Why cant x-ray and gamma-ray telescopes use the same designs as visible-light telescopes?

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April 20, 1999: 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.

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.

Right: The electromagnetic spectrum. Radio has long wavelengths and low energies, while gamma rays have very short wavelengths and high energies.

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.

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.

Left: Galileo Galilei (1564-1642), Italian astronomer, mathematician and physicist.

The glass lenses in the Galileo telescope weren't very clear, however - they were full of little bubbles and had a greenish tinge due to the iron content of the glass. Also, the shape of the glass lenses gave the field of view very fuzzy edges.

The magnification of Galileo's telescope could only be improved by focusing the light farther behind the primary lens, which resulted in longer and longer telescopes. But once

telescopes reached 140 feet in length they became almost useless for observation. It was impossible to keep the lenses properly aligned at such long lengths. Longer telescopes also required larger lenses, and after a lens reached 1 meter (3.28 ft.) in diameter it would deform, sagging under its own weight.

Right: Johannes Hevelius' 150-ft. telescope (Machina Coelestis, 1673). Reprinted with permission by the Royal Astronomical Society, London.

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.

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.

Left: Sir Isaac Newton (1642-1727), English mathematician and physicist.

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.

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 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.

Left: The "Jansky Antenna" doesn't look much like modern aerials, i.e., TV antennas or satellite dishes, because it was designed to receive shortwave signals coming over the horizon.

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. Resolution is also a factor:

low-frequency radio waves would be unfocused and fuzzy in smaller telescopes. 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 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. An example of a modern radio telescope is The Very Large Array in New Mexico (right), composed of 27 antennas electronically combined to give the resolution of an antenna 36 kilometers (22 miles) across.

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 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.

Left: The NASA Infrared Telescope Facility 3.0 meter telescope at the summit of Mauna Kea, Hawaii. Photo courtesy of Ernie Mastroianni.

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 rudimentary 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.

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 from Kenya on Dec. 12, 1970. Because this date was the 7th anniversary of Kenya's independence, the satellite was named Uhuru (Swahili for "freedom").
 

Left: X-rays can be reflected using a combination of paraboloidal mirrors and hyperboloidal mirrors. This is the arrangement of mirrors in the Chandra X-ray Observatory, scheduled for launch in July, 1999.

X-ray telescope mirrors are coated with gold or other metals; Uhuru's mirrors were coated with beryllium, for instance. 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.

Right: Illustration of a crystal gamma-ray detector. The electrons expelled by gamma-rays act like a trigger on an alarm, letting the detector know when gamma rays are passing through.

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, a year before Compton's death. The Compton Gamma Ray Observatory, launched in 1991 and still orbiting the Earth today, was named in his honor.

Left: Arthur Holly Compton (1892-1962) won the 1927 Nobel Prize in physics for his work with gamma rays.

Physics 101

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, a radio astronomer now consulting for the Science@NASA web site.

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.

Right: A mosaic of different astronomical phenomena at various wavelengths.

Phillips says that telescopes designed for different parts of the electromagnetic spectrum often look dramatically unlike one another. "Low-frequency radio telescopes look wildly different from microwave telescopes, even though both study the radio portion of the spectrum," he states. "And low-frequency radio telescopes don't bear the remotest resemblance to X-ray telescopes."

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 Christmas tree," says Weisskopf. "What we want is a system that can look at all of the emissions simultaneously." Matched telescopes could be aligned to look at

Left: If an all-wave telescope were made, it would be on the wish list for both professional and amateur observers. On the tree, the planets are represented in order of their distance from the Sun (at the top).

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.

Right: Skylab space station orbiting Earth in 1973.

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. There are no instruments designed to just study globular clusters, for instance.

Phillips agrees that wavelength specialization is prevalent in the scientific community, but he believes this attitude is changing. Up until the last thirty years, many astronomers built their own telescopes, thereby focusing all their attention on one portion of the spectrum. Today, engineers build the telescopes based on what the astronomers want to study. Because astronomers no longer build the telescopes, they are more willing to look at other wavelengths.

"Astronomers are now becoming more multi-wave literate in order to solve astrophysical puzzles," says Phillips.

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. This happened recently due to an accident with satellite equipment. The Compton Gamma Ray Observatory (left) once stored data on its satellite tape recorders and dumped data about gamma-ray bursts down to ground stations several times each day. However, in 1992 the tape recorders failed. Since there was no way to save the data, it had to be transmitted instantly. The bright side of this accident was that this instant transmission of information made it possible for other types of telescopes to have immediate, 'real-time' burst alerts. On Jan. 23, 1999, gamma-ray and visible light robotic telescopes coordinated an observation of a gamma-ray burst. When the gamma-ray burst was detected, the GRO quickly sent the information out through the Internet. The Robotic Optical Transient Search Experiment (ROTSE), a visible light

telescope, used this information to lock on to the burst 22 seconds after it began. This allowed scientists to see for the first time a gamma-ray burst explosion in the visible light portion of the spectrum.

Right: The Robotic and Optical Search Experiment (ROTSE), operated by Carl Akerloff at the University of Michigan.

"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."

Such serendipitous events as the GRO storage tape failure often reveal new ways of looking at old science, causing scientists to rethink ideas previously taken for granted.

Although Phillips says an all-wave telescope is not currently a topic of serious discussion in the scientific community, satellites with coordinated telescopes have worked well in the past. Perhaps the success of Skylab and other multi-wavelength observatories, combined with the happy accident of the GRO, will inspire new and revolutionary ideas about telescopes.

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Author: Leslie Mullen
Curator: Linda Porter
NASA Official: Gregory S. Wilson

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