At the fringes
By Ashley Yeager
In 1609, when Galileo Galilei first turned his two-centimeter telescope to the sky, he was able to observe twice the level of detail of objects compared with his unaided observations. The telescope allowed him to resolve impact craters on Earth’s moon as well as much fainter objects, such as moons orbiting Jupiter. The observations revolutionized astronomy and triggered astronomers’ desire to engineer and construct telescopes with increasing light-gathering potential and angular resolution, or the ability to observe fine detail of and distances between cosmic objects.
Currently the largest optical telescopes, including the twin Kecks, are ten meters in diameter—500 times the size of Galileo’s telescope—which means their angular resolution should be roughly 500 times better than those first used in the 1600s. Larger telescopes can collect more light and measure smaller apparent distances as well as observe finer detail.
Astronomers have quickly realized, however, that the mirrors needed for increasingly larger telescopes are not easy to finance, engineer or construct. One alternative to larger, individual telescopes is to create arrays of them.
Keck Observatory is one of the first major observatories to develop this array concept by linking its twin, ten-meter telescopes. This technique, called interferometry, is based on the concept of interference, which in physics is the addition of two or more waves that results in a new wave pattern. The phenomenon affects light waves, sound waves, ocean waves, and even seismic waves from earthquakes.
Applied to astronomy, interference involves light waves. Astronomers can collect the waves originating from a single source, such as a star, with two separate telescopes and then manipulate the waves to interfere with each other. The result is a specific pattern called fringes.
The resolution achieved with an interferometer is based on the distance between the telescopes. The twin Keck telescopes are separated by 85 meters. To maximize the Keck Interferometer’s resolution, the light each telescope collects is first corrected for atmospheric distortion using adaptive optics. The two individual light beams are then directed down to the basement of the Observatory where they are relayed along tracks of flat mirrors positioned throughout the length of the 85-meter chamber that separates the telescopes. The path that the light travels can be adjusted to delay the wave from one telescope so that it aligns to correctly interfere with the incoming wave from the other telescope.
During an interferometry run, merging waves from the Keck I and Keck II telescope results in fringes, an alternating pattern of light and dark rings around the central image of the light source. When the waves are perfectly aligned, the fringes appear to stand still—even though the beams are traveling at the speed of light. This is known as the standing wave phenomenon, says Keck Interferometer specialist Julian Woillez. At this point, the telescopes become a virtual 85-meter telescope called the Keck Interferometer, or KI.
The Keck Interferometer saw “first fringes” in 2001.
In 2002 and 2003, astronomers used the system to study the young protostar DG Tau and its orbiting dust disc. The interferometry data showed that the star and the disk were separated by a gap of 18 million miles. By comparison, Mercury sits roughly 36 million miles from the Sun. Since the star is located about 450 light years from Earth distinguishing the apparent distance between the two objects was a significant achievement.
In 2009, astronomers used the Keck Interferometer to discover an extended, double-layered dust disk orbiting 51 Ophiuchi, a star that is 410 light-years from Earth. Based on the data, the observers calculated that if the debris disks orbited the Sun, the inner cloud would extend roughly from the position of Mercury’s orbit to just past the edge of the asteroid belt. The outer disk would originate just before Saturn’s orbit and extend to a distance ten times farther than the edge of the Kuiper belt.
Studying dust disks around other stars is an important part of NASA’s Origins program, which focuses on understanding how planets form and finding planets orbiting other stars. NASA funded the Keck Interferometer to provide astronomers with access to instruments capable of studying stars and their orbiting debris disks.
“It’s from these disks of dust and gas that planets form, so measuring angular distances and also emissions from the disks gives us clues about how stars and planets, even those similar to Earth, form,” says Rafael Millan-Gabet, an astronomer at the NASA Exoplanet Science Institute.
Seeking to make even more precise measurements of stellar dust disks, engineers at the Keck Observatory and the Jet Propulsion Laboratory in Pasadena, California collaborated to develop an interferometry instrument called the Nuller. It destructively interferes with or “nulls” light emitted from a target star. Blocking the starlight allows astronomers to study the light of the star’s much fainter debris disk and to look for the planets at the beginning of their formation.
To Planets and Beyond
Aside from dust around stars, astronomers also use the Keck Interferometer to study material orbiting supermassive black holes.
In 2003, a team used the two Keck telescopes to observe the inner regions of galaxy NGC 4151, which is 40 million light years from Earth. The galaxy’s central, supermassive black hole is estimated to be 10 million times the mass of the Sun. With the resolving power of the Keck Interferometer, astronomers studied the core of NGC 4151 and observed emissions that likely originated from material escaping the galaxy’s central supermassive black hole. The observations marked the first instance of an optical/infrared interferometer observing objects outside the Milky Way.
Peter Wizinowich, Keck Observatory’s interferometer team leader, and engineers from the Observatory and the Jet Propulsion Laboratory are currently fine-tuning the Keck Interferometer’s capabilities.
In 2006, Keck Observatory received funding from the National Science Foundation’s Major Instrumentation program to begin the ASTrometric and phase-Referenced Astronomy upgrade on the Keck Interferometer. The project, called ASTRA, will give the Interferometer even higher resolution. The first phase of ASTRA was completed in 2008, and the next phase, which will enable even higher measurements on even fainter targets, will be completed in 2010. In the future, the entire interferometer system might be modified even further to include data taken with other telescopes on Mauna Kea.
The project to link the Subaru, Gemini, Canada-France-Hawaii and twin Keck telescopes, which all have existing adaptive optics systems, with interferometry is called the Optical Hawaiian Array for Nanoradian Astronomy or OHANA.
Named for the Hawaiian word for family and extended family, OHANA will achieve ultra-high resolution observations of the near-infrared universe and be 80 times more accurate than a single 10-meter Keck telescope. The light from each telescope will be brought together using fiber optics.
The Keck Interferometer was used to perform the first demonstration of the fiber optic combining approach in 2005. The incoming infrared light was transported over a simulated baseline of roughly 500 meters and then coherently combined and successfully measured.
In August 2009, the fiber optics-based OHANA setup was used for science observations with the Keck Interferometer. The astronomers pointed the two telescopes at the inner regions of young stellar objects in the Ophiuchus star forming region. The resulting fringes from these and other observations suggest it that it may one day be possible to combine the light of several of the Mauna Kea telescopes into a grandiose 800-meter telescope.
The next phase of OHANA is to develop the optics to bring together the light of the Gemini and Canada-France-Hawaii telescopes to the KI system.