Hubble Space Telescope Term Paper

  • Length: 11 pages
  • Subject: Astronomy
  • Type: Term Paper
  • Paper: #30233308

Excerpt from Term Paper :

Hubble Space Telescope

As the world's first Earth-orbiting reflecting telescope, the Hubble Space Telescope (HST) represents the culmination of the dreams of many astronomers and the fulfillment of a hope that began in the 1950's when the United States launched instruments into space in order to study the Earth's atmosphere. Our planetary speck in space, namely the Earth, is only one of hundreds of billions of planets that surely orbit other star systems; our Milky Way galaxy, composed of billions of stars and other astronomical bodies, is but one out of billions of other galaxies. Thus, in order to increase our knowledge of the universe, the Hubble Space Telescope serves a vital function, for since its launch in 1990, despite several crucial problems, it has revealed a universe full of mysterious bodies, nebula, star systems and galaxies and has expanded the possibilities that humankind is not alone in the universe.

The first scientific idea for a telescope such as the HST came about in 1946 when Lyman Spitzer issued a proposal for a space telescope with a primary mirror between sixteen and fifty feet in diameter. Throughout the 1960's and 1970's, with the ultraviolet observatories OAO-2, OAO 3 and the IUE in the planning stages, the space telescope remained only a dream, due to the lack of technology. Yet Spitzer did not give up, for by the 1970's he had offered another proposal for a space telescope which would be able to provide images of very faint objects as contrasted to ground-based telescopes which suffer from distortion due to the Earth's atmosphere. According to Spitzer, such a telescope would not "supplement our present ideas of the universe we live in, but rather would uncover new phenomena not yet imagined and perhaps would modify profoundly our basic concepts of space and time" (42).

Thus, as the years passed, many astronomers came to understand that such an instrument would provide new opportunities for the present and for the future. In 1965, the National Academy of Sciences issued a report which focused on the construction of a space-based telescope; in 1970, the NAS convened an Astronomy Survey Committee and placed all new efforts on the California Institute of Technology to set priorities for the construction of the telescope. Between 1973 and 1977, the Space Telescope Working Group was formed to discuss the project in detail and to make certain that the space telescope contained the best instruments possible within certain budgetary and design restrictions.

By the early 1980's, plans were well underway for the construction of the Hubble Space Telescope. However, there were several engineering problems that had to be overcome, namely, the effects of gravity and moisture regarding the telescope's primary mirror and its internal structure. First of all, gravity greatly affected the lightweight trusses that formed the internal skeleton of the HST which were in danger of sagging just enough to affect the fine alignment of all the integral parts of the telescope. On the Earth's surface, there is no practical way to support every part of an object equally well, but with the primary mirror, the engineers managed to design a method to support it at various points which reduced the overall stress upon it by gravity. As long as the mirror remained on the ground, it was being affected by gravity via bending, but once in space, gravity would be virtually eliminated.

The mirror itself was designed by the Perkin-Elmer Corporation which worked for three years to solve the problem of creating such a mirror on the ground. Made of special ultra-low expansion glass, the mirror consisted of honeycomb cores with most of the glass behind the reflecting surface removed in order to decrease its total weight. Another essential property of the mirror, made by the Corning Glass Company, was its weldability which allows a single slab of glass to be fused from individual honeycombs. As a result, the mirror at ninety-four inches in diameter weighs under one ton; in comparison, the 200-inch mirror for the Mount Palomar telescope weighs more than fourteen tons and has only a little bit better than twice the light-gathering power of the HST mirror. The telescope's mirror system is based on the Ritchey-Chretien design which "affords wider and flatter fields of view than traditional Cassegrain systems" ("Hubble Space Telescope," and even though "the Hubble Space Telescope has a mirror 15 times smaller than an Earthbound telescope, it can. . . resolve detail almost 100 times finer" (Nemiroff, Internet).

Thus, by 1985, the HST was complete and represented new frontiers in optics, detectors and pointing accuracy. Yet as it stood ready for launch in early 1990, the scientists who would decide if the launch was to take place faced a dilemma, being the coming of the solar maximum or solar activity which includes the appearance of sunspots and active regions as well as solar flares. In March of 1989, one of the largest groups of sunspots ever observed became apparent and a solar flare emitted huge numbers of energetic particles that caused brilliant displays of the northern lights. For the telescope, this meant higher densities of gas in the atmosphere which could affect the amount of drag on the instrument's launch rocket.

However, an even bigger setback occurred in 1986 with the Challenger explosion disaster which took the lives of all the astronauts aboard the space shuttle. As a result, the initial launch for the Hubble Space Telescope was delayed but during this time, the engineers at NASA used this to their advantage, for they ran numerous tests on the HST, such as checking all the alignments related to the primary mirror and double-checking all supporting devices like internal computers, the skeletal structure and other important systems. After all of this delay, the Hubble Space Telescope was finally launched aboard the STS-31 mission of the space shuttle Discovery on April 24, 1990.

Overall, the HST, at the time of its launch, stood as an engineering marvel, symbolized by its scientific instruments, the ninety-four inch primary mirror and the detectors which would analyze all of the images. For the HST engineers, the CCD detectors were the most crucial aspect of the instrument, for they were designed to sense the light of an image and then transmute it into electrical impulses. Of course, these detectors would work at their optimum best with a long focal length which would create a sharper and larger image, along with the smallest pixels in order to produce an image as clear as possible.

Yet the scientist at NASA were restricted by the fact that in order to fit inside the space shuttle, the HST could be no longer than forty-two feet. However, the engineers decide to use an optical system similar in design to a Newtonian reflector, and in the case of the Hubble, a large primary mirror reflects the incoming light which is then reflected to a secondary mirror supported sixteen feet ahead of the primary by a truss rod arrangement which blocks a small portion of the incoming light. In essence, the light reflected from the primary travels around the secondary mirror and is not crucially diffused. After being reflected from the secondary mirror, the light passes through a small hole in the center of the primary where the light is detected and analyzed by the detectors. The actual image produced is located a few feet behind the primary and the total effective focal length equals one hundred and ninety feet. This system is also referred to as a Cassegrain and is very common in Earth-bound telescopes.

The HST also contains four additional mirrors placed behind the small hole in the primary mirror. Three of these much smaller mirrors are used to direct part of the incoming light to fine guidance sensors which help in pointing the telescope in the proper direction. A fourth mirror directs a portion of the light to the wide-field planetary camera which captures wide-field images. In addition, the HST contains instruments used to analyze the color of light and its total intensity. These include the beam to high resolution photometer, beam to high resolution spectrograph, beam to faint object camera and the beam to faint object spectrograph.

After all of this hard work, design and engineering to create the Hubble Space Telescope, soon after its launch in 1990, scientists at NASA discovered that something was terribly wrong with the instrument. On May 20, 1990, the HST sent its first images to the Goddard Flight Center in Greenbelt, Maryland, the Marshall Space Flight Center in Huntsville, Alabama and the Space Telescope Science Institute in Baltimore, Maryland, being the light from a star cluster over 1300 light years away.

Of course, no one expected these images to be very spectacular, for they were meant to be an evaluation of the telescope's performance. Star images normally look like bright spots surrounded by a little scattered light, due to most of the star's light being focused by the telescope's mirrors into a little core,…

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