(The following information is from "NASA Fact Sheet," "Hubble Space Telescope's First 125 Days" published by the NASA Marshall Space Flight Center, Huntsville, Alabama, September 1990.)
Hubble Space Telescope's First 125 Days
NASA's Hubble Space Telescope, although still undergoing checkout in orbit, is already furnishing scientists with exciting new information about the cosmos‹and offering the world a preview of great discoveries to come. Just months after launch, it provided a remarkably clear look at a distant star cluster once believed to be a single giant star. Displaying a capability for resolution far exceeding that of Earth-based optical telescopes, the Hubble Space Telescope provided an image of the star cluster R-136, some 160,000 light years from Earth. The image - so clear that more than 60 stars could be seen easily - is what one scientist has called "the finest family portrait of stars outside our galaxy." This image, and the significant new astronomical data it reveals, clearly shows that the Hubble Space Telescope, despite a spherical aberration caused by its primary mirror, is an invaluable scientific instrument that's giving astronomers exactly what they wanted: a much better look at the universe than they ever had before.
Hubble Space Telescope is a complex, supersophisticated, one-of-a-kind device that pushes current technology to its limits. Even with its one imperfection the Hubble observatory is an extremely powerful telescope with unique capabilities that will surely result in many significant scientific discoveries. When NASA corrects for the telescope's spherical aberration early in its 15year operational life, Hubble will perform exactly as it was designed to, allowing scientists to strip away even more of the veil of darkness that shrouds the universe in mystery.
Deployment and Activation
At 3:38 p.m. EDT on April 25, 1990, the crew of Space Shuttle Discovery freed Hubble into its orbit around Earth, sending it on a 15-year search for other worlds, other galaxies and the very origins of the universe itself.
Shortly after astronaut mission specialist Steven Hawley gently grasped the space telescope with Discovery's 50-foot mechanical arm to lift the 12-1/2-ton observatory out of the cargo bay, Hubble began operating on its own nickel-hydrogen batteries. The first task in deployment was to extend the telescope's solar array panels which would keep the batteries charged. The first array rolled out without a hitch, but the second panel proved to be more troublesome, initially refusing to unfurl. Another try, however, proved successful. With both solar arrays fully extended and feeding power to the batteries, and with the telescope's dish antennae fully deployed, Hawley released the Shuttle's grip on Hubble, setting the telescope into orbit.
The activation and fine-tuning of the telescope's on-board instruments, called "orbital verification," proceeded well, with only a few minor problems arising, including a delay in activating the telescope's high-gain antennae that transmit vast amounts of data and photographs. The only problem that controllers could not solve was a very slight vibration, a "microjitter," as the telescope moved from sunlight into darkness and darkness into sunlight. Since determining the jitter's duration and frequency, engineers are developing software to create an opposite effect in the guidance and pointing system to nullify the vibration.
First Light: The First Pictures
By May 21, controllers had entered another phase of orbital verification when they began receiving the first series of photographs from the telescope. What was supposed to be a simple engineering test turned out to be a pleasant surprise for scientists, imaging a double star in the Carina system. The photo sequence provided bright, crisp images against the black background of space, much clearer than pictures of the same target taken by ground-based telescopes. Although the telescope's focusing had yet to be fine-tuned, the photo sequence thrilled observers with the promise of more discoveries to come.
Controllers then began moving the telescope's mirrors to better focus images. Although the focus sharpened slightly during the six times the mirrors were moved, the best image achieved was a pinpoint of light encircled by a hazy ring or "halo." Controllers concluded that the telescope had a "spherical aberration," a mirror defect, only 1/25th the width of a human hair, that prevents Hubble from focusing all light to a single point.
At first, some scientists believed the spherical aberration would cripple the 43-foot long telescope, but they were proved wrong. Engineers began running a battery of tests to determine which mirror ‹primary or secondary‹was causing the spherical aberration. Pictures taken with the on-board Faint Object Camera suggested that the focusing problem rested with the primary mirror, the large reflector that first captures starlight. By late August, an investigation into the cause of the problem determined that a "null corrector," an optical device that was used as a precise guide in grinding and polishing the nearly 8-foot-in-diameter mirror, contained a spacing discrepancy. The discrepancy caused the mirror to be ground too flat by two microns, an extremely small error in a mirror so large, but an error that resulted in the wrong prescription for the optics, preventing the Hubble from achieving the expected focus. The good news is that the error caused a "pure" spherical aberration, a problem relatively easy to correct much like the way an eye doctor corrects poor vision with spectacles.
The spherical aberration's greatest impact is on the telescope's two visible light cameras, the Faint Object Camera and the Wide Field/Planetary Camera. The Faint Object Camera is designed to photograph stars farther away than any photographed before; the Wide Field/Planetary Camera will give astronomers a clearer view of galaxies and young stars as well as photograph entire hemispheres of planets within our own solar system.
The mirror's ability to focus all the light gathered to fine pinpoints is critical to proper function of the two cameras. Even so, a great deal of science can be accomplished even before the spherical aberration is corrected. In fact, the Wide Field/Planetary Camera is now providing images of the universe that no ground-based telescope can achieve. As the saying goes, a picture is worth a thousand words. And that picture came during the aftemoon of August 7, still early in the telescope's verification period, when the Wide Field/Planetary Camera photographed the R-136 star cluster in the star cloud known as 30 Doradus. The photograph was taken during an engineering test, but what scientists saw laid to rest much of their concern over the observatory's ability to perform.
