My OperaHubble Space Telescope
Monday, April 24, 2006 12:25:33 PM
On this day in 1990 the Hubble Space Telescope was launched. So here's a post on it.
Main information
Type of Orbit : Circular
Orbit height : 607.2 km (330 nautical miles)
Orbit period : 96.8 min
Orbit velocity :7554 m/s
Acceleration due to gravity : 8.169 m/s²
Angular momentum : 5.28×1010 m²/s
Launch date : April 24, 1990
Deorbit date : Around 2010
Mass : 11,000 kg
Telescope style:Ritchey-Chretien reflector
Diameter:2.4 m (7.87 feet)
Collecting area:approx. 4.3 m² (46.28 feet²)
Effective focal length: 57.6 m (189 feet)
Instruments
NICMOS :infrared camera/spectrometer
ACS: optical survey camera
WFPC2: wide field optical camera
STIS: Optical spectrometer/camera (failed)
The HST is named in memoriam of Edwin Hubble.
Proposals and precursors
The history of the Hubble Space Telescope can be traced back as far as 1946, when astronomer Lyman Spitzer wrote a paper entitled Astronomical advantages of an extra-terrestrial observatory. In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes: First, the angular resolution (smallest separation at which objects can be clearly distinguished) would be limited only by diffraction, rather than by the turbulence in the atmosphere which causes stars to twinkle and is known to astronomers as seeing. At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.1 arcsec for a telescope with a mirror 2.5 m in diameter. The second major advantage would be that a space-based telescope could observe infrared and ultraviolet light, which are strongly absorbed by the atmosphere.
Left Image : Lyman Spitzer, 'father' of the Space Telescope
Spitzer devoted much of his career to pushing for a space telescope to be developed. In 1962 a report by the US National Academy of Sciences recommended the development of a space telescope as part of the space program, and in 1965, Spitzer was appointed as head of a committee given the task of defining the scientific objectives for a large space telescope.
Space-based astronomy had begun on a very small scale following World War II, as scientists made use of the developments in rocket technology that had taken place. The first ultraviolet spectrum of the Sun was obtained in 1946. An orbiting solar telescope was launched in 1962 by the UK as part of the Ariel space program, and 1966 saw NASA's launch of the first Orbiting Astronomical Observatory (OAO) mission. OAO-1's battery failed after three days, terminating the mission, but OAO-2 carried out ultraviolet observations of stars and galaxies from its launch in 1968 until 1972, well beyond its original planned lifetime of one year.
The OAO missions demonstrated the important role space-based observations could play in astronomy, and 1968 saw the development by NASA of firm plans for a space-based reflecting telescope with a mirror 3 m in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope, with a launch slated for 1979. These plans emphasised the need for manned maintenance missions to the telescope to ensure such a costly program had a lengthy working life, and the concurrent development of plans for the reusable Space Shuttle indicated that the technology to allow this was soon to become available
Construction and engineering
Once the Space Telescope project had been given the go-ahead, work on the program was divided between many institutions. Marshall Space Flight Center 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 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.
Optical Telescope Assembly (OTA)
The mirror and optical systems of the telescope were the most crucial part, and were designed to exacting specifications. Telescopes typically have mirrors polished to an accuracy of about a tenth of the wavelength of visible light, but because the Space Telescope was to be used for observations ranging from ultraviolet to near-infrared with ten times better resolution than the best previous telescopes, its mirror needed to be polished to an accuracy of 1/20 of the wavelength of visible light, or about 30 nanometres.
Perkin-Elmer intended to use extremely sophisticated computer-controlled polishing machines to grind the mirror to the required shape, but in case their cutting-edge technology ran into difficulties, Kodak was commissioned to construct a back-up mirror using traditional mirror-polishing techniques. Construction of the mirror began in 1979, using ultra-low expansion glass. To keep the mirror's weight to a minimum it consisted of inch-thick top and bottom plates sandwiching a honeycomb lattice.
