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Saturday, April 24th, 2010trademarks | Posted by Editor - Blog News Service

20th Anniversary of the Hubble Space Telescope

20th Anniversary of the Hubble Space Telescope

20th Anniversary of the Hubble Space Telescope

The Hubble telescope, launched on 24 April 1990, has had a chequered history, requiring major repairs early in its career due to one of its focusing mirrors being incorrectly made. However, since then it has returned some of the most incredible images ever taken of space, including the famous Pillars of Creation image and the Deep Field.

Google’s Doodles – the images on its home page – change frequently in response to major events. 23 April saw it being given a St George’s Day makeover, showing a mounted St George and a cartoon dragon.

The first Google doodle was of the Burning Man Festival of 1998. The doodle was designed by Larry Page and Sergey Brin, Google’s founders, to notify users of their absence in case the servers crashed.

Doodles have been produced for the birthdays of several noted artists and scientists, including Isaac Einstein, Albert Einstein and Leonardo da Vinci.

Last year a series of doodles showing UFOs and crop circles caused lengthy speculation before it was revealed that they were celebrating the birthday of science fiction author HG Wells.

The Hubble Space Telescope (HST) is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, Hubble is one of the largest and most versatile, and is well-known as both a vital research tool and a public relations boon for astronomy. The HST is a collaboration between NASA and the European Space Agency, and is one of NASA’s Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.

Space telescopes were proposed as early as 1923. Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster. When finally launched in 1990, scientists found that the main mirror had been ground incorrectly, severely compromising the telescope’s capabilities. However, after a servicing mission in 1993, the telescope was restored to its intended quality. Hubble’s orbit outside the distortion of Earth’s atmosphere allows it to take extremely sharp images with almost no background light. Hubble’s Ultra Deep Field image, for instance, is the most detailed visible-light image ever made of the universe’s most distant objects. Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.

Hubble is the only telescope ever designed to be serviced in space by astronauts. Four servicing missions were performed from 1993–2002, but the fifth was canceled on safety grounds following the Space Shuttle Columbia disaster. However, after spirited public discussion, NASA administrator Mike Griffin approved one final servicing mission, completed in 2009. The telescope is now expected to function until at least 2014, when its ’successor’, the infrared James Webb Space Telescope (JWST), is due to be launched.

Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required new theories to explain them. Among its primary mission targets was to measure distances to Cepheid variable stars more accurately than ever before, and thus constrain the value of the Hubble constant, the measure of the rate at which the universe is expanding, which is also related to its age. Before the launch of HST, estimates of the Hubble constant typically had errors of up to 50%, but Hubble measurements of Cepheid variables in the Virgo Cluster and other distant galaxy clusters provided a measured value with an accuracy of 10%, which is consistent with other more accurate measurements made since Hubble’s launch using other techniques.

While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future. Astronomers from the High-z Supernova Search Team and the Supernova Cosmology Project used the telescope to observe distant supernovae and uncovered evidence that, far from decelerating under the influence of gravity, the expansion of the universe may in fact be accelerating. This acceleration was later measured more accurately by other ground-based and space-based telescopes, confirming Hubble’s finding. The cause of this acceleration remains poorly understood;the most common cause attributed is dark energy.

The high-resolution spectra and images provided by the HST have been especially well-suited to establishing the prevalence of black holes in the nuclei of nearby galaxies. While it had been hypothesized in the early 1960s that black holes would be found at the centers of some galaxies, and work in the 1980s identified a number of good black hole candidates, it fell to work conducted with Hubble to show that black holes are probably common to the centers of all galaxies] The Hubble programs further established that the masses of the nuclear black holes and properties of the galaxies are closely related. The legacy of the Hubble programs on black holes in galaxies is thus to demonstrate a deep connection between galaxies and their central black holes.
Main article: Comet Shoemaker-Levy 9

The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 was fortuitously timed for astronomers, coming just a few months after Servicing Mission 1 had restored Hubble’s optical performance. Hubble images of the planet were sharper than any taken since the passage of Voyager 2 in 1979, and were crucial in studying the dynamics of the collision of a comet with Jupiter, an event believed to occur once every few centuries.

Other major discoveries made using Hubble data include proto-planetary disks (proplyds) in the Orion Nebula; evidence for the presence of extrasolar planets around sun-like stars;[85] and the optical counterparts of the still-mysterious gamma ray bursts.[86] HST has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto and Eris.
Main articles: Hubble Deep Field and Hubble Ultra Deep Field

A unique legacy of Hubble are the Hubble Deep Field and Hubble Ultra Deep Field images, which utilized Hubble’s unmatched sensitivity at visible wavelengths to create images of small patches of sky that are the deepest ever obtained at optical wavelengths. The images reveal galaxies billions of light years away, and have generated a wealth of scientific papers, providing a new window on the early Universe.

Impact on astronomy

Many objective measures show the positive impact of Hubble data on astronomy. Over 8,000 papers based on Hubble data have been published in peer-reviewed journals, and countless more have appeared in conference proceedings. Looking at papers several years after their publication, about one-third of all astronomy papers have no citations, while only 2% of papers based on Hubble data have no citations. On average, a paper based on Hubble data receives about twice as many citations as papers based on non-Hubble data. Of the 200 papers published each year that receive the most citations, about 10% are based on Hubble data.

