© 1994, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112.
by Sally Stephens, Astronomical Society of the Pacific
The small room at the Space Telescope Science Institute in Baltimore was packed, even though it was the middle of the night. Astronomers and technicians strained to get a good view of the monitor that would soon show them the first picture taken with the newly "fixed" Hubble Space Telescope (HST). At 1:00 a.m., on Dec. 18, 1993, the image was radioed from the telescope to the ground. Tension gave way to cheers and exuberant shouts as the image of a star appeared on the monitor, a star without any of the smeared light astronomers had come to expect from the telescope's flawed main mirror. According to Edward Weiler, Hubble Space Telescope Program Scientist, the HST had not only been fixed, but "fixed beyond our wildest expectations."
The Repair Mission
"The Trouble with Hubble is Over"
Activity: "Name That Angle"
But the telescope's 2.4-meter (94-inch) main mirror had been ground to the wrong specifications, and was too flat near the edge by about 1/50th the width of a human hair. This miniscule error meant that only a small fraction, about 15 percent, of the light gathered by the HST was properly focused to a sharp point. The remaining 85 percent was spread out into a large, fuzzy halo and was essentially unusable. This problem, well known to astronomers, is called spherical aberration.
Astronomers quickly devised computer programs to remove this wasted light from images taken by the telescope, and the Hubble produced spectacular pictures of dust rings in the centers of galaxies that may be hiding massive black holes from view, peered into the heart of clusters of stars, and followed a storm on Saturn. But many projects, especially those involving faint objects, could not be done, and astronomers were haunted by the idea of discoveries that might have been made had the mirror not been flawed.
For example, astronomers had always planned to replace the HST's Wide Field and Planetary Camera (abbreviated WF/PC, and usually called "Wiff-Pick"), designed to look at relatively bright objects with a wide field of view, during the first servicing mission. The new instrument would have "faster" electronic detectors, which, because they capture the same amount of light in half the time, can image fainter objects. After the HST mirror's aberration was discovered, scientists realized they could polish the mirrors in the new camera to a different "prescription" than originally planned to properly refocus the light. Replacing the old WF/PC would correct the HST mirror's problem, but only for that one instrument.
Unfortunately, replacements were not readily available for the other Hubble instruments, since they had not been scheduled for early upgrades. So scientists and engineers devised COSTAR (Corrective Optics Space Telescope Axial Replacement), with three small mirrors, about the size of dimes and quarters, each polished to compensate for the main mirror's flaw. After installation, a set of mechanical arms, no longer than a human hand, would put one of the mirrors in front of the opening that admits light into each of the other three scientific instruments, properly refocusing the light entering each one. In a sense, COSTAR put "eyeglasses" in front of each instrument to correct the telescope's vision, although the eyeglasses were mirrors, not lenses.
To make space for COSTAR, the shuttle astronauts had to remove one of the original Hubble instruments, the phonebooth-sized High Speed Photometer, which measured the brightness of celestial objects. This instrument was chosen to be sacrificed because it did proportionally less science than any of the other four instruments.
Working in pairs on alternating days, four spacewalking shuttle astronauts worked on the Hubble Space Telescope. Jeffrey Hoffman and Story Musgrave installed the new WF/PC and replaced two pairs of faulty gyroscopes. The HST needs three gyros to point at and "track" stars during observations. It originally carried six gyroscopes. But the failure of three gyros since launch left the telescope with no back-ups. The repair gave the Hubble a full complement of healthy gyros.
Astronauts Kathryn Thornton and Tom Akers gently installed COSTAR, and replaced the HST's solar power panels. Shortly after launch, scientists discovered that the Hubble's solar panels expanded and contracted more than expected every time the telescope passed from the warmth of day into the cold of night and vice versa. This happened twice during every 96-minute orbit, and caused a "jitter" in the telescope that interfered with its ability to point at stars correctly for a short time after each temperature change. As a temporary fix, engineers created special computer programs that compensated for almost all of the jitter, allowing the telescope to function normally.