Mirror polishing began in 1979 and continued until May 1981. NASA reports at the time questioned Perkin-Elmer's managerial structure, and the polishing began to slip behind schedule and over budget. To save money, NASA halted work on the back-up mirror and put the launch date of the telescope back to October 1984. The mirror was completed by the end of 1981 with the addition of a reflective coating of aluminum 75 nm thick and a protective coating of magnesium fluoride 25 nm thick, which increased the mirror's reflectivity in ultraviolet light.
However, doubts continued to be expressed about Perkin-Elmer's competence on a project of this importance as their budget and timescale for producing the rest of the OTA continued to inflate. In response to a schedule described as "unsettled and changing daily," NASA postponed the launch date of the telescope until April 1985. Perkin-Elmer's schedules continued to slip at a rate of about one month per quarter, and at times delays reached one day for each day of work. NASA was forced to postpone the launch date until first March and then September 1986. By this time the total project budget had risen to $1.175 billion.
Spacecraft systems
The spacecraft in which the telescope and instruments were to be housed was another major engineering challenge. It would have to adequately withstand frequent passages from direct sunlight into the darkness of Earth's shadow which would generate major changes in temperature, while being stable enough to allow the extremely accurate pointing of the telescope that would be required. A shroud of multi-layered insulation keeps the temperature within the telescope stable, and surrounds a light aluminium shell in which the telescope and instruments sit. Within the shell, a graphite-epoxy frame keeps the working parts of the telescope firmly aligned.
While construction of the spacecraft in which the telescope and instruments would be housed proceeded somewhat more smoothly than the construction of the OTA, Lockheed still experienced some budget and schedule slippage, and by the summer of 1985, construction of the spacecraft was 30% over budget and three months behind schedule. An MSFC report said that Lockheed tended to rely on NASA directions rather than take their own initiative in the construction
Instruments
When launched, the HST carried five scientific instruments: the Wide Field and Planetary Camera (WF/PC), Goddard High Resolution Spectrograph (GHRS), High Speed Photometer (HSP), Faint Object Camera (FOC) and the Faint Object Spectrograph (FOS). WF/PC was a high-resolution imaging device primarily intended for optical observations. It was built by NASA's Jet Propulsion Laboratory, and incorporated a set of 48 filters isolating spectral lines of particular astrophysical interest. The instrument contained four CCD chips, three of which were 'wide field' chips while the fourth was the 'planetary camera' (PC). The PC took images at a longer effective focal length than the WF chips, giving it a greater magnification.
The GHRS was a spectrograph designed to operate in the ultraviolet. It was built by the Goddard Space Flight Center in conjunction with Ball Aerospace, and could achieve a spectral resolution of 90,000 [6]. Also optimised for ultraviolet observations were the FOC and FOS, both of which were also capable of the highest spatial resolution of any instrument on Hubble. Rather than CCDs these three instruments used photon-counting digicons as their detectors. FOC was constructed by ESA, while the Martin Marietta corporation built the FOS.
The final instrument was the HSP, designed and built at the University of Wisconsin. It was optimised for visible and ultraviolet light observations of variable stars and other astronomical objects varying in brightness. It could take up to 100,000 measurements per second with a photometric accuracy of about 2% or better .
The "sixth instrument" used for scientific observations on the Hubble are its three Fine Guidance Sensors (FGS), which have a dual purpose. The first is to keep the telescope accurately pointed during an observation; and the second is to perform astrometric science on selected stars. Since there are three FGS, and only two are needed to point the telescope, the third can be used to acquire stars and measure their positions to within a millisecond of arc. The original specifications required that measurements on a single star have a precision of 0.002 arcseconds; with repeated measurements, parallax and proper motion precisions down to about 0.0002 arcseconds have been achieved. The FGS is the most complex optical instrument flown to date. In the presence of primary mirror spherical aberration, the precision achieved was non-trivial. Extensive characterization, combined with secondary mirror compensation, has provided enough optical recompense to achieve this precision.
