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September-October 2013

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Windows Into Other Worlds

They are humanity's exquisitely precise, perceptive eyes that capture views of worlds unimaginably distant, lapping up images from as far away as the edge of the known universe: massive, multimillion-dollar telescopes that bring us views of stars, galaxies, nebulae, planets, moons, and other features of the vast cosmos in which our planet is barely a speck. They also bring data to us that allow researchers to further the understanding of the origins of all that exists. These modern facilities stand isolated in some of the planet's most exotic locations—in the middle of the Pacific Ocean atop the world's largest volcano, deep in the planet's highest and driest desert, on the windswept Tibetan plateau, and even amid a sea of Antarctic ice. Many of these places are so difficult to access that construction is immensely expensive and even sometimes dangerous, yet the scientific benefits far outweigh the risks and expense. What makes these places so special for viewing the heavens? The air. Scientists have these observatories built at locations with climates and under columns of the atmosphere that are ideal for the most lucid views of all the features of space they seek to study. But identifying these locations is nearly a science unto itself. A full site survey requires an analysis of far more than just the average number of cloudy days at a given location.

“Any site for a major (night-time) observatory must be reasonably free from light pollution, and must have a climate that enables observations during a large fraction of the year,” states Dainis Dravins, professor of astronomy at Lund Observatory, which is at Sweden's Lund University. Dravins, who completed portions of his graduate work in the United States at the California Institute of Technology, is an expert in the fields of observational astrophysics and precision observation techniques relating to solar and stellar surfaces. He is also renowned for his expertise on observatory locations, having visited numerous observatories throughout the world, and having served on advisory boards for the European Southern Observatory, the European Space Agency, and the European Research Council. “Year-round or even round-the-clock observing, however, is generally not feasible (except maybe for some radio observatories),” he continues. “In some places (Arizona, for example) observatories close during the rainy summer months, while some solar telescopes (such as on the Canary Islands) close during some winter months. And radio telescopes for short-wavelength microwave are not useful during months when air humidity is high, etc.”

Dravins's overview reveals an important point for any discussion among non-astronomers and non-astrophysicists about observatories: Telescopes come in many, many flavors. Instruments used to observe other worlds aren't limited to the type with which most of us are familiar—what we simply call a “telescope,” but which experts in the field call a “visible light” or a “visible wavelength telescope.” A visible wavelength telescope detects that slice of the electromagnetic spectrum to which the human eye's retina is sensitive (a wavelength typically between 400 and 700 nanometers), which is just a small band of the entire electromagnetic spectrum. Cosmic bodies, however, emit all types of radiation, so astronomers and astrophysicists explore virtually all bands of the electromagnetic spectrum to study the universe and its constituent parts, often combining research data from two or more bands of observed spectra in order to study a specific cosmic body or in an attempt to discover ones never before detected. Modern terrestrial-based astronomy employs highly specialized telescopes to study most types of radio waves, infrared radiation, visible wavelength radiation, submillimeter radiation (extremely high-frequency radiation), and microwave radiation.

The Telescope Defined

In the early days of space study, astronomers built observatories near their sponsoring institutions, typically on the outskirts of university towns. Those early telescopes, although they were relatively feeble in their ability to study the heavens by today's standards, functioned exactly as today's massive icons of extraterrestrial exploration do. Simply defined, a telescope—of any type—gathers electromagnetic radiation and focuses that radiation onto a receiver. Inexpensive backyard hobby telescopes gather and focus visible light with a series of lenses, directing it through an eyepiece to the system's receiver—a human retina. A radio telescope gathers radio waves with a reflective parabolic dish, the shape of which focuses those reflected waves onto its receiver (an antenna), or a sub-reflector that directs the waves to the system's antenna, at which point the radio energy is amplified and processed.

Ideally, absolutely nothing would stand between a cosmic body and the observing telescope: A perfect vacuum through which radiation emitted by the distant object, cascading for untold millions of millennia at physics' top speed, would meet the instrument absolutely unfettered. While electromagnetic radiation from distant galaxies, stars, and other cosmic bodies may travel virtually unhindered toward earth, as soon as this radiation beams into the upper atmosphere, a dance of distortion begins.

