Just four years ago, the Hubble Space Telescope was a symbol of Big Science run amok. Members of Congress, stand-up comedians and movies like "Naked Gun 2 1/2" ridiculed the complex orbiting instrument as a $2 billion "technoturkey" with all the prowess of Mr. Magoo.

But the Hubble's public image, as well as its images, has undergone a phenomenal redemption. Since astronauts compensated for a devastating flaw in the telescope's main mirror, Hubble has become the most familiar symbol of astronomy in the same way that the late Carl Sagan became astronomy's most famous voice.

The Hubble has literally helped to rewrite the textbooks on planets, stars, galaxies and the evolution of the universe. "Observing with the Hubble is like eating every day at Sardi's," said astronomer Sandra M. Faber of the University of California at Santa Cruz. "It's an absolutely unending feast."

And so it might seem strange that NASA is about to risk another reversal of fortune to give its astronomical flagship even more observing muscle. In the pre-dawn hours Tuesday, a crew of veteran astronauts is set to blast into orbit aboard the shuttle Discovery, chase and grapple the 12.5-ton observatory and replace some of its 1970s technology with state-of-the-art devices.

The stakes are, as they say, astronomical. Edward J. Weiler, who has been Hubble's chief scientist throughout its roller-coaster history, confesses to some anxiety. "If it ain't broke, don't fix it. Some people think we may be violating that cardinal rule, and in a way we are."

But when the Hubble was built by NASA, with the participation of the European Space Agency, it was designed to be upgraded regularly. The improvements are necessary to enable the device to stay on the forefront of science and accomplish its most lofty goals through the year 2005.

The telescope, already the most complex and expensive scientific instrument of all time, enables researchers to examine the cosmic zoo in visible light with more clarity than ever before, including some objects whose light began the journey toward Earth billions of years ago at a time when the stars had barely begun to form. Hubble provides astronomers with an unprecedented vantage point above Earth's atmosphere, which either bends or blocks all incoming light. The telescope can detect objects several billion times as faint as the human eye can see, and at wavelengths -- or colors of light -- that don't reach the ground. The Hubble sees so sharply, said Faber, that "it's like moving the universe 10 or 20 times closer to our doorstep."

The Hubble's visual acuity has created such demand that only one of every 10 proposals to use the telescope can be accepted. Since its launch in early 1990, some 2,000 lead scientists (plus their teams) in more than two dozen countries have used the Hubble to make more than 110,000 observations. Seven years into the Hubble's planned 15-year operating life, scientists have produced 1,346 papers based on its observations.

"The overall impact of Hubble has been tremendous. It has lived up to its promise," said G. Wayne Van Citters of the National Science Foundation (NSF). At astronomy meetings, the latest Hubble observations "are almost always the center of focus," said Rodger Thompson of the University of Arizona.

Independent of their scientific content, many Hubble images are simply dazzling, revealing the cosmos as unexpectedly bizarre and in some cases strangely organic looking. Through its imagery, the Hubble has penetrated the public consciousness as a purveyor of cosmic wonder and a metaphor for keen vision. Hubble photographs show up on magazine covers, refrigerator doors, T-shirts and school bulletin boards. The Hubble's output is the subject of several books.

The Space Telescope Science Institute (STScI) in Baltimore, the research center for the Hubble, has logged a stream of queries from students, children, lawyers, homemakers, movie producers and even prisoners. A typical one came from a nurse in Los Angeles, who said a Hubble image that she happened to see in a magazine had "changed her life, showed her how trivial her everyday problems are," said Ray Villard, a spokesman for STScI.

Some callers complain about a dearth of images. "They think we're concealing information," he said. "Some believe there's a Hubble control room at the Vatican, with armed guards. Some believe we can look down at Earth, like a spy satellite. They thought we might solve the O.J. mystery."

Then there are those who think they can hook into Hubble as if it were satellite TV. "One guy wanted to buy time on Hubble for pay per view,' " Villard said. "Charge people to look at Mars."

It's all a stark contrast with the bleak days before the telescope was repaired, when it was depicted in "Naked Gun 2 1/2" on a wall of shame, alongside the Titanic and the Hindenburg, as one of history's great disasters. "If there was anything that was a shock to the system at NASA headquarters, it was that scene," Villard said.

More recently, Hubble has been boffo in Hollywood, with respectful references in TV shows and films such as "Mars Attacks." The weekly tabloids portray Hubble as an instrument not only of science but of miracles: "Heaven Photographed by Hubble Telescope," blared a 1996 headline in the Weekly World News, which ran a photo of a shining city floating among the stars. "There was a guy in Louisiana who broke my heart," Villard said. "He called up and wanted a copy of the picture. I had to tell him it wasn't real."

