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Frequently Asked Questions

What is the world’s largest telescope?

The largest visible-light telescopes currently in operation are the Keck I and II telescopes on Mauna Kea in Hawaii. Each uses an array of 36 computer-controlled mirror segments to provide the light-gathering power of a single 10-meter (33-foot) mirror.

The Hobby-Eberly Telescope at McDonald Observatory near Fort Davis, Texas, houses the world's largest telescope mirror. Because of the way HET is designed, astronomers use only 9.2 meters of the 11-meter (36-foot) mirror at any one time, making HET the world’s third-largest telescope. Scheduled upgrades to the telescope, however, will improve its performance to that of a 10-meter telescope, which is the equivalent of the twin Kecks.

The largest single-mirror telescope is the 8.3-meter Subaru Telescope, operated by the National Astronomical Observatory of Japan atop Mauna Kea. It saw first light in 1999.

Two larger ground-based mirrors are in the planning stages: the Giant Magellan Telescope and the Thirty-Meter Telescope. The first would consist of eight individual mirrors working together, while the latter would consist of a large segmented mirror. Each would have an effective aperature of roughly 30 meters (100 feet), giving them as much surface area as a small office building.

Hubble Space Telescope looks at the nether regions of the universe with a 2.4-meter mirror. The James Webb Space Telescope, which NASA plans to launch as early as 2013, will have an eight-meter (25.6-foot) primary mirror.

The largest refracting telescope in the world is at Yerkes Observatory in Williams Bay, Wisconsin. Instead of a mirror, it gathers light with a 40-inch glass lens.

Astronomers also gather radio waves from space using dish-shaped antennas, the largest of which is the Arecibo Observatory in Puerto Rico. Featured in the movie “Contact,” Arecibo's dish is 1,000 feet in diameter.

Who invented the telescope?

The true inventor of the first telescope is somewhat difficult to nail down. However, the first person to apply for a patent on a telescope was Hans Lippershey, a lensmaker in the Netherlands, in 1608.

The first astronomical use of the telescope is easier. After learning of the new device, the great Italian scientist Galileo Galilei designed and built his own. He turned his finest telescopes toward Jupiter, the Moon, and Venus in 1609 and 1610.

Why are many observatories located on mountaintops?

Almost all of the world’s finest ground-based observatories are located on mountains, for a variety of reasons. First and foremost, starlight appears less distorted in the thin atmosphere on mountaintops. (Space-based telescopes such as Hubble and Spitzer Space Telescope circumvent the disturbing effects of the atmosphere by flying above it.)

At high altitudes, there is less atmosphere to absorb infrared energy, which reveals details about some of the coldest objects in the universe, such as clouds of gas and dust and the disks of dust that give birth to planets.

Mountaintops also have unobstructed views of the horizon in all directions. Lastly, most cities and towns — with their accompanying light pollution — are situated in valleys and plains, so remote mountaintops are among the last places on Earth to find the dark skies so sought after by astronomers.

Are any observatories open to the public?

Many observatories around the world are at least partially accessible to the public, with some, such as McDonald Observatory in West Texas, even allowing regular public access to some of their research telescopes.

Listing of observatories with public telescope viewing

What do astronomers do at an observatory?

Before using a research telescope, an astronomer submits a proposal detailing project goals and equipment needs for approval by a committee of other astronomers. In some cases, even when a project is approved, the astronomer never visits the observatory at all. Instead, telescope specialists operate the instruments and gather data for the astronomer.

When an astronomer does travel to an observatory, the engineers, electricians, opticians, computer scientists, cooks, and crew who live there prepare for the astronomer's “observing run,” which typically lasts a few nights. The astronomer sleeps through the day, then spends a few hours before sunset preparing for the observations. After dinner, when night falls, the observations begin.

The astronomer spends the entire night pointing the telescope at distant objects — planets, stars, nebulae, or galaxies — and collecting the faint trickle of light from each object. A computer stores the data for later analysis. If clouds spoil the observations, the astronomer must submit a new proposal and hope for clear skies next time.

The astronomer never actually looks through the telescope, although most telescopes have a video system to display the area of the sky at which the telescope is aimed. Instead, the astronomer generally remains in the lighted, heated control room and monitors both the telescope and instruments as they collect and record data.

After completing an observing run, an astronomer may spend months or years analyzing and interpreting the results. Meanwhile, back at the observatory, the staff prepares for the next astronomer.

How do modern telescopes work?

Telescopes gather light in one of two ways. Reflecting telescopes focus light with a series of mirrors, while refracting telescopes use lenses. For research purposes, reflecting telescopes have become the standard because of the relative ease of constructing and working with large mirrors. The lenses needed for refracting telescopes present endless engineering problems and must be extremely pure throughout their entire volumes, while mirrored surfaces require ultra-fine precision only on the surface.

Modern telescopes gather information from the electromagnetic spectrum far beyond the range of visible light. Telescopes that survey radio, X-ray, and gamma-ray wavelengths have dramatically broadened our understanding of the universe. Radio telescopes — huge wire-mesh dishes designed to focus radio signals from space — have helped to map the spiral arms of our galaxy, while gamma-ray observatories high in Earth orbit have captured the high-energy signals of exotic objects such as black holes and gamma-ray bursts.

More details on how telescopes work

Is Hubble Space Telescope the only telescope in space?

