets, and safely view the sun. The filter screws into the barrel of the eyepiece.
Another add-on component is a dew shield, which prevents moisture condensation. For taking photographs, conventional lens and film cameras or CCD devices/digital cameras are used. Some astronomers use telescopes to make scientific measurements with photometers (devices to measure the intensity of light) or spectroscopes (devices to measure the wavelengths and intensities of light from an object).