Keck and Company
By the year 2000, four powerful new ground-based telescopes will have come on-line. Together, they promise to provide exciting new views of the universe.
For almost four years now, the 10-meter Keck Telescope has poked at the sky in its search for answers. The Keck Telescope and its twin brother, Keck II--known by some as "Side-Keck"--live together near the summit of Mauna Kea, Hawaii, at an altitude of almost 4200 meters.
It's a good place for telescopes to live. This location places the telescopes above most of the atmosphere's obscuring water vapor and is high enough that the atmosphere itself begins to thin out, allowing for very clear observing conditions. And thanks to new technologies, such as adaptive optics, the telescopes can adjust themselves to make up for most any problem a turbulent atmosphere still presents.
The presence of two telescopes rather than one also makes a difference. "They will do two-element interferometry so we can tell if a star is binary or single," said Cornell astronomy professor Riccardo Giovanelli. Interferometry is the technique of combining the light of an object received at two different places and comparing the light's wavelengths to obtain higher resolution of the object than otherwise possible. In this way, two 10-meter telescopes ten meters apart would give the resolution of an object viewed by a single 50-meter telescope.
For years, many of the most significant ground-based discoveries came from the 5-meter Hale telescope on Mount Palomar, and from several 3- and 4-meter telescopes spread over the world. To many people, these telescopes seemed capable of doing everything. Why, then, did astronomers see a need for the Keck system?
The answer proves rather simple for astronomers to explain: the sheer size of the new class of telescopes represents a significant advantage in studies of objects’ light spectra. Spectroscopy allows astronomers to determine velocity, mass, and composition of stellar objects. In spectroscopic studies, light is broken down into its component colors. In order to yield a good spectra, a telescope must be able to gather a lot of light. Older telescopes’ smaller light collection capacity limited spectrographic studies to reasonably bright stars. But the Keck 10-meter telescopes can gather four times more light than the Palomar 5-meter. The newer telescopes thus will allow scientists to do studies on much fainter objects such as brown dwarves, white dwarves, very distant main sequence stars, and unperturbed comets in the Oort cloud surrounding the solar system.
Designers built Hale and other pre-Keck telescopes around technology which ignores the constant motion of the atmosphere due to the mixture of warm air with cool air, a fact of nature that limited such telescopes to resolutions of one arc second, or 1/3600 of a degree. Astronomers measure the sizes of celestial objects by determining how much of the sky an object spans in degrees. There are 180 degrees from horizon to horizon. As a reference, a quarter, face-on five kilometers away, has an angular diameter of one arc second. Conventional telescopes can just barely tell that the quarter is there, much less make out any detail. Keck, however, can achieve resolutions of 0.1 arc seconds, a full order of magnitude better.
Designers built old telescopes within nearly closed domes. In these
domes, warm air degrades the resolution. Furthermore, unlike their newer, more flexible
counterparts, the mirrors on these telescopes are too stiff to compensate for the dance of
starlight caused by the atmosphere.
The new domes have much better ventilation, allowing cool outside air
to enter. Designers have placed heat-producing instruments and control rooms outside of
the dome. Finally, and most importantly, the new telescopes have thinner mirrors which
mechanical control devices called actuators can warp thousands of times each second to
account for the atmosphere's turbulence. This technology, known as adaptive optics, allows
images to appear almost as if there was no atmosphere in the way.
The Keck telescopes are the world's largest. The primary mirror, with a diameter of 10 meters, is actually not a single mirror, but a set of 36 smaller hexagonal mirrors arranged together in such a way that they form a single large mirror. This feature brings scientists several advantages.
Having several small mirrors allows designers to save a lot of weight in making the mirrors thin. Conventionally, large single mirrors have to be very thick to maintain their shape as scientists tilt them. By comparison, Keck's mirror is only 7.5 centimeters thick. This allows the entire telescope to weigh only 245 metric tons. If that sound like a large number, consider the 5-meter Hale telescope on Mount Palomar, which weighs 500 metric tons. Lighter telescopes are less likely to warp and sag under gravity's constant downward pull. Furthermore, because technicians can tilt the segments independently of each other by small increments, the telescopes can see finer details than any ground-based telescope. Finally, it's a lot easier to repair or replace one damaged mirror panel rather than a huge, single disk.
Right now, the Keck telescopes are in a class apart: no other Earth-based instrument matches their size and technology. But this situation won't last for long: scientists are currently building several other adaptive telescopes with impressive new technology.
