Expanding Horizons
First Light
Gentleman Astronomers
More Glass
Sky High
1968: Joining the Moon Race
1972: The Challenges of Lunar Astronomy
1990: Astronomy On High
1993: Piecing Together a Giant Telescope
1997: Giant Performance, Discount Price

Telescope History

Sky High

Other than the full Moon, nothing in the night sky inspires more poetry than the twinkling of the stars. It inspires astronomers to say a few choice words, too, but few of them are printable. That’s because the same effect that causes the stars to twinkle also makes them look like fuzzy blobs through a telescope. Even worse, the atmosphere absorbs many wavelengths of energy, making it impossible to observe much of what’s happening in the universe.

The solution is to raise a telescope and its instruments as high above the ground as possible. In the early part of the 20th century, that meant building observatories atop tall mountains. Later, it meant lofting them with balloons. And by the 1970s, it meant lofting them into Earth orbit or beyond, providing a crystalline view of planets, stars, galaxies, and the rest of the universe.

Since then, space agencies around the world have launched dozens of telescopes into space, and their results have been spectacular.

Even so, space-based telescopes are expensive to build and operate so only a few are working at any given time. So most astronomical observations are still done from the ground. But like their orbiting cousins, today’s ground-based telescopes are built with Space Age technology.

Advances in mirror making, electronic detectors, and computer control systems have made it possible to build bigger telescopes at a fraction of the price of those built in the early 1900s. These new telescopes see deeper into space than older instruments, provide sharper views of astronomical objects, and parse light into its individual wavelengths more efficiently.

These powerful new instruments are opening new scientific frontiers, allowing astronomers to look into regions of the universe where no one has looked before.


Joining the Moon Race

Smith telescopeMcDonald Observatory joins the Moon Race with an innovative new reflector.


The Challenges of Lunar Astronomy

Apollo 16 astronauts operate the first telescope on the Moon, a camera that looks at ultraviolet wavelengths.


Astronomy on High

Hubble Space Telescope enters orbit.Hubble repair


Piecing Together a Giant Telescope

Keck domesThe 10-meter Keck Telescope, which has a segmented mirror, enters service in Hawaii.


Giant Performance, Discount Price

HET mirrorThe Hobby-Eberly Telescope, built for a fraction of the cost of other giant telescopes, is dedicated at McDonald Observatory.

As darkness begins to fall, the Hobby-Eberly Telescope is prepared for its night’s work. [Martin Harris/McDonald Observatory]

The world's largest optical telescope mirrors were inspired by the world's largest radio telescope.

In 1983, Penn State astronomer Daniel Weedman proposed a giant new optical telescope that would operate like the Arecibo radio telescope in Puerto Rico. The giant Arecibo dish is built in a bowl-shaped depression in the hills, and always stares straight up into the sky. But a tracking device suspended above its reflective surface moves back and forth, allowing the telescope to track astronomical objects as Earth rotates on its axis.

HET mirror

The Hobby-Eberly Telescope primary mirror reflects its support structure and the interior of its dome. [Martin Harris/McDonald Observatory]

Weedman's original proposal called for a mirror made of 10 segments, each 3 meters (10 feet) in diameter. The mirror array would remain fixed so that it always looked straight up, but a tracker would move across the mirror to follow astronomical objects as they passed overhead.

Since the telescope itself wouldn't move, it wouldn't need the same type of support structure as conventional designs, greatly reducing its weight and cost. And its method of tracking astronomical objects would make it well suited for surveys, in which it captures fairly short but high-quality observations of many different targets every night.

Over the following year, Weedman and fellow Penn State astronomer Larry Ramsey conferred with several telescope experts and modified the original design. Their new concept was named the Spectroscopic Survey Telescope. When it was completed a decade later, it was renamed the Hobby-Eberly Telescope (HET).

Under the revised concept, instead of pointing straight up (at the zenith, in astronomical parlance), the mirror would be tilted at an angle of 55 degrees relative to the horizon. It would be mounted in such a way that it could rotate 360 degrees, allowing it to see about 70 percent of the sky. The assembly at the top of the telescope would move across the mirror, allowing HET to track individual targets for about an hour. This design would provide the performance of a giant telescope at a fraction of the price of older designs.

Penn State teamed with The University of Texas at Austin to build the telescope atop Mount Fowlkes at McDonald Observatory. Development began in the late 1980s, and the telescope was dedicated in 1997, after three other partners joined the project team.

HET's main mirror consists of 91 individual hexagonal segments, each of which is 1 meter (39.5 inches) across. Three small computer-controlled devices attached to each segment constantly adjust their position to maintain the correct shape for the overall mirror, which is 11 meters (36 feet) in diameter — larger than any other telescope mirror in the world. Because of the way the telescope tracks astronomical objects, though, only 9.2 meters of the mirror area is used for each observation. That makes HET and a near twin based on the HET design, the South African Large Telescope, the third-largest telescopes in the world.

The mirror forms a spherical shape, like a slice of a ball. That design allowed all 91 mirror segments to have the same shape, which greatly reduced construction costs. But a spherical mirror smears the light from its targets. Corrective lenses in the tracker assembly compensate for this effect and bring the view into sharp focus.

The tracker uses fiber optics to feed light to spectrographs, which can be attached to the telescope or located in a climate-controlled room beneath it. These instruments split the light into its component wavelengths or colors, yielding details on a target's composition, motion, and more.

HET as it appeared shortly after it entered service. Later, it was coated with a chrome-colored tape to reflect more sunlight, and louvers were inserted around the base to improve airflow. These improvements allowed the telescope to more quickly adjust to nighttime temperatures, improving its performance. [McDonald Observatory]

More information

Hobby-Eberly Telescope