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Not All Is Flawed in Hubble Images

Despite a defect in its mirror, the space telescope is taking useful photographs of remote galaxies

By Robert C. CowenStaff writer of The Christian Science Monitor / December 16, 1992



BOSTON

HUBBLE Space Telescope is performing better than astronomers expected when they first learned that its mirror is flawed.

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Recently reported observations by the telescope show an ability to resolve detail and to image faint distant galaxies that astronomers had thought only a near-perfect mirror could achieve.

For example, the image reproduced here represents what astronomer Daniel W. Weedman of Pennsylvania State University at University Park calls "one of the most dramatic pictures" Hubble has taken. The photograph shows what astronomers call an accretion disk of swirling dust and gas circling the core of an energetically active galaxy, just as theory says it should.

Earlier this month, it was announced that Carnegie Institution astronomer Alan Dres- sler and colleagues have resolved images of individual galaxies that are members of remote galaxy clusters. Dr. Dressler, who works at the Carnegie Observatories offices in Pasadena, Calif., said, "The results have important implications for theories of how galaxies have evolved since the beginning of the universe 15 billion years ago."

This kind of study is a primary goal of the Hubble-telescope program. But astronomers had thought that the mirror's flaw foreclosed that possibility.

Ray Villard, spokesman for the Space Telescope Science Institute in Baltimore, notes that "it's fair to say" that Hubble astronomers now are getting some of the research opportunities they thought they had lost.

Hubble's mirror has an optical flaw called spherical aberration. This means the 2.4-meter (7.9-foot) primary mirror can't bring enough of the light it gathers into a single focus. It was supposed to concentrate 70 percent of a star's light. Instead, only 15 percent of that light forms a sharp image. With the help of computer processing, astronomers have been able to use that 15 percent to study a wide range of relatively nearby objects and distant bright objects. But it seemed much too skimpy to allow th em to resolve some of the finer details or to image the faint distant galaxies that are one of their prime targets.

Dr. Villard recalls that many astronomers simply gave up thinking about such goals. Dressler says, "I almost gave up, too." He explains, however, that he and his colleagues - Augustus Oemier of Yale University in New Haven, Conn., James E. Gunn of Princeton University in New Jersey, and Harvey Butcher of the Netherlands Foundation for Research in Astronomy - decided to give distant galaxy study a try. And, he adds, "It worked."

Making the most of Hubble's largely blurry images is like working with a partly rotten apple. Dressler explains, "We just cut away the bad part and used the rest." This left enough good data to resolve images of galaxies that form two galaxy clusters, each of which is 4 billion light-years away.

This shows the galaxies as they were 4 billion years ago, since it has taken their light that long to reach us. There seem to be relatively more spiral galaxies in those clusters than in the relatively nearby galaxy clusters astronomers have been able to study up to now.