Important discoveries
Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required whole 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 Hubble, 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 the age of the universe, it also threw doubt on its future. Astronomers using the telescope to observe distant supernovae uncovered evidence that far from decelerating under the influence of gravity, the universe may in fact be accelerating. This acceleration was later measured more accurately by other ground-based and space-based telescopes which confirmed Hubble's finding, but the cause of this acceleration is currently very poorly understood.
The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 was very 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; and the optical counterparts of the still-mysterious gamma-ray bursts.
A unique legacy of Hubble is the set of Deep Field Images, which utilized Hubble's unmatched sensitivity at visible wavelengths. These are the most sensitive observations ever made at visible wavelengths, and have generated a wealth of scientific papers, providing a new window on the early Universe.
Many objective measures show the enormous 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 very 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 cost about 100 times as much to build and maintain . Even before Hubble was launched, speckle imaging and aperture synthesis could provide higher resolution than Hubble would ever achieve, but these techniques did not have the sensitivity of Hubble to faint objects. The development of adaptive optics in recent years now means that ground-based telescopes can take images of relatively faint objects with resolution similar to that of Hubble images, at much lower cost, and this has been a key consideration in the debate about the future of space telescopes . However, ground-based telescopes are still not capable of produce visible images which are as sensitive as the Hubble Ultra Deep Field.
Images of Hubble
Left :A WFPC2 image of a small region of the Tarantula Nebula in the Large Magellanic Cloud
Right : The Horsehead Nebula
Above : Pillars of creation where stars are forming in the Eagle Nebula
Main information
Type of Orbit : Circular
Orbit height : 607.2 km (330 nautical miles)
Orbit period : 96.8 min
Orbit velocity :7554 m/s
Acceleration due to gravity : 8.169 m/s²
Angular momentum : 5.28×1010 m²/s
Launch date : April 24, 1990
Deorbit date : Around 2010
Mass : 11,000 kg
Telescope style:Ritchey-Chretien reflector
Diameter:2.4 m (7.87 feet)
Collecting area:approx. 4.3 m² (46.28 feet²)
Effective focal length: 57.6 m (189 feet)
Instruments
NICMOS :infrared camera/spectrometer
ACS: optical survey camera
WFPC2: wide field optical camera
STIS: Optical spectrometer/camera (failed)
The HST is named in memoriam of Edwin Hubble.
Proposals and precursors
The history of the Hubble Space Telescope can be traced back as far as 1946, when astronomer Lyman Spitzer wrote a paper entitled Astronomical advantages of an extra-terrestrial observatory. In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes: First, the angular resolution (smallest separation at which objects can be clearly distinguished) would be limited only by diffraction, rather than by the turbulence in the atmosphere which causes stars to twinkle and is known to astronomers as seeing. At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.1 arcsec for a telescope with a mirror 2.5 m in diameter. The second major advantage would be that a space-based telescope could observe infrared and ultraviolet light, which are strongly absorbed by the atmosphere.
Left Image : Lyman Spitzer, 'father' of the Space Telescope
Spitzer devoted much of his career to pushing for a space telescope to be developed. In 1962 a report by the US National Academy of Sciences recommended the development of a space telescope as part of the space program, and in 1965, Spitzer was appointed as head of a committee given the task of defining the scientific objectives for a large space telescope.
Space-based astronomy had begun on a very small scale following World War II, as scientists made use of the developments in rocket technology that had taken place. The first ultraviolet spectrum of the Sun was obtained in 1946. An orbiting solar telescope was launched in 1962 by the UK as part of the Ariel space program, and 1966 saw NASA's launch of the first Orbiting Astronomical Observatory (OAO) mission. OAO-1's battery failed after three days, terminating the mission, but OAO-2 carried out ultraviolet observations of stars and galaxies from its launch in 1968 until 1972, well beyond its original planned lifetime of one year.
The OAO missions demonstrated the important role space-based observations could play in astronomy, and 1968 saw the development by NASA of firm plans for a space-based reflecting telescope with a mirror 3 m in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope, with a launch slated for 1979. These plans emphasised the need for manned maintenance missions to the telescope to ensure such a costly program had a lengthy working life, and the concurrent development of plans for the reusable Space Shuttle indicated that the technology to allow this was soon to become available
Construction and engineering
Once the Space Telescope project had been given the go-ahead, work on the program was divided between many institutions. Marshall Space Flight Center 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 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.
