It takes more than one kind of telescope to see the light

By Joyce Evans,2014-11-13 15:17
8 views 0
It takes more than one kind of telescope to see the light

    It takes more than one kind of telescope to see the light!

    Why we need different types of telescopes to look at outer space…

    By studying the electromagnetic emissions of objects such as stars, galaxies, and black holes, astronomers hope to come to a better understanding of the universe. Although many astronomical puzzles can only be solved by comparing images of different wavelengths, telescopes are only designed to detect a particular portion of the electromagnetic spectrum. Astronomers therefore often use images from several different telescopes to study celestial phenomena. Shown below is the Milky Way Galaxy as seen by radio, infrared, optical, X-ray and gamma-ray telescopes.

The Multi-Wave Milky Way Galaxy





    Gamma ray

    Different types of telescopes usually don't take simultaneous readings. Space is a dynamic system, so an image taken at one time is not necessarily the precise equivalent of an image of the same phenomena taken at a later time. And often, there is barely enough time for one kind of telescope to observe extremely short-lived phenomena like gamma-ray bursts. By the time other telescopes point to the object, it has grown too faint to be detected.

    So why haven't scientists created a telescope designed to look at everything at once?

    "Nature has determined the design of our telescopes," says Dr. Martin Weisskopf, an astrophysicist at NASA's Marshall Space Flight Center. The differing wavelengths among the various energies create different instrumental needs. This results in dissimilar, incompatible detecting devices.

    Telescopes rely on the interaction between energy and matter. The atomic matter that forms the telescope has to somehow interpret the energy emitted from astronomical objects. This energy is in the form of electromagnetic waves. Although the first telescope was created 400 years ago, we didn't have a complete picture of the electromagnetic spectrum until the early part of this century. As our knowledge of physics improves, scientists are able to develop increasingly superior telescopes. But as

    the technology advances and becomes more specialized, differences among telescope designs become more pronounced

    The Development of Telescopes

    Most of the universe is invisible to us because we only see the visible light portion of the electromagnetic spectrum. When most people think of telescopes they think of visible light, or optical, telescopes.

Optical telescopes:

    When the first optical telescope appeared in the 1570s, the design was simple - one concave and one convex lens fitted inside a tube. The tube acted as a receiver, or 'light bucket'. The lenses bent, or refracted, the light as it passed through the glass and thus made the scene appear 3 to 4 times larger. Galileo improved upon the design and by 1609 had developed a 20-power refracting telescope. Galileo made the telescope famous when he discovered the valleys and mountains of the moon and spotted four of Jupiter's satellites.

Reflecting telescopes:

    Isaac Newton invented the first reflecting telescope in 1671. By using a curved mirror to reflect and focus the light inside the tube, he was able to reduce the length of the telescope dramatically. The reflecting telescope solved another problem inherent in the refracting telescope: chromatic aberration.

    Chromatic aberration the property of a lens whereby the light of

    different colors is focused at different places

    In 1672, Newton described how white light is actually a mixture of colored light. Each color has its own degree of refraction, so curved lenses split white light into the colors of the spectrum. This chromatic aberration caused central images in refracting telescopes to be surrounded by rings of different colors. Planets seen through a refracting telescope would appear to be encircled by a rainbow.

    By 1730, Newton's reflecting telescope had caught on with the scientific community. Even today, large optical telescopes are based upon Newton's basic design. Yet another bonus of Newton's reflecting telescope is that it can also be used to study ultraviolet and infrared light. The Hubble Space Telescope, famous for its stunning optical images of the universe, also works in the ultraviolet and infrared parts of the spectrum.

     Radio telescopes:

     But it wasn't until the 1930s that astronomers even began looking for other parts of the electromagnetic spectrum. Karl Jansky inadvertently discovered galactic emissions of radio waves in 1933. Working at Bell Telephone Laboratories, Jansky was trying to find what caused short-wave radio interference in Trans-Atlantic (across the Atlantic Ocean) communications. By building a rotating radio telescope to look at the horizon, he eventually discovered that most of the static resulted from engine ignition noise and distant lightning storms. But Jansky also discovered that some radio noise was coming from the center of the Milky Way Galaxy.

    Like optical telescopes, radio telescopes have reflectors and receivers. Most radio telescopes need to be large in order to accommodate radio's longer wavelengths and lower energies. Radio telescopes also need to be large in order to overcome the radio noise, or "snow," that naturally occurs in radio receivers. We generate a large amount of noise pollution on Earth as well, so smaller telescopes would lose some

    astronomical radio signals amid our daily production of rock music, television broadcasts and cellular phone calls.

    Radio and optical telescopes can be used on Earth, but some resolution is lost due to Earth's atmosphere. By viewing from the other side of the sky, the Hubble Space Telescope allows astronomers to see the universe without the distortion and filtering that occurs as light passes through the Earth's atmosphere.