One endeavor currently in progress with U.S. government support is the construction of two identical 8-meter telescopes, collectively known as Gemini. One will be placed on Mauna Kea, and the other in Cerro Pachon, Chile. Taken in combination, the locations of these two instruments will allow astronomers to see all astronomical objects, regardless of their location on the celestial sphere. While use of the Keck telescopes is restricted to a small number of universities, any American astronomer will be able to use the Gemini telescopes.
The Gemini telescopes also will incorporate dozens of new design features. The dome which houses each of these telescopes has several ventilation gates. The control buildings are separated from the unheated enclosure. Designers built the telescopes 20 meters above ground level--above the turbulent boundary layer caused by ground-heated air. Even if turbulence occurs, the telescopes can cope with it: their primary mirrors are thin and, as a result, slightly flexible. Since the mirrors are flexible, they can be pushed and pulled from the bottom to deform and compensate for atmospheric turbulence.
Meanwhile, the European Space Agency has yet another super telescope under way: the Very Large Telescope in Parranal, Chile. This telescope--actually a set of four 8.2-meter telescopes--can also adapt to turbulence in the atmosphere. In addition, it will be able to combine light in such a way as to allow the telescopes to behave as a single 16-meter telescope. It will come on-line around the turn of the century as the world's most powerful--although not largest--instrument.
Finally, there is the Subaru Telescope, an 8.3-meter Japanese telescope currently under construction. The Subaru, scheduled for completion in 1998, is an infrared (IR) telescope and will be the largest of its kind. As an infrared telescope, it is essential that its surrounding temperature conditions remain perfect: the air around it must be cool, and practically no infrared-absorbing water should be in the air. That is why it is also being built on Mauna Kea. Infrared studies let astronomers infer compositions and temperatures of objects. IR technology allows for observations cool objects, such as brown dwarfs, and objects very far away, such as galaxies in formation.
So why do astronomers want such large, high-resolution telescopes? Isn't the Hubble Space Telescope, flying high above the earth, free of fuzzy atmospheric effects, enough? It seems that space scientists are truly after the technology of big telescopes. While probably not capable of achieving Hubble's resolution, these instruments will gather much more light and, as a result, see dimmer objects. "For faint objects such as Uranus, Neptune or faint satellites, asteroids and comets, we just need the gathering power," said Glenn Orton, a senior research scientist at NASA's Jet Propulsion Laboratory.
Coordinating efforts, astronomers eventually may use Hubble to observe details in an object, and use the giant Earth-based telescopes to observe the spectra of these objects. In any case, the recent advent of these super telescopes may open a new era in astronomy.
Eldar Noe is a junior in astronomy. He recently transferred from the Florida Institute of Technology, attracted by the tales of the wonderful weather in the northeastern United States. By now he's pretty sure the tales were a lie.
Fuertes Tells Time, Scans the Heavens
Like most early observatories, Cornell University's Fuertes Observatory was designed with time-keeping in mind. When it opened in 1921, Fuertes--located on Cornell's North Campus near Helen Newman Hall--kept accurate time by recording when certain stars passed overhead. At the time, astronomers maintained several transit telescopes--fixed on a north-south horizontal axis--for time-telling purposes. The Observatory also housed a gas chamber room where a pendulum clock kept time, unaffected by temperature change. Accurate time-keeping is important for measurements of longitude and latitude needed for surveying land, plotting the exact locations of ships at sea, and tracking animal migrations.
The University named the observatory in honor of ornithologist Louis Agassiz Fuertes, who donated money to build an earlier observatory located on Cornell's central campus. "The observatory was quite a fancy place...housing three telescope domes," said Cornell astronomy professor Philip Nicholson. When the campus grew, the University demolished this observatory in 1903 to make way for the Barton Hall field house.
Prior to both Fuertes and the earlier observatory, Fuertes
helped build a wooden observatory with two telescope domes in the late 1800s at the
northernmost end of Cornell's Goldwin Smith Hall. The University quickly replaced this
structure with an agricultural studies building. "It was probably removed because of
its unsightly appearance," said Nicholson, partly in jest.
For a long time, the history of Fuertes was something of an enigma: it wasn't until last
year that Nicholson knew the date the observatory opened. Nicholson learned that besides
for telling time, Cornell used the building to teach navigation to World War II naval
cadets. The building still contains some vintage equipment from that time.
NASA's 24-satellite Global Positioning System has taken over
Fuertes’ original purpose of keeping accurate time but the observatory lives on as a
place for small-scale research and stargazing. Designers built Fuertes to hold a 12-inch
refractor telescope--which uses magnifying lenses to gaze at distant objects--and two
reflector telescopes that use mirrors rather than lenses. Today Cornell uses Fuertes for
teaching introductory astronomy classes. The observatory is open to the public for viewing
on clear Friday nights.
--Rob Peterson