View the large, annotated HST illustration
In the quest for better resolution, astronomers built the first mountaintop observatory only a hundred years ago, on Mount Hamilton, California, under its dark, stable, clear skies. Since then, higher and darker mountain locations have been sought for astronomical observatories. But there are always three major problems with any earthbound observatory: first, only a small portion of the whole spectrum of energy can get through the atmosphere -- visible light. Most other forms: ultra-violet, infra-red, etc., are absorbed by the atmosphere. Second, the atmosphere blurs objects and makes them appear to twinkle. Third, the sky is never perfectly dark. The more sensitive the instrument, the more background light is seen, caused by light scattered in the atmosphere, as well as some actual glow. Astronomers have compared skywatching from the bottom of the atmosphere to birdwatching from under water. Now a major astronomical observatory has its vantage point high above the earth's life-giving, but optically troublesome atmosphere. The HST is the largest and most sensitive telescope ever lifted into orbit. Its launch came after seven years of delays, aboard the Space Shuttle Atlantis April 24, 1990.
HST Repair Missions
After about a week of work from the ground with the HST's Fine Guidance Sensors, and adjusting the position of its secondary mirror, first light for the HST came on May 20, 1990: during an engineering test, it was pointed toward the open star cluster NGC 3532 in the constellation Carina. The Wide Field / Planetary Camera captured a one-second test exposure image, which was telemetered to scientists at the NASA Goddard Space Flight Center. It was determined then that bringing the telescope into perfect focus was going to be impossible. During subsequent investigations, it was found that the world's smoothest mirror was designed to the wrong specification, due to an error in the test equipment used to control figuring and polishing. Original plans called for replacing the HST's modular instruments as new ones could be developed. The first was the Wide-Field/Planetary Camera, built by Caltech's Jet Propulsion Laboratory, in 1993. It contained corrective optics necessary to bring the HST's performance up to the initial specification (see inset on first page). Even before these repairs, special computer processing was able to remove some effects of the flaw for bright, high-contrast objects.
NASA's Office of Space Science and Applications is responsible for the overall Space Telescope Program. NASA's Goddard Space Flight Center (GSFC) at Greenbelt, Maryland, is responsible for the HST's scientific instruments, mission operations, and the Space Telescope Science Institute (STSI). The STSI, located at Johns Hopkins University in Baltimore, Maryland, is operated by the Association of Universities for Research in Astronomy, which is a consortium of twenty American universities. Lockheed Missiles and Space Company, Inc., was the prime contractor responsible for the HST's Support Systems Module, and overall systems engineering. Perkin-Elmer was the prime contractor for the Optical Telescope Assembly, including the 94.5-inch primary mirror. Perkin-Elmer was subsequently acquired by Hughes Aircraft Company, a subsidiary of General Motors, and is now known as Hughes-Danbury Optical Systems.
The people of the STSI at Johns Hopkins are the liaison between the HST and the world's community of professional (and amateur) astronomers who will be using the telescope. Proposals for using the HST are submitted to STSI. Approved proposals are scheduled piecemeal into an overall observing schedule. Since the telescope slews slowly from one target to another, and it takes time for the different instruments to be selected and configured, observations are grouped such that slewing and configuration times are minimized. These observing schedules are relayed to GSFC, where sequences of detailed instructions for the telescope and its instruments and subsystems are generated. These instruction sequences are uplinked to the spacecraft through NASA's Tracking and Data Relay Satellite (TDRS). As the HST carries out its operations and observations, science data from the observations, along with engineering data from the spacecraft subsystems and instruments, are transmitted back through TDRS (or stored on tape recorders on board if TDRS isn't immediately available). The received data are then relayed through GSFC back to investigators at STSI for scientific analysis and publication.
HST has produced many advances in astronomical knowledge as this Fact Sheet goes to press in December, 1994. Some highlights which have appeared in the press include: R Aauarii, which is a binary star system consisting of a cool red giant star orbiting with a hot white dwarf star. Huge jets of plasma were observed with the Faint Object Camera streaming out of the star pair during a recent Nova-type eruption -- an exciting and unprecedented observation. A rare storm visible in the cloudtops of Saturn erupted, and was observed by HST's Wide Field/Planetary Camera with clarity impossible from Earth. Its images rivaled those returned from the Voyager spacecraft during Saturn encounter. The clearest views ever obtained of Pluto and its satellite Charon were the work of HST's Faint Object Camera. Unprecedented detail is being revealed by the same instrument in the immediate vicinity of Supernova 1987A, a star in the nearby Large Magellanic Cloud (a satellite galaxy to the Milky Way) which destroyed itself in a spectacular explosion in 1987. Observations of variable stars in other galaxies are refining knowledge of the distance between galaxies. Many other observations are continuing to increase our knowledge of the universe -- knowledge which will most likely challenge current cosmological theories, much as Galileo did when he first turned a telescope to the heavens.
HST's Optical Path
The HST is a Cassegrain Telescope: a reflector wherein the light from the large, concave, primary mirror is directed onto a smaller secondary mirror, which then brings the light to a focus at a point behind the primary mirror, after passing through a hole in the center of the primary. In the HST, additional flat mirrors direct the light into the desired instruments. The Ritchey Chretein design, which the HST uses, is an optical configuration of their surfaces which keeps images in focus over the widest possible field of view.
View the large, annotated HST illustration
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