Friday, July 31, 2009

Hubble Space Telescope - A Knol Useful to Know More about Optical Instruments

Selected Passages from the Article about Hubble Telescope

The Hubble Space Telescope was launched in 1990, a joint project of NASA (the National Aeronautics and Space Administration of the United States) and ESA (the European Space Agency). It carries a 2.4-m (94-inch) telescope that feeds several different instruments. It is in low Earth orbit, allowing it to be reached and serviced by astronauts, a process that made it work properly in the first place and that continues to allow its updating.

History of Hubble
The Hubble Space Telescope is the descendant of a planned Large Space Telescope, but during the 1980's it was downsized in planning both for psychological reasons, so the word "Large" wouldn't go in the proposal to Congress, and to allow it to fit in the payload bay of a space shuttle. Lyman Spitzer of Princeton University and John Bahcall of the Institute for Advanced Study, also located in Princeton, were principal scientists who worked not only scientifically but also politically to see the project advance.

The mirror was made by the Perkin-Elmer Company in Connecticut, and was said to be the most perfectly and smoothly shaped telescope mirror ever. The telescope mirror was completed and the telescope almost ready to be launched when the space-shuttle Challenger exploded in 1984. With space-shuttle launches suspended, the telescope was put into storage.

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The telescope was launched on April 24, 1990, from Cape Canaveral on a space shuttle. It was named after Edwin P. Hubble, the astronomer at the Palomar Observatory who had discovered the important cosmological law about the expansion of the Universe, linking its rate of expansion linearly with distance. It received the name in recognition of the prospectively important cosmological work the telescope could do, and in the hope that it could refine Hubble's Law—in particular, Hubble's constant, the constant of proportionality between redshift and distance—to a higher accuracy than had been previously possible.

Science with Hubble
The Hubble Space Telescope has been used by astronomers to study objects in the Universe near and far, excepting only the Sun (which is too bright, but which was indeed detected through the back of the mirror!). A main reason for the launch of Hubble is that from its perch outside Earth's atmosphere, it can have resolution about 7 times finer than normal ground-based resolution from good telescope sites; that is, it can see detail about 7 times finer. It had often been loosely said that it could therefore see 7 times farther into space, but large ground-based telescopes had already been observing objects so far in the outer solar system that it was impossible to see 7 times farther.

In the years since Hubble's launch, ground-based capabilities have advanced, and Active Optics has allowed Hubble's resolution to be achieved in certain limited but growing circumstances from ground-based telescopes. Still, Hubble can attain its high resolution for all objects it observes without complicated post-processing.

Note, however, that Hubble is only one 2.4-m telescope, not large by today's standards. The twin Keck telescopes in Hawaii, for example, have mirrors each 10 m in diameter, about 4 times in diameter and 16 times in area compared with Hubble's. Thus many astronomical projects, particularly those that require collecting as much light as possible, are better carried out with this new generation of large, mountain-based telescopes, still leaving many, many projects best achieved with the high-resolution of Hubble.

Hubble Space Telescope By Jay M. Pasachoff, Astronomer,Williams College, Williamstown, MA, and Chair of the International Astronomical Union's Working Group on Solar Eclipses

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