While Hubble Space Telescope is certainly the most famous observatory in space, it is by no means the only one. There have been dozens of space-based telescopes, including past missions like the highly successful Compton Gamma-Ray Observatory, which helped astronomers unravel some of the mysteries of gamma-ray bursts, and the currently operating Chandra X-Ray Observatory, which is exploring the violent regions around black holes and other high-energy phenomena.

Another member of NASA’s Great Observatories program is the Spitzer Space Telescope, which was launched in August 2003. It explores the infrared glow of stellar nurseries, planet-forming disks around newborn stars, and interstellar dust clouds.

The Kepler mission, launched in 2009, will spend three years looking for Earth-like planets in Earth-like orbits around Sun-like stars. It will scan 100,000 stars in the constellations Lyra and Cygnus in hopes of finding planetary transits, in which the star’s light dims slightly as a star passes across its disk.

NASA also has launched many smaller observatories through its Explorer program. These missions have probed the “afterglow” of the Big Bang (COBE and WMAP), the ultraviolet light from other galaxies (GALEX and EUVE), and the violent explosions known as gamma-ray bursts (SWIFT).

How do astronomers use space-based telescopes?

Procedures for operating satellite telescopes are somewhat different from traditional ground-based telescope observing runs. Telescopes orbiting in space are operated remotely from control stations on the ground, where specially trained staff point the telescope at the specific targets requested by the astronomers. This “queue-based” observing makes efficient use of the telescope’s time — several different projects can be done in a single day. As a result, many observatories have started using this approach for ground-based telescopes as well, such as the Hobby-Eberly Telescope at McDonald Observatory.

How do astronomers find planets outside our solar system?

Astronomers have discovered far more planets outside our solar system, known as extrasolar planets, than within it. Astronomers can not only detect a planet’s presence, they can deduce a lower limit to its mass and distance from its parent star, and the length of its year.

The most successful search method used to date is called the radial velocity method. As a star is tugged to and fro by a planet’s gravitational pull, astronomers measure a slight shift in the frequency of the star’s light.

Astrometry is another detection method. It is sometimes called positional astronomy. Astronomers measure a tiny shift in a star’s position on the sky caused by the gravitational pull of a planet. They can use this information to calculate the planet’s mass and orbit.

If a planet passes directly between a star and the observer, it blocks out a tiny portion of the star’s light. This so-called transit method looks for repeated dips in a star’s light to confirm the presence of an orbiting planet.

A fourth detection method, called gravitational microlensing, comes from one of Einstein’s insights in his theory of general relativity: Gravity bends space. When a planet passes in front of a more-distant star, the planet’s gravity will behave like a lens to temporarily focus light from the star. This should cause a sharp increase in brightness and a change in the apparent position of the star.

Do I need a telescope for stargazing?

The best equipment for stargazing remains the least expensive — a pair of wide-open eyes and a mind craving the expansive beauty of the night sky. There is simply no substitute for drinking in the entire sky with a gaze that stretches from horizon to horizon under the sparkling arch of the Milky Way, as humans have done for eons.

Beyond that, the next best piece of equipment for the star hunter is a good pair of binoculars. They are inexpensive, highly portable, and strike a wonderful balance between magnification and light-gathering power; the wide view provided by a pair of binoculars also counters the inevitable confusion of finding a target in a crowded starfield.

Telescopes inevitably enter the minds of stargazers, and there are many fine models available for the casual observer that balance power, portability, and price. Most of the major telescope manufacturers publish free guides to their products, providing the basic information critical to making a wise purchase. Potential buyers should consider the conditions available at their stargazing site, however; even the best telescopes are wasted on sites with light pollution or other poor seeing effects.

More details

How do astronomers measure distances to stars and galaxies?

Astronomers have developed several techniques to indirectly measure the vast distances between Earth and the stars and galaxies. In many cases, these methods are mathematically complex and involve extensive computer modeling.

Parallax is the visual effect produced when, as an observer moves, nearby objects appear to shift position relative to more-distant objects. This common event is easily reproduced; hold your finger out at arm’s length, and look at your fingertip first with one eye closed, then the other. The “motion” of your fingertip as seen against background objects is caused by the change in your viewing position — about three inches from one eye to the other.

As Earth orbits the Sun, astronomers invoke this same principle to determine the distance to nearby stars. Just like your fingertip, stars that are closer to us shift positions relative to more-distant stars, which appear fixed. By carefully measuring the angle through which the stars appear to move over the course of the year, and knowing how far Earth has moved, astronomers are able to use basic high-school geometry to calculate the star’s distance.

Parallax serves as the first “inch” on the yardstick with which astronomers measure distances to objects that are even farther.

For example, they use a class of variable stars known as Cepheids, which pulsate in and out like beating hearts. There is a direct relationship between the length of a Cepheid’s pulsation and its true brightness. Measuring a Cepheid’s apparent brightness — how bright it looks from Earth — allows astronomers to calculate its true brightness, which in turn reveals its distance. For this technique to work correctly, though, astronomers must first use the parallax method to get the distances to some of the closer Cepheids. This allows them to calibrate a Cepheid’s true brightness, which then can be used to calculate its distance. Cepheids are especially bright stars, so they are visible in galaxies that are tens of millions of light-years away.

For more-distant galaxies, astronomers rely on the exploding stars known as supernovae. Like Cepheids, the rate at which a certain class of supernovae brighten and fade reveals their true brightness, which then can be used to calculate their distance. But this technique also requires good calibration using parallax and Cepheids. Without knowing the precise distances to a few supernovae, there is no way to determine their absolute brightness, so the technique would not work.

More on the cosmic distance scale