The Ultimate Portable Telescope

Sally Stephens

A picture of the Milky way taken from a space telescope.

Courtesy of 2Mass/Gene Kopan/Robert Hurt

Putting a telescope on a jet plane to make precise observations doesn’t seem like a particularly good idea. Turbulence and engine vibrations would shake the telescope, often violently. Winds would rush past at 800 kilometers per hour, tearing the lens from its distant quarry.

And yet, airplanes have carried ever more powerful telescopes aloft for nearly 40 years. The incomparable view, say scientists, is well worth the hassle. From the outer edges of the atmosphere, NASA’s flying telescopes have glimpsed parts of the universe invisible to their earthbound counterparts. Their decades of discoveries have added valuable details to our understanding of the universe. Now a new addition to the family, to begin observations in 2005, promises to reveal even more about the origins of the stars themselves.

Flying telescopes are effective precisely because they fly. By soaring into the upper atmosphere, above 40,000 feet, they can escape much of the water vapor present at lower elevations. To an astronomer, water vapor is the enemy. It absorbs nearly all of the infrared light—also known as thermal radiation or just heat—emitted by celestial objects. That leaves ground-based telescopes practically blind to this segment of the spectrum—a serious handicap for those studying clouds of gas and dust that give rise to stars and hide the centers of galaxies.

Tiny particles within these clouds scatter visible light, leaving their interiors shrouded in mystery. But the longer wavelengths of infrared are not affected by dust. Researchers hoping to probe deep into these clouds, therefore, must take their telescopes to the skies.

Scientists have recognized the value of airplanes in astronomy for years. In the late 1940s and early 1950s, British astronomer D. E. Blackwell studied the zodiacal light, a faint glow caused by sunlight scattered by dust in our solar system, from the open door of a cargo plane. In the 1960s, several scientists observed total solar eclipses from the air. Above the aerosol and dust particles in the atmosphere that scatter light, the eclipse appeared darker than it would have from the ground, making it easier to pick out faint features in the solar corona.

In the mid-1960s, astronomer Gerard Kuiper put a small infrared telescope on a Convair CV 990, named Galileo, that NASA was using to study Earth. The telescope was mounted inside the Convair’s cabin, looking out a window. Though window glass reduced the amount of infrared light the telescope could receive, Kuiper and his team still detected water ice in Saturn’s rings, and proved that Venus’ clouds were not made of water vapor (we now know they’re actually sulfuric acid). Tragically, Galileo collided with a Navy plane in April 1973. Sixteen of the 17 people onboard both planes died, including everyone on Galileo. More bad luck followed. Gerard Kuiper died by year’s end. And in 1983, Galileo’s successor caught fire when a tire blew out on take-off. Fortunately, there was no loss of life.

Yet, scientists from many different disciplines who had flown on Galileo and its kin continued to dream of a plane that would do nothing but infrared astronomy. In 1967, they got it. A team led by astronomer Frank Low modified a twin-engine, six-passenger Lear Jet to carry a 12-inch telescope. Virtually all the jet’s passenger seats were removed and replaced with electronic equipment. The telescope, located in place of the window in an emergency exit, was sealed off from the rest of the cabin so it could be opened to the atmosphere, with no window glass to interfere.

“While observing, you sat on the floor,” recalls Al Harper, from the University of Chicago’s Yerkes Observatory. “One person would sit by the front door of the plane operating the electronics. The telescope operator would sit with knees tucked up under the telescope. It was rather intimate.”

The Lear Jet Observatory (LJO) could stay at high altitudes for just about an hour. Despite the short flights, astronomers were able to confirm that Jupiter and Saturn generate substantial amounts of internal heat. They also began to use this infrared eye to look deep into interstellar clouds of gas and dust.

Tantalized by the LJO's success, astronomers wanted a larger airborne telescope that could remain in the air for long periods of time. In 1974, a modified C-141 Starlifter transport began ferrying a 36-inch telescope to high altitudes. The Kuiper Airborne Observatory (KAO) could remain aloft for seven luxurious hours at a time, giving astronomers the chance to make extended observations.

The passenger accommodations improved as well. Instead of crouching on the floor, astronomers rode in seats. Mounted before them were several rows of monitors and computer consoles. The whole assembly was located right next to the pressurized bulkhead that separated the scientists from the telescope—and from the stratosphere outside. Like its predecessor, KAO’s telescope was sealed off from the cabin in a recessed cavity. It opened to the sky when at observation altitude. Oxygen masks hung like stalactites from the ceiling, a constant reminder of the dangers of high altitude work. At 41,000 feet or higher, you have just a few seconds to don a mask in the event of sudden decompression before you pass out from lack of oxygen.

The KAO lacked much insulation. As a result, it was noisy and cold. Astronomers needed microphones and headphones to talk to each other above the roar of the plane’s engines. Scientists donned warm sweaters and down parkas—some even huddled under blankets—to ward off the minus 40 ºC temperatures of the stratosphere that seeped into the plane during the long flights.

The KAO flew for 20 years. With its telescope, astronomers studied the distribution of water and organic molecules in star-forming regions and interstellar space, and probed disks around young stars that may hold clues to planetary formation. They detected water molecules in comets, confirming they really are “dirty snowballs.” They discovered a ring of gas at the center of our galaxy. And they showed that nine thin rings encircle the planet Uranus.

The venerable KAO was retired in 1995, sacrificed to free up money NASA needed to build its successor.