Infrared & Ultraviolet telescopes:

    Infrared and ultraviolet light are affected more dramatically by the Earth's atmosphere. Their telescopes must therefore always be positioned high above the ground or in space. Infrared telescopes are placed on mountaintops, far above the low-lying water vapor that interferes with infrared light.

    Ultraviolet telescopes have to be placed even higher than infrared telescopes. The Earth's stratospheric ozone layer, located 20 to 40 kilometers above the Earth's surface, blocks out UV wavelengths shorter than 300 nanometers. By the 1940s, scientists were launching rockets with early UV detectors onboard.

    The Earth's atmosphere scatters or absorbs high-energy radiation, protecting us from the damaging effects of UV, X-rays and gamma rays. The atmosphere does such a good job that telescopes designed to detect these portions of the electromagnetic spectrum have to be positioned outside the atmosphere.

X-ray & Gamma-ray telescopes:

    Studies of astronomical objects in high energy X-rays and gamma rays began in the early 1960s. Although high altitude balloons and rockets can provide X-ray and gamma ray data, the best results come from satellites orbiting completely outside the Earth's atmosphere. NASA's first X-ray telescope was launched on Dec. 12, 1970.

    X-ray telescope mirrors are coated with gold or other metals. The mirrors have shallow angles of reflection because X-rays are so short they only reflect at angles almost parallel to the rays themselves. At steeper mirror angles the rays won't reflect - instead they will penetrate the mirror like a bullet embedding itself in a wall.

    Because gamma rays are even shorter than X-rays, there is no way to prevent them from passing right through a detection device. Since mirrors can't be used to focus gamma rays, a method had to be developed for detecting gamma rays indirectly.

    The American physicist Arthur Holly Compton discovered that gamma rays would expel electrons as they moved through a detector. Modern gamma-ray detectors use crystals or liquids that are triggered by these expelled gamma-ray electrons to record the passing gamma rays as flashes of light. The first gamma-ray satellite, Explorer XI, was launched in 1961. The Compton Gamma Ray Observatory launched in 1991 and is still orbiting the Earth today.

Why so many telescopes?

    Most objects give off several frequencies of energy simultaneously. Your body, for instance, glows in thermal infrared down to radio. But in order to get astronomical data about different wavelengths, scientists have to use several different types of telescopes. There is no such thing as an 'all-wave' telescope. The problem with having one telescope able to detect the entire electromagnetic spectrum lies in the differences in detection techniques.

    "Telescopes are designed with one goal in mind: to build a device that interacts with radiation coming from the cosmos," says Dr. Tony Phillips.

    Different energy wavelengths interact with matter in different ways. Radio waves will reflect from a metal that X-rays pass right through. These differences in the interaction between matter and energy have resulted in telescopes designed to only accommodate very specific wavelengths.

    With present technology, it is not possible to build one telescope able to efficiently survey the entire electromagnetic spectrum. Scientists follow established laws of physics in building telescopes, and an all-wave telescope would have to break those laws.

    "That's the wall that keeps us from building one device for everything," says Phillips.

    Because it is not currently possible to create an all-wave telescope, the next choice is to create a device that uses many telescopes at once. "What we want is a system that can look at all of the emissions simultaneously." Matched telescopes could be aligned to look at the same thing at the same time. A device containing all the different types of telescopes would have to be a satellite so that X-rays and gamma rays could be detected.

    Several multi-wavelength observatories have already flown - Skylab, the Solar Maximum Mission, and the Solar and Heliospheric Observatory (SOHO). The Skylab space station in particular is hailed as a good model for conducting multi-wavelength studies in space. Launched in 1973, Skylab had eight coordinated telescopes located on its Apollo Telescope Mount (ATM). The eight telescopes studied the Sun's spectrum from X-ray almost down to infrared, all with very high quality resolution. Skylab was coordinated with ground-based astronomers as well. Whenever ground observers detected active solar prominences, flares, or mass ejections, they would notify the astronauts, who would then point their telescopes to record the event.

    The problem in developing this type of technology today, says Weisskopf, is two-fold. Money is the most immediate impediment. It would cost several billion dollars just to create a high quality combined optical and X-ray telescope.

    More difficult to tackle is the social mind-set of scientists. Scientists are often trained to specialize; to study only one segment of the electromagnetic spectrum. Hence we have many X-ray astronomers, radio astronomers, and so on, with fewer scientists following a multi-wavelength approach. Facilities and instruments are built to study only portions of the spectrum, rather than phenomena as a whole.

    Because specialized telescopes are so well developed and are still strongly supported by scientists, the most logical approach would be to coordinate the telescopes already in existence.

    "You want to ask, 'Why didn't everyone do it this way from the beginning?'" Weisskopf grins. "It's because everyone got in their own cars and started driving, and many were just following the cars in front of them."

Report this document

For any questions or suggestions please email