The atmosphere may be viewed in many ways, but to astronomers, telescope designers, and to those who seek the best locations for viewing the heavens, it's a sort of combination between a lens and a filter—one that degrades the quality of the stream of radiation through refraction (bending radiation rays), scattering (deflecting them), and absorption (making them go away altogether). And, of course, the greater the distance through the atmosphere this radiation must travel, the greater the distortion. Furthermore, the degree of degradation increases closer to the earth's surface due to the greater density of the atmosphere relative to that of higher elevations. A great example of this degradation is a star twinkling. Technically called scintillation and seeing, stars appear to twinkle when their light passes through an ever-changing column of air of varying temperatures (and hence densities and refractive indices), refracting the light rays relative to a viewer. Technically, “scintillation” refers to a star's observed change in brightness, while “seeing” refers to its observed change in image sharpness and position. If viewed through a powerful telescope, not only would the size of the star be magnified, but so would this distortion, rendering not only an inaccurate image of the star, but an inaccurate and ever-changing position of it. The deep red and orange hues of the sun that we see as it sets is an example of atmospheric scattering, where particles in the atmosphere deflect shorter wavelengths of the shower of sunlight, allowing only the longer yellow, orange, and red wavelengths to pass through to a viewer on the ground. For an accurate portrayal of distant objects, astronomers seek the full range of radiation emitted by them. Another factor is that certain bands of the electromagnetic spectrum never even reach the ground, because constituents in the atmosphere absorb them.

Each of the various components of the atmosphere affect different ranges of the electromagnetic spectrum differently. While many locations on the planet prove ideal for a range of different types of telescopes (for instance on the summit of Mauna Kea, which has an infrared telescope, submillimeter array telescopes, and visible wavelength telescopes), there is no “one location fits all.” In general, though, higher locations prove better, because placing an observatory at a higher altitude reduces the amount of distorting atmosphere through which pristine cascades of electromagnetic radiation must pass before being detected. “In the early 20th century, observatories started to be placed on more remote mountain tops that offered more clear nights or days, and where the air was less turbulent, permitting sharper images to be recorded (for example, Mount Wilson in the San Gabriel mountains of California),” Dravins notes.

An Ideal Location

Today, the world's major observatories, while scattered throughout the globe in places like the Canary Islands, northern Chile, South Africa, and a part of Australia, all share key important traits. As astronomer and atmospheric optics expert Dr. Andrew T. Young explains, “All these places have a characteristic climate caused by a nearby cold ocean current, which produces a dry climate, and a persistent, strong thermal inversion at the coast, which suppresses turbulence. The large oceanic area upwind, combined with this inversion, guarantees low aerosol content; so the air is very clear.” Young goes on to note that the Mauna Kea Observatory, on the Island of Hawaii, is a notable exception. The Mauna Kea Observatory, the largest in the world for optical, infrared, and submillimeter astronomy, is ideal due to a strong thermal inversion that keeps the air at its 13,796-foot high summit dry and cloud free. Furthermore, virtually no human-created air pollution makes its way to this isolated location in the middle of the Pacific Ocean.

Young also notes an indirect benefit of the climate of these places: “It's clear that most modern observatories have one thing in common: a similar climate, of the Mediterranean type. The dry climates have discouraged people from trying to settle in these places; they're all deserts, or semi-deserts, where you can't raise enough crops to live on. So they all have low populations, and consequently little air pollution.”

Also related to large populations of people, light pollution poses significant problems for astronomy. Dravins explains that there are different types of artificial light pollution: for example, low-pressure sodium lamps that emit yellow light and high-pressure sodium lamps, which emit a more whitish light. The yellowish low-pressure lights can be filtered out, as they emit light in a narrow spectral region; the high-pressure whitish lights, however, “cannot be filtered out since the light comes in all colors,” he states. Sparse population due to a dry climate also benefits radio astronomy, where lack of stray radiation emitted from cellular phones, radar, remote controls, and other devices ensures the quality of signals that the ultrasensitive telescopes receive.

Professor Dravins adds that not all light interference can be blamed on humans. He points to “airglow,” a phenomenon that can occur anywhere that is caused when solar radiation excites atoms in the atmosphere, and “some faint aurorae, even at non-polar latitudes, where the aurorae may not be readily recognized as such.” He also points to foreground light from sunlight striking dust floating in our solar system: “From a dark site, this can be observed after sunset or before sunrise as zodiacal light. The mere fact that this is seen with the unaided eye tells us that its brightness must be comparable to that of the night sky itself.”