No Hubble image has resonated with the public quite as spectacularly as its widely published "pillars of creation" in the Eagle Nebula, released in late 1995. The images reveal the glint of embryonic stars forming on the surface of monstrous columns of interstellar gas, all bathed in a ghostly light. The eerie shapes remind some of sea monsters rising from the deep, or of stalactites in a vast cave. CNN, after televising the images, reported callers who claimed they saw ghosts, heaven or the face of Jesus in those shapes.

The towering columns of infant stars in their swaddling represent, in a sense, "the placenta of the cosmos," said astronomer Richard Berendzen of American University. "Whoever said we'd be permitted to see such things. . . . It's an exquisite gift. It evokes deep emotions."

NASA administrator Daniel S. Goldin has called for a steep reduction in the cost of future space observatories, saying Hubble-class investments are no longer acceptable. But for the present, various scientific panels have endorsed the orbital telescope as a vital astrophysical asset -- even considering its cost.

Hubble's fame has caused periodic strain in the science community. NASA news briefings on Hubble discoveries have generated private complaints among some astronomers that the events sometimes foster "Hubble hype," exaggerating the telescope's importance in astronomical research.

Douglas Duncan of the University of Chicago and Adler Planetarium said that while Hubble clearly has produced important findings, some ground-based astronomers "feel slighted. . . . The big ground telescopes don't have the public relations operation that Hubble has." Many visitors to Adler believe that anything important that happens in astronomy, such as recent discoveries of planets around stars beyond the sun, came from Hubble even when it didn't, he said.

Even some of those associated with the project acknowledge that, as the project struggled to get launched, there were exaggerated claims. Later, they say, the extent of the Hubble's troubles also was overblown.

But supporters of the NASA approach say that, since the telescope's 1993 repair, scientific discipline has reigned. They argue that the periodic Hubble briefings set a praiseworthy example of how to explain clearly to taxpayers what their money has bought and why the research matters -- a practice they say more scientists should copy.

The NSF's Van Citters said the problem is not so much hype as an incomplete picture of how the Hubble's advent has dissolved the barriers between ground- and space-based astrophysics, putting them in harness together. "People use the Hubble data to make important discoveries," he said, then they turn to big telescopes on the ground to do follow-up work that Hubble can't do. And that follow-up work, by its nature, often does not generate dramatic images that attract media attention.

"It's yin and yang," said Faber. She is on one of several teams that are using the ground-based Keck in Hawaii -- the world's most powerful telescope -- to study a rich soup of previously unsuspected galaxies that were first detected by the Hubble last year in a tiny patch of seemingly empty sky.

Sidney Wolff, head of the National Optical Astronomy Observatories (with telescopes in Arizona, New Mexico and Chile), said the combination of the Hubble with today's advanced ground-based instruments really is "revolutionizing astronomy. I'm not sure it's the greatest revolution since Galileo -- but it's close." And, she added, "I'm the last one to engage in hype."

John Bahcall of the Institute for Advanced Study at Princeton, a leading Hubble advocate since 1971, recalls the early days when "the few of us {interested in space-based astronomy} were regarded as a cult. . . . Now I can't imagine any astronomer approaching any {research} problem without involving space observations." Bahcall compared the telescope to one of his human children. "There was that terrible period of adolescence when it looks like you'll never survive it. Now it's fair to say that, like every proud parent, I'm astonished at how well it has performed. . . . The fellow who puts gas in my car always asks me, what's the latest thing from the Hubble.' " CAPTION: The 12.5-ton Hubble Space Telescope can detect objects several billion times as faint as the human eye can see. CAPTION: Space Telescope's Images Bring Universe Into Focus

Since 1990, the Hubble Space Telescope has circled the Earth 5,000 times a year. Here's some of what the telescope has accomplished:

Improved knowledge of the size and age of the universe.

Provided decisive evidence of the existence of super-massive black holes at the centers of galaxies and shown that they are probably common throughout the universe, dwelling at the cores of most, if not all, galaxies.

Made the deepest-ever look into the universe (in visible wavelengths), revealing thousands of galaxies and enabling astronomers to trace aspects of cosmic evolution. Detected "protogalaxies" close to the time the universe formed.

Revealed clearly the galactic environments in which quasars (the most luminous objects known, believed to be powered by black holes) reside.

Elucidated more clearly the various processes by which stars form.

Traced the buildup of heavier elements via star formation in the early universe. Such elements are necessary for planets and life to exist.

Revealed the complexities of gas shells around dying stars.

Detected many disks of dust around young stars in the Orion Nebula, suggesting that planetary systems may form commonly in Earth's home galaxy, the Milky Way.

Provided an unprecedented, detailed look at a once-in-a-millennium collision between Jupiter and a train of comet fragments.

Studied the "weather" on major planets, including dust storms and changing seasons on Mars.

Discovered oxygen atmospheres on Jupiter's moons Europa and Ganymede.

-- Kathy Sawyer CAPTION: A Hubble image shows multiple comet impact sites on Jupiter. Hubble has been used to make more than 110,000 observations.