Still, scientists worried that the jitter would weaken the solar panels, causing them to break before the telescope's mission was done. Without enough power, the telescope could not operate. This concern prompted the astronauts to replace the solar panels with new ones designed to reduce the amount of jitter caused by the extreme temperature changes in Earth orbit.
The five spacewalks went so smoothly that the astronauts were frequently an hour ahead of the timeline established beforehand for each repair. Moving with extreme care and precision, the astronauts accomplished all of the tasks asked of them. On Dec. 13, 12 days after launch, the shuttle returned to Earth, its mission completed. At the time, one scientist said, "We have just completed eye surgery on the Hubble Space Telescope. It will be a matter of six to eight weeks before we can remove the bandages, figuratively speaking, from the patient and determine whether the operation was a success or not."
First astronomers had to calibrate the new gyroscopes, allow the WF/PC's new electronic detectors to cool to operating temperatures and align the Hubble's secondary mirror to make sure light properly entered the improved WF/PC. Then they would focus on aligning COSTAR's mirrors.
To their delight, astronomers found that the astronauts had been extraordinarily gentle to the telescope and its instruments; nothing had been bumped out of place. The corrective optics, which had been tested, retested, and tested again on the ground behaved exactly as expected. The telescope worked, in the words of one scientist, "like a dream." Astronomers and engineers worked feverishly for very long hours over the holiday season, aligning mirrors and analyzing information sent down by the telescope. About a month after the servicing mission, much sooner than expected, astronomers knew that the repairs had succeeded.
As astronomers analyzed data sent from Hubble throughout December, they were surprised at how good the images really were. There is a limit to how sharp light in a telescope can be made to focus. This so-called diffraction limit is based on the fact that light waves will inevitably spread out when they encounter an object in their path, like a mirror. This limit is different for every telescope and depends solely on the width of the main mirror and the wavelength of light under consideration. The resolution of most telescopes (the degree to which fine detail can be observed, or resolved) is normally much higher than the diffraction limit, due to the blurring of light by the Earth's atmosphere and imperfections in the optics.
The Hubble's spherical aberration spread a star's light out into a fuzzy halo four arc seconds across. [One arc second is 1/3600 of a degree, and corresponds to how far apart the headlights on a car would look if the car was 480 kilometers (300 miles) away.] Now, with the corrective optics in place, 60-70 percent of the light is concentrated within 1/10 of an arc second, very near the sharpest focus possible for a 2.4-meter telescope, essentially at the telescope's diffraction limit.
According to James Crocker, COSTAR team leader, the new Hubble images are "as perfect as engineers can achieve and as physical laws will allow." In fact, the corrected Hubble's vision is so precise that, if the telescope were in Washington, D.C., with an hour-and-a-half-long observation (the length of one HST orbit), it could detect the light from a firefly in Tokyo, half a world away. "And," Crocker continued, "if they were ten feet apart, we could see there were two fireflies."
For the first time, astronomers could follow the galaxy's spiral arms into the innermost regions of the galaxy. And, because of the clearer image, they were able to see extremely faint stars in M100's outer regions that will allow them to accurately determine its distance. The faint glow of these stars had always been swamped by light from nearby brighter stars blurred by the Earth's atmosphere. But with the repaired Hubble's sharp focus, the brighter stars looked like true points of light, allowing astronomers to see their fainter neighbors for the first time.
Another Hubble test image revealed thousands of stars in a cluster of young, hot stars, where before astronomers had only been able to detect the hundred or so brightest stars. And these were just "raw" images, with no computer enhancements of any kind. The telescope's new-found ability to see faint objects and structure within a few tenths of an arc second of bright stars delighted scientists. The telescope is now every bit as good as it was supposed to be when it was launched. In fact, in some ways it's even better; the improved WF/PC detectors make that instrument more sensitive than the original.
During the coming year, 1,200 to 1,500 astronomers will use the refurbished telescope to learn more about the universe in which we live. They will look for evidence of massive black holes in the centers of galaxies, continue the search for planets around other stars, and begin to calibrate the distance scale to far-away galaxies. Given the extraordinary success of the repair mission, astronomers have high hopes that the Hubble will finally deliver on its promise to revolutionize our understanding of astronomy.
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