ASHINGTON, Sept. 16 — Even in astronomy, there are some tough acts to follow. After parting a curtain to the universe with the Hubble Space Telescope and allowing millions to experience previously hidden wonders in space, what do you do for an encore?

The National Aeronautics and Space Administration last week revealed its choice for the next stage in expanding human vision into deep space. The agency announced that it would build the long-discussed Next Generation Space Telescope, selecting the design of a team led by TRW Inc., for a successor to Hubble to be launched in 2010.

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The new observatory, while only half as big as the 24,000-pound Hubble, will have a primary, light-gathering mirror 20 feet in diameter compared with the existing telescope's 8-foot reflector. With a mirror that has a light gathering area six times as large as the Hubble's and a suite of more sensitive instruments, the new telescope should be able to detect objects a hundredth the brightness Hubble can see in visible light and one four-hundredth the brightness in the infrared part of the light spectrum.

Following recommendations from astronomers who suggested a bold new concept, NASA said it would build an observatory that would look back into time and space for some of the first light produced in the universe. Unlike Hubble, this telescope will be sent into orbit far from Earth and should be able to detect and analyze the faint, warm light produced when the first stars and galaxies formed a few hundred million years after the Big Bang, the theoretical beginning of the universe some 14 billion years ago, scientists say.

The observatory will also be used to study the formation of planets and to search for the hidden dark matter that is thought to make up most of the matter in the universe.

The new telescope will not be just a larger version of Hubble, a traditional telescope that mostly views objects in the visible light part of the electromagnetic spectrum that is seen by the human eye. The observatory will be optimized to see in the infrared, best for detecting faint light moving rapidly away from the observer that has shifted into the red, thermal part of the spectrum.

Dr. Alan Dressler, an astronomer with the Observatories of the Carnegie Institution in Pasadena, Calif., who took part in studies of what should follow Hubble, said scientists wanted more than just a bigger space telescope.

"The Hubble Space Telescope raised the ante," Dr. Dressler said. "The desire was to make a huge leap, to go for something bold that would really be a breakthrough. So the goal became to see the first light of stars and the emerging of galaxies. This is the birth of the modern universe we live in today."

Although the new telescope is optimized for infrared viewing, astronomers and NASA agreed that the observatory had to have the ability to produce visible light pictures at least as good as Hubble's, to assure public support. "NASA has tried for years to recapture the public imagination about space and, to everyone's surprise, the public got caught up in the images from Hubble," Dr. Dressler said. "We were conscious of this public perception in making our recommendations."

Dr. Marcia Rieke of the University of Arizona, the principal investigator for the observatory's primary instrument, a near-infrared camera, said pictures from the new telescope taken in the visible light range should be much better than Hubble's. "The telescope will be best in the infrared area, but it can extend into visible light and do just fine," she said.

Heartened by the success of Hubble, NASA readily embraced recommendations from the Association of Universities for Research in Astronomy and the National Academy of Sciences for the first-light telescope.

However, in a time of tightening budgets, the agency's challenge is to build and operate the new telescope for substantially less cost than Hubble. With its periodic hardware upgrades, the Hubble itself has cost more than $2 billion, not to mention operating costs of at least that much when the bill for space shuttle flights to the telescope for maintenance is included. Since its launching in 1990, NASA has sent four shuttle service missions to the observatory and a final upgrade and repair flight is scheduled for 2004.

TRW and its partners, including Ball Aerospace and Eastman Kodak, are to build, test and operate the new observatory for a year under an $824.8 million contract that does not include launching costs. NASA officials said the ultimate cost of the observatory, planned to last at least 5 years and perhaps 10, could be about $1.2 billion.

One way in which the new telescope will be cheaper to operate than Hubble is that it is not designed to be repaired or serviced once launched. Because of this, engineers are emphasizing high reliability and backup capabilities for all critical systems, said John C. Mather, the project scientist at the Goddard Space Flight Center in Greenbelt, Md.

To isolate the telescope from infrared interference from the Earth and the Sun, the observatory will be launched on an expendable rocket on a three-month journey that will take it to an area 940,000 miles from Earth called L2 for Lagrange Point 2. At a spot like this, the gravity of the Earth and the Sun balance each other out and a spacecraft can keep a stable position with just a few rocket adjustments. The L2 spot is located on the side of the Earth in the direction away from the Sun, with the planet always positioned between the Lagrange point and the Sun.

The spacecraft, whose three main instruments consist of multiwavelength cameras and spectroscopic devices that break down light to be analyzed, also will have a multilayered sunshade the size of a tennis court to insulate it from light and heat produced by the Sun and Earth that could interfere with its observations. The shade and the remote location allow the observatory to cool to below minus 378 degrees Fahrenheit, allowing other cooling systems aboard to chill detectors even more for the best infrared readings.

For the 20-foot diameter mirror to fit into a rocket nose cone, it will be built as 36 hexagonal segments that are folded over into three panels at launch and unfurled into its circular shape once in space. These flexible panels will have multiple, computer-controlled actuators on their backsides that can alter the shape of the mirrors to make sure reflected light is perfectly focused on a collector.

To cut costs and reduce risk of technical problems, NASA has been supporting research into lightweight mirrors, star detectors and other relevant technologies. The telescope initially was planned to have a 26-foot-diameter mirror, but the size was reduced to 20 feet to cut costs by reducing complexity and fabrication time.

Mr. Mather said delays in developing the mirror technology increased cost estimates and pushed launch time from 2008 to 2010. "The mirror is the hardest problem," he said, "We saw that at the beginning so it's no surprise."

The Hubble's biggest problem was also the mirror: it was found to be defective after launching and later had to be corrected by installation of special lenses. NASA does not want a repeat of that embarrassment.

In a year, the agency must make the critical decision of what material to use for the new telescope's mirror. The candidates are a metal mirror made of beryllium or one made of some form of glass.

Mr. Mather said NASA would take its time selecting the best material and would thoroughly test the entire observatory as a unit before launching it.

"Some things you just can't rush," he said.