What Does the Future Hold?

One important question to ask is, with the Hubble Space Telescope and other space-based observatories, is terrestrial-based astronomy doomed to obsolescence? Absolutely not, as Dravins explains: “There is really no contradiction nor conflict between ground-based and space-based observations. They are highly complementary and are used for different purposes. Indeed many or most observational astronomers (myself included) regularly use both ground-based and space-based telescopes and their data.” Some fields of astronomical research require a space-based platform, however, as ultraviolet radiation and X-rays do not pass through the atmosphere. “To detect these, one must go into space,” Dravins explains.

While visible wavelength astronomy can be conducted by both earth-based and space-based observatories, raw economics and logistics limit the capabilities of space-based telescopes. “The Hubble is a truly great space telescope, but its mirror is ‘only’ 2.4 meters in diameter, corresponding to a modest or medium-size ground-based instrument,” Dravins notes. He adds that the Hubble is superior to any earth-based telescope for capturing images absolutely free of distortion because it doesn't have an atmosphere interposed between it and the subject it is observing. “But for other purposes it is inferior, for example, if one wants to examine the spectrum and analyze the properties of some faint and distant source,” he explains. The key factor for such an undertaking is light-gathering capability, and a 2.4-meter telescope in space won't collect much more light than a similar instrument on the earth. “Such studies are therefore made with today's largest optical 8-10 meter class telescopes [such as the European Southern Observatory's Very Large Telescope in Chile, or the Keck telescopes on Hawaii].” Dravins adds that scientists have plans for extremely large earth-based telescopes, including the Thirty Meter Telescope for Mauna Kea, and the 40 meter European Extremely Large Telescope, to be built in Chile. Such instruments are so massive that placing them in space would prove virtually impossible.

Similarly, most radio telescopes will likely continue to remain earth-based. In March 2013, the world's largest radio observatory ever constructed began operation: the Atacama Large Millimeter/Sub-Millimeter Array (ALMA). Consisting of 66 moveable radio telescopes, the observatory was built on the Chajnantor plateau at roughly 16,400 feet in the Atacama Desert of northern Chile to study the early dynamics of the universe. The $1 billion complex was built in this location due to the incredibly dry air, as water vapor absorbs electromagnetic radiation in the bands this facility was constructed to study. And like the massive 10-, 30-, and 40-meter diameter visible wavelength telescopes, placing such a facility in space would prove impossibly expensive.

Further mitigating the effects of the atmosphere, the process of “adaptive optics,” a technology developed initially in the 1990s, further refines the quality of terrestrial-based imagery. Implemented at all major observatories, adaptive optics measure the optical changes made by the atmosphere, and it corrects for them in real-time (within one or two milliseconds). Professor Dravins notes, however, that all major contemporary observatories sit at similar latitudes, roughly around 30 degrees north and south of the equator, and this poses problems for adaptive optics, as jet stream currents can snake into these latitudes and “deposit significant amounts of kinetic energy in the high atmosphere, producing turbulence in clear air, and—due to their high velocities—cause these turbulence patterns to fly over the observatories in a very short time—too short for comfortable measurements of the adaptive optics. This has prompted searches for observatory locations outside the jet-stream latitudes. The most promising seem to be in the highlands in eastern Antarctica.”

Dr. Young notes another benefit to basing observatories in Antarctica: “The Antarctic locations are close enough to the pole to be beyond the jet-stream latitudes, of course. But another advantage of the high Antarctic plateau is the low density of the air there: Because it's so cold, the atmosphere shrinks down toward sea level, and the telescopes are above more of it than they would be at the same height in warmer places.” However, building massive observatories in Antarctica poses obvious logistical and safety challenges.

As technology advances and scientists continue to develop ever more powerful telescopes of all types, the search will continue to find the clearest atmospheric windows through which astronomers and astrophysicists can aim these incredibly sensitive instruments to further unlock the secrets of the cosmos, and possibly discover the origin of everything.

ED DARACK is an independent author and photographer. Learn more at www.darack.com.

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