Although the HST has clearly had a significant impact on astronomical research, the financial cost of this impact has been large. A study on the relative impacts on astronomy of different sizes of telescopes found that while papers based on HST data generate 15 times as many citations as a 4 m ground-based telescope such as the William Herschel Telescope, the HST costs about 100 times as much to build and maintain.

Making the decision between investing in ground-based versus space-based telescopes in the future is complex. Even before Hubble was launched, specialized ground-based techniques such as aperture masking interferometry had obtained higher-resolution optical and infrared images than Hubble would achieve, though restricted to targets about 108 times brighter than the faintest targets observed by Hubble. Since then, advances in adaptive optics have extended the high-resolution imaging capabilities of ground-based telescopes to the infrared imaging of faint objects. The usefulness of adaptive optics versus HST observations depends strongly on the particular details of the research questions being asked. In the visible bands, adaptive optics can only correct a relatively small field of view, whereas HST can conduct high-resolution optical imaging over a wide field. Only a small fraction of astronomical objects are accessible to high-resolution ground-based imaging; in contrast Hubble can perform high-resolution observations of any part of the night sky, and on objects that are extremely faint.

Usage

Anyone can apply for time on the telescope; there are no restrictions on nationality or academic affiliation. Competition for time on the telescope is intense, and the ratio of time requested to time available (the oversubscription ratio) typically ranges between 6 and 9.

Calls for proposals are issued roughly annually, with time allocated for a cycle lasting approximately one year. Proposals are divided into several categories; ‘general observer’ proposals are the most common, covering routine observations. ‘Snapshot observations’ are those in which targets require only 45 minutes or less of telescope time, including overheads such as acquiring the target; snapshot observations are used to fill in gaps in the telescope schedule that cannot be filled by regular GO programs.

Astronomers may make ‘Target of Opportunity’ proposals, in which observations are scheduled if a transient event covered by the proposal occurs during the scheduling cycle. In addition, up to 10% of the telescope time is designated Director’s Discretionary (DD) Time. Astronomers can apply to use DD time at any time of year, and it is typically awarded for study of unexpected transient phenomena such as supernovae. Other uses of DD time have included the observations that led to the production of the Hubble Deep Field and Hubble Ultra Deep Field, and in the first four cycles of telescope time, observations carried out by amateur astronomers.

Amateur observations

The first director of STScI, Riccardo Giacconi, announced in 1986 that he intended to devote some of his Director Discretionary time to allowing amateur astronomers to use the telescope. The total time to be allocated was only a few hours per cycle, but excited great interest among amateur astronomers.

Proposals for amateur time were stringently peer reviewed by a committee of leading amateur astronomers, and time was awarded only to proposals that were deemed to have genuine scientific merit, did not duplicate proposals made by professionals, and required the unique capabilities of the space telescope. In total, 13 amateur astronomers were awarded time on the telescope, with observations being carried out between 1990 and 1997. One such study was Transition Comets — UV Search for OH Emissions in Asteroids. The very first proposal, A Hubble Space Telescope Study of Post Eclipse Brightening and Albedo Changes on Io, was published in Icarus, a journal devoted to solar system studies. After that time, however, budget reductions at STScI made the support of work by amateur astronomers untenable, and no further amateur programs have been carried out.

Hubble data

Transmission to Earth

Hubble data was initially stored on the spacecraft. When launched, the storage facilities were old-fashioned reel-to-reel tape recorders, but these were replaced by solid state data storage facilities during servicing missions 2 and 3A. Approximately twice daily, The Hubble Space Telescope radios data to a satellite in the geosynchronous Tracking and Data Relay Satellite System (TDRSS).[103] The TDRSS then downlinks the science data to one of two 60-foot (18-meter) diameter high gain microwave antennas located at the White Sands Test Facility in White Sands, New Mexico.[103] From there they are sent to the Goddard Space Flight Center and finally to the Space Telescope Science Institute for archiving.

These data are then transmitted to the Space Telescope Operations Control Center (STOCC) located in Greenbelt, Maryland.

Archive

All Hubble data is eventually made available via the archives of STScI. Data is usually proprietary—available only to the principal investigator (PI) and astronomers designated by the PI—for one year after being taken. The PI can apply to the director of the STScI to extend or reduce the proprietary period in some circumstances.

Observations made on Director’s Discretionary Time are exempt from the proprietary period, and are released to the public immediately. Calibration data such as flat fields and dark frames are also publicly available straight away. All data in the archive is in the FITS format, which is suitable for astronomical analysis but not for public use. The Hubble Heritage Project processes and releases to the public a small selection of the most striking images in JPEG and TIFF formats.

Pipeline reduction

Astronomical data taken with CCDs must undergo several calibration steps before it is suitable for astronomical analysis. STScI has developed sophisticated software that automatically calibrates data when it is requested from the archive using the best calibration files available. This ‘on-the-fly’ processing means that large data requests can take a day or more to be processed and returned. The process by which data is calibrated automatically is known as ‘pipeline reduction’, and is increasingly common at major observatories. Astronomers may if they wish retrieve the calibration files themselves and run the pipeline reduction software locally. This may be desirable when calibration files other than those selected automatically need to be used.

Data analysis

Hubble data can be analysed using many different packages. STScI maintains the custom-made STSDAS (Space Telescope Science Data Analysis System) software, which contains all the programs needed to run pipeline reduction on raw data files, as well as many other astronomical image processing tools, tailored to the requirements of Hubble data. The software runs as a module of IRAF, a popular astronomical data reduction program.

source:wikipedia

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