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HST, or more commonly known as the Hubble Space Telescope, the telescope is currently orbiting Earth and was named after Edwin Hubble who was a famous Astronomer.Its position being outside the Earth's atmosphere provides significant advantages over ground-based telescopes — images are not blurred by the atmosphere, there is no background from light scattered by the air, and the Hubble can observe ultra-violet light that is normally absorbed by the ozone layer in observations made from Earth.
The Hubble is the only telescope ever designed to be serviced in space by astronauts. To date, there have been four servicing missions, with a fifth and final mission planned for October 2008. Servicing Mission 1 took place in December 1993 when Hubble's imaging flaw was corrected. Servicing Mission 2 occurred in February 1997 when two new instruments were installed. Servicing Mission 3 was split into two distinct missions: SM3A occurred in December 1999 when urgently needed repairs were made to Hubble; and then SM3B followed in March 2002 when the Advanced Camera for Surveys (ACS) was installed.
Seven astronauts are currently training at the Goddard Space Flight Center for the final Hubble Servicing Mission (SM4) scheduled for October 2008. This mission will extend the operating life of HST until at least 2013 making it more powerful than ever. The purpose of SM4 is to install several new instruments. The Cosmic Origin Spectrograph (COS) will be the most sensitive ultraviolet spectrograph ever flown on HST. The Wide Field Camera3 (WFC3) will have a much broader range of imaging, from early and distant galaxies beyond Hubble’s current reach, to more nearby galaxies and their star formation histories. These two working together will make Hubble's imaging capabilities more spectacular than ever before. Also being installed is a refurbished Fine Guidance Sensor (FGS) as well as gyroscopes, battery modules and new outer blanket layer.
Once the Space Telescope project had been given the go-ahead, work on the program was divided between many institutions. Marshall Space Flight Center (MSFC) was given responsibility for the design, development, and construction of the telescope, while the Goddard Space Flight Center was given overall control of the scientific instruments and ground-control centre for the mission. Marshall commissioned the optics company Perkin-Elmer to design and build the Optical Telescope Assembly (OTA) and Fine Guidance Sensors for the space telescope. Lockheed was commissioned to construct the spacecraft in which the telescope would be housed.
The design of the telescope had always incorporated servicing missions, and astronomers immediately began to seek potential solutions to the problem which could be applied at the first servicing mission, scheduled for 1993. While Kodak and Itek had each ground back-up mirrors for Hubble, it would have been impossible to replace the mirror in orbit, and too expensive and time-consuming to bring the telescope temporarily back to Earth for a refit. Instead, the fact that the mirror had been ground so precisely to the wrong shape led to the design of new optical components with exactly the same error but in the opposite sense, to be added to the telescope at the servicing mission, effectively acting as "spectacles" to correct the spherical aberration.
Because of the way the instruments were designed, two different sets of correctors were required. The design of the Wide Field and Planetary Camera (WF/PC) included relay mirrors to direct light onto the eight separate CCD chips making up its two cameras, and an inverse error could be built into their surfaces that would completely cancel the aberration of the primary. This fix was built into the replacement Wide Field and Planetary Camera 2 (which, however contained only four rather than eight CCDs due to schedule and budgetary pressures). However, the other instruments lacked any intermediate surfaces which could be figured in this way, and so required an external correction device.
Many objective measures show the positive impact of Hubble data on astronomy. Over 4,000 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 which 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 vs. space-based telescopes in the future is complex. 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. The field of view over which high-quality adaptive optics corrections is limited however, especially in optical colors. HST retains the unique ability to do high-resolution optical imaging over a wide field. Even before Hubble's launch, ground-based speckle imaging could provide higher resolution images of bright objects than Hubble can achieve.
All Hubble data are eventually made available via a public archive at http://archive.stsci.edu/hst. Data are 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.
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