Working under floodlights and their helmet headlamps, Endeavour astronauts floating in the night over Mexico early today replaced the Hubble Space Telescope's busiest astronomical camera with a new model that should correct a blur caused by the observatory's misshapen mirror.
"It looks like it's in there. Very nice," said astronaut Jeffrey A. Hoffman after he and his spacewalking partner, Story Musgrave, slid the new Wide-Field/Planetary Camera into the telescope as if they were placing it in a giant drawer.
The camera performs half of Hubble's observations and functions as its "wide-angle" and "telephoto" lens. If it works as advertised, it should give astronomers broad views of the heavens nearby, as well as finely detailed looks at the universe's faintest and most distant objects.
Those most difficult observations have been beyond the reach of the old Wide-Field/Planetary Camera because of Hubble's blurry vision.
Unlike the astronauts' prior two spacewalks, which repaired Hubble's most pressing mechanical problems, this morning's work and another tonight and early tomorrow are designed to correct the observatory's blurred vision.
Late tonight and early tomorrow, astronauts Kathryn Thornton and Tom Akers are scheduled to use the fourth of five spacewalks on this mission to install COSTAR, a package of robot arms and mirrors that give three other astronomical instruments on the telescope a cure for the mirror flaw.
The design of COSTAR -- which stands for Corrective Optics Space Telescope Axial Replacement -- was the brainchild of Dr. Jim Crocker of the Space Telescope Science Institute in Baltimore and a Colorado optical scientist.
The mission's first two spacewalks Sunday and early yesterday went off with hardly a hitch, to the delight of NASA officials, who badly need an unqualified success.
"We've been up to bat twice and the crew has hit two home runs," said Joe Rothenberg, Hubble project director at the Goddard Space Flight Center in Greenbelt, referring to the replacement of the telescope's gyroscopes Sunday and electricity-generating solar panels yesterday.
Even the shuttle has been trouble-free on its fifth space flight. "Endeavour has been a great repair ship. It hasn't caused us one hiccup at all to worry about," said the lead flight director, Milton Heflin.
NASA was embarrassed in 1990 by the discovery that Hubble's primary mirror had been ground too flat by a measurement equal to one-fiftieth the width of a human hair. The mistake didn't cripple the telescope, but it has prevented astronomers from seeing the faintest and most distant objects -- observations needed to answer some of science's most fundamental questions.
Officials and engineers hope the human repairs on an orbiting spacecraft will help them sell Congress and the American public on the viability of a permanently manned space station.
Tonight's assignment for the astronauts is to install the 640-pound COSTAR package.
COSTAR's job is to insert small, precision-ground mirrors into the light stream from Hubble's flawed primary mirror. The mirrors are shaped to correct the big mirror's focusing error and positioned to relay the light in three separate beams to the telescope's Faint Object Camera, Faint Object Spectrograph and Goddard High Resolution Spectrograph.
Getting COSTAR inside the telescope will be a little like sliding a big refrigerator into a small closet, with Ms. Thornton maneuvering the instrument from her perch on the end of the shuttle's robot arm.
To make room for COSTAR in the telescope's crowded lower level, scientists and engineers agreed that one of the four scientific instruments housed there had to be sacrificed.
The loser was the High Speed Photometer (HSP), designed to measure rapid changes in the intensity and color of starlight.
After removing and stowing the 487-pound High-Speed Photometer for its return to Earth, Ms. Thornton will attach a portable handhold to COSTAR and draw it from its housing in the shuttle's payload bay.
Swiss astronaut Claude Nicollier, at the arm's controls, will then maneuver Ms. Thornton and her massive parcel to the side of the telescope. With Colonel Akers, an Air Force officer, helping, the three will align COSTAR with a set of guide rails and slowly slide the device into the telescope. Once latched in place, COSTAR will be connected to the HSP's cables, and the service bay will be closed.
COSTAR was conceived by the late Dr. Mark Buttema, an optical scientist with Ball Aerospace and Communications in Boulder, Colo., and Dr. Crocker, an electrical engineer at the Space Institute. It was then built by Ball.
Its designers were challenged to find the most efficient, reliable way to correct Hubble's flawed optics for three separate instruments, and to get it designed and built in just 28 months. They hit upon the idea of deploying a collection of 10 mirrors into the telescope's light stream from a movable optical bench.
The bench emerges like a periscope from COSTAR's "black box" after installation, slipping through a triangular hole no bigger than a wedge of pie.
Its mirrors are mounted on the end of four mechanical arms that flip out from the bench's "mast." They move into the light stream from Hubble's primary mirror, canceling out its optical error, and relaying the sharpened image to the appropriate instruments. Each arm is minutely adjustable by remote control from the ground for fine-focusing.