Now, the latest in the series of flying telescopes sits inside a hanger in Waco, Texas, poised to make its debut. Known as the Stratospheric Observatory for Infrared Astronomy (SOFIA), it is a Boeing 747 modified to house a 2.5-meter telescope. When finally operational in 2005, it will carry the largest telescope ever to leave the surface of the Earth.

To provide SOFIA's telescope with access to the sky, engineers cut a 4-meter-wide hole in the left side of the plane, behind the wing. The hole, big enough for an 18-wheel semi to drive through, must accommodate the mirror support assembly (half a meter wider than the mirror itself) and still allow room for the telescope to turn slightly sideways. To ensure the structural integrity of the plane, technicians have strengthened virtually the entire rear fuselage, especially the area around SOFIA’s telescope and “viewing hole.” Despite the modifications, wind tunnel tests indicate the plane will fly essentially as if there were no hole in its side.

The telescope itself was made in Germany under the auspices of DLR, Germany’s version of NASA and a partner in SOFIA. The heart of the telescope is its primary mirror, effectively 2.5 meters across. Telescope mirrors are light buckets that scoop up light from celestial objects. The bigger the mirror, the more light it can collect, and, therefore, the fainter the objects it can see.

Size matters with airborne telescopes, but so does weight. Planes with heavier loads use more fuel, which cuts into the length of time they can remain in flight. So technicians put the SOFIA mirror’s 4-ton block of glass on a diet. For nearly one-and-one-half years, they painstakingly drilled 120 holes into the back of the mirror, removing 85 percent of the glass. This side now resembles a honeycomb, with seven-millimeter-thick walls surrounding pockets of air. The honeycomb structure preserves the stiffness of the mirror, while reducing its overall weight. The mirror is now a svelte 960 kilograms.

Opticians then spent a year slowly grinding a parabolic shape into the mirror’s front side. This configuration helps focus light onto the telescope’s detectors. The grinding process was so precise that the finished mirror deviates from a perfect parabola by less than 0.0001 mm.

In a system full of mind-boggling details and precise engineering, perhaps the most incredible feature will be the system holding SOFIA's telescope steady during flights. Imagine pointing a laser beam at a penny 16 kilometers away and not letting it waver for hours at a time. That’s how accurate and steady SOFIA's telescope must be. But what about all the air turbulence, engine vibrations, and wind gusts?

It was amazing to me the first time I noticed the back of the KAO's telescope turning this way and that,” remembers Yvonne Pendleton, of the NASA Ames Research Center, SOFIA's home base. “Then I looked at the monitor that depicted our stellar image and realized the image was quite stationary. It was suddenly obvious to me that the plane was moving around the telescope and not the other way around.”

A combination of low-tech padding and high-tech instrumentation makes this feat possible. SOFIA’s observing apparatus is shaped roughly like a dumbbell. The telescope is located at one end, while the detectors that record the infrared light and counterweights are on the other; the two are connected by a tube. Twelve pressurized air bladders arranged around the tube will absorb nearly all of the airplane’s vibrations. Sudden gusts of wind that swirl into the telescope cavity from outside will be countered by three gyroscopes attached to the telescope’s frame. If the gyroscopes detect even the slightest motion, electromagnetic motors will quickly nudge the telescope back into place.

The telescope and mirror, weighing nearly as much as the entire Lear Jet Observatory, have now been installed in the aircraft. In summer 2005, after SOFIA proves it can fly safely with a large hole in its side, the plane will travel to its home base at NASA’s Moffett Field in Mountain View, California. Flights to verify that the telescope and its detectors are working properly will follow. SOFIA will not begin taking "real" data until fall 2005. With three to four flights a week planned once SOFIA is operational, astronomers' then-year absence from the stratsphere will finally come to an end.

"SOFIA will have a sharper infrared view of the universe than any previous telescope," says SOFIA chief scientist and University of California, Los Angeles astronomer Eric Becklin. With SOFIA, scientists hope to resolve features in star-forming regions and galaxy centers that, up to now, have appeared as indistinct blobs. Researchers hope to find and study very young, very distant galaxies, visible only at long infrared wavelenghts. Astronomers will also search the interstellar medium, which emits most of its energy at infrared wavelengths, for complex organic molecules and other atoms thought to be necessary for the formation of life.

"SOFIA is about as capable as a space-based telescope," says Dan Backman, manager of the SOFIA Education and Public Outreach programs, "but it comes home every night." That convenience makes the observatory a mechanic's dream. If an instrument malfunctions, it can be repaired or replaced on the ground. That's not the case with Chandra or Hubble, which require a visit from the Space Shuttle. Also, says Becklin, "we can install the latest and greatest detectors and instrumentation at any time." In fact, SOFIA's instruments will be constantly upgraded throughout its 20-year lifetime.

But perhaps SOFIA's greatest asset is its flexibility. Airplanes can fly anywhere in the world to observe an event at any time and in any part of the sky. For example, to observe what would happen when Uranus moved in front of a distant star, an event not visible in the Northern Hemisphere, the KAO flew out of Australia. The expedition discovered previously unknown rings around the planet. That kind of flexibility is impossible in telescopes anchored to the ground and difficult for tightly scheduled space telescopes. And that makes SOFIA, the latest and largest in a distinguished history of airborne observatories, the ultimate portable telescope.

Sally Stephens is a freelance science writer in San Francisco. She recently completed a high school curriculum for the SETI Institute and is currently working on a children's book.