The instruments have been in the media spotlight in the 24-hour countdown to this afternoon's scheduled launch of the shuttle Atlantis, with engineers and scientists extolling the virtues of the gadgets and aerospace companies passing out gadget-glorifying press packets.

CAPE CANAVERAL, Fla. — The Hubble Space Telescope is not a single instrument. It's an agglomeration of instruments. It's a big, cavernous spacecraft capable of housing an ensemble of devices.

The instruments have been in the media spotlight in the 24-hour countdown to this afternoon's scheduled launch of the shuttle Atlantis, with engineers and scientists extolling the virtues of the gadgets and aerospace companies passing out gadget-glorifying press packets.

Consider, for example, the Wide Field and Planetary Camera 2, the subject of an hour-long briefing Sunday by some of NASA's top scientists. The WFPC2 — "Wiff-pick 2," in NASA vernacular — captured many of the images that made the Hubble famous. It is now going to be swapped out (replaced by Wiff-pick 3) and lugged back to Earth aboard the shuttle. It will then have a valedictory scientific moment, as NASA plans to study the abundance of micrometeorite impacts on a surface of the instrument that has been exposed to space for 15 years.

Ed Weiler, NASA's head of space science, recounted the bust-and-boom history of the Hubble, and said that this one camera was essentially what saved the telescope.

The telescope had — and still has, to this day — a flawed mirror. Although the mirror is perfectly smooth, Weiler said, "It was also perfectly wrong. It had the wrong curve." The announcement of the "spherical aberration" was humiliating for NASA and for a time made Hubble a symbol of incompetence.

But then came the first Hubble repair mission, in 1993, and astronauts swapped the original WFPC with a new version that had its own "flaws" that precisely canceled out the mirror's aberrations. Since then, 50 to 60 percent of all of Hubble's scientific work has been done with the WFPC2, Weiler estimated.

The camera took the iconic Eagle Nebula image known as the "Pillars of Creation." It photographed Comet Shoemaker-Levy breaking up and colliding with Jupiter. It was used for the Hubble Deep Field image that captured the light of thousands of oddly shaped galaxies from the early universe.

The final assignment for the camera has been to photograph a beautiful planetary nebula, Kohoutek 4-55, an exploded red giant star, its galaxy-like structure glowing in red, green and blue, like a gem found in space.

"It's time to retire it. It's time for it to come home," said John Trauger, the principle investigator for the camera. "We're going to celebrate."

Each Hubble instrument has its own purpose, design, personality and supporting cast (scientists, engineers, bureaucrats, aerospace executives) back on Earth.

One new instrument aboard the shuttle and bound for the Hubble is the Cosmic Origins Spectrograph. The question it will address is a grand one, as framed by principle investigator Jim Green of the University of Colorado: "What's the large-scale structure of the universe?"

The spectrograph will be able to discern the way ordinary matter is distorted gravitationally by the presence of tendrils of dark matter that (theorists believe) are present throughout the cosmos and, unseen, bind the galaxies together in a kind of dark-matter foam.

This new instrument, which weighs 850 pounds, will take the place of an instrument called COSTAR. The COSTAR instrument includes a set of mirrors that helped correct Hubble's initially flawed vision, but which are no longer necessary because the corrections are now built directly into each of the instruments aboard the spacecraft.

While the scientific instruments have gotten a lot of attention, so have the customized tools that the astronauts will use to do their unprecedented repair jobs in space.

For example, engineers from NASA contractor ATK have had to figure out how a spacewalking astronaut can remove 111 screws on a panel covering an instrument called the Space Telescope Imaging Spectrograph, which contains a failed power converter that the astronauts want to replace.

The astronaut will use a customized power tool that can be quickly refitted with different drill bits. The challenge is that the screws mustn't get loose and become projectiles. So ATK designed a "fastener capture plate" that can be clamped onto the panel, with each screw falling into a small Plexiglas compartment. The engineers had to study blueprints of the panel to ensure that the capture plate aligns correctly with each of the 111 screws.

"There was never any thought that we'd have to repair this instrument," ATK engineer Matt Ashmore said as he demonstrated the various tools used for the job.

When the screws are removed and the plate comes off, the next challenge is to grab the very sharp electronic card and remove it — without endangering the astronaut. So ATK built a "Card Extraction/Insertion Tool" that keeps the astronaut's gloved hands well away from the sharp edges of the card.