As recently as 1980, astronomers believed that R- 136 was a single star, but new techniques in the early 1980s gave observers a view of eight distinct objects where they had thought only one existed. Recent ground-based images suggested there might be upwards of 27 stars in the cluster. But when scientists saw Hubble's image, they counted within the cluster more than 60 of the youngest and heaviest stars ever viewed. The dense cluster is located within the Large Magellanic Cloud, a neighboring galaxy, and is about 160,000 light years from Earth. A light year is the distance light travels in a year's time through a vacuum, roughly 5.878 trillion miles.
The photograph proved that with the aid of computer restoration, the Wide Field/Planetary Camera has significant capability right now. Computer restoration eliminates some of the blurred light around the center of the star image, effectively sharpening the focus. Computer restoration would have been used in any event, even on the sharpest of photographs provided by perfect mirrors. But the technique has proved a more useful tool to reduce the effect of the spherical aberration. Despite the telescope's problem in obtaining a sharp focus, Hubble's imaging of bright targets is still 10 times better than the best images produced through ground-based telescopes on a clear night. The reason? Simply put, Earth's turbulent atmosphere,which deflects and distorts the light from stars and other distant objects, greatly limits a ground-based telescope's ability to see into the cosmos. This means that even before repairs, Hubble offers great opportunities to scientists.
Further Scientific Capabilities
Although the spherical aberration has most affected the Wide Field/Planetary Camera and the Faint Object Camera, its impact on the telescope's ability for spectroscopy, the observation and study of starlight, has been minimal because the spectrographic instruments - the Faint Object Spectrograph and the High Resolution Spectrograph - do not require finely focused light to perform many of their designed tasks. Light from celestial objects is a mixture of wavelengths that indicate the presence of various chemical elements. Atoms of elements emit or absorb light in unique ways according to physical conditions such as temperature and pressure. By studying the light, scientists can learn whether an object is hot or cold, its density, and its chemical composition. The Faint Object Spectrograph analyzes the light from individual stars and nebulae, enabling scientists to determine a target's physical and chemical properties. The High Resolution Spectrograph will examine ultraviolet light emitted by celestial objects, enabling scientists to observe interstellar space that can't be detected any other way. Ultraviolet light cannot be studied through earthbound instruments because Earth's atmosphere obstructs the light. The spherical aberration's main effect on spectroscopy will be in "crowded field" observations in which the light images from so many stars overlap. Scientists can still perform about 50 percent of all planned spectrographic observations without any repairs.
The High Speed Photometer is another instrument that can perform about half of its scheduled observations even with the spherical aberration. The High Speed Photometer, which operates somewhat like the light meter of a 35mm camera, measures the intensity and wavelength of light from a particular region of space. The instrument will resolve many mysteries concerning white dwarfs, neutron stars, binary stars with gas streams, and pulsars.
The telescope's Fine Guidance Sensors are virtually unaffected by the spherical aberration. Two of the sensors lock onto target stars to maintain the telescope's pointing stability. A third sensor will be used for astrometry, the precise measuring of the distance between target stars and other stars.
The Plan to Fix Hubble
Hubble's on-board instruments are modular, designed for quick and simple replacement, much like changing tapes in a video cassette recorder. Since replacing Hubble's mirror while in space is out of the question, and bringing the telescope back to Earth for repair is not practical‹reentry through Earth's atmosphere would subject the telescope's sensitive optics to contamination‹scientists plan to compensate for the spherical aberration by modifying the telescope's replacement instruments, commonly referred to as "second-generation" equipment. The first instrument scheduled for replacement, even before the discovery of the mirror flaw, was the Wide Field/ Planetary Camera. Once a precise prescription to correct the spherical aberration is determined in late 1990, the Wide Field/Planetary Camera will be equipped with small corrective mirrors or lenses that are expected to restore the telescope to its original wide field capabilities. The new camera will be installed within the next three years. Until replacement, the current Wide Field/Planetary Camera will be limited to the study of brighter, uncrowded star fields. Subsequent Shuttle missions will replace other instruments equipped to compensate for the spherical aberration, enabling Hubble to carry out most of its planned observations over the telescope's 15-year lifetime.
Observing Schedule Revised
Now that scientists have had an opportunity to consider what lies ahead, they realize that Hubble's problem with its mirror represents only a delay for some observations. Over its scheduled 15-year life, the telescope will provide all that scientists had hoped it would‹and more. With second-generation equipment, the space telescope will see light emitted shortly after the Big Bang that created our universe, light that began its joumey some 12 billion years ago. Unmatched in its ability to capture faint light, the telescope will reveal the age and size of the universe, what it is made of, and how it will end. Until then, a shifting of priorities is in order. Astronomers will gather scientific data originally scheduled for later in the telescope's mission. In time, the Hubble telescope will reveal whether expansion of the universe is slowing or speeding. Constant expansion could mean eventual dissipation into nothingness; contracting could culminate in a fiery collapse. Other observations will include a look at the remnant of a supernova in the Crab nebula, using the Faint Object Spectrograph to analyze its remains, the very stuff that makes up life.
High above Earth's atmosphere, the space observatory will unveil some of the secrets of quasars and black holes and provide discoveries about the universe that scientists haven't even dreamed of.