Optical Telescope Assembly (OTA)
The mirror and optical systems of the telescope were the most crucial part, and were designed to exacting specifications. Telescopes typically have mirrors polished to an accuracy of about a tenth of the wavelength of visible light, but because the Space Telescope was to be used for observations ranging from ultraviolet to near-infrared with ten times better resolution than the best previous telescopes, its mirror needed to be polished to an accuracy of 1/20 of the wavelength of visible light, or about 30 nanometres.
Perkin-Elmer intended to use extremely sophisticated computer-controlled polishing machines to grind the mirror to the required shape, but in case their cutting-edge technology ran into difficulties, Kodak was commissioned to construct a back-up mirror using traditional mirror-polishing techniques. Construction of the mirror began in 1979, using ultra-low expansion glass. To keep the mirror's weight to a minimum it consisted of inch-thick top and bottom plates sandwiching a honeycomb lattice.
Mirror polishing began in 1979 and continued until May 1981. NASA reports at the time questioned Perkin-Elmer's managerial structure, and the polishing began to slip behind schedule and over budget. To save money, NASA halted work on the back-up mirror and put the launch date of the telescope back to October 1984. The mirror was completed by the end of 1981 with the addition of a reflective coating of aluminum 75 nm thick and a protective coating of magnesium fluoride 25 nm thick, which increased the mirror's reflectivity in ultraviolet light.
However, doubts continued to be expressed about Perkin-Elmer's competence on a project of this importance as their budget and timescale for producing the rest of the OTA continued to inflate. In response to a schedule described as "unsettled and changing daily," NASA postponed the launch date of the telescope until April 1985. Perkin-Elmer's schedules continued to slip at a rate of about one month per quarter, and at times delays reached one day for each day of work. NASA was forced to postpone the launch date until first March and then September 1986. By this time the total project budget had risen to $1.175 billion.
Spacecraft systems
The spacecraft in which the telescope and instruments were to be housed was another major engineering challenge. It would have to adequately withstand frequent passages from direct sunlight into the darkness of Earth's shadow which would generate major changes in temperature, while being stable enough to allow the extremely accurate pointing of the telescope that would be required. A shroud of multi-layered insulation keeps the temperature within the telescope stable, and surrounds a light aluminium shell in which the telescope and instruments sit. Within the shell, a graphite-epoxy frame keeps the working parts of the telescope firmly aligned.
While construction of the spacecraft in which the telescope and instruments would be housed proceeded somewhat more smoothly than the construction of the OTA, Lockheed still experienced some budget and schedule slippage, and by the summer of 1985, construction of the spacecraft was 30% over budget and three months behind schedule. An MSFC report said that Lockheed tended to rely on NASA directions rather than take their own initiative in the construction
Instruments
When launched, the HST carried five scientific instruments: the Wide Field and Planetary Camera (WF/PC), Goddard High Resolution Spectrograph (GHRS), High Speed Photometer (HSP), Faint Object Camera (FOC) and the Faint Object Spectrograph (FOS). WF/PC was a high-resolution imaging device primarily intended for optical observations. It was built by NASA's Jet Propulsion Laboratory, and incorporated a set of 48 filters isolating spectral lines of particular astrophysical interest. The instrument contained four CCD chips, three of which were 'wide field' chips while the fourth was the 'planetary camera' (PC). The PC took images at a longer effective focal length than the WF chips, giving it a greater magnification.
The GHRS was a spectrograph designed to operate in the ultraviolet. It was built by the Goddard Space Flight Center in conjunction with Ball Aerospace, and could achieve a spectral resolution of 90,000 [6]. Also optimised for ultraviolet observations were the FOC and FOS, both of which were also capable of the highest spatial resolution of any instrument on Hubble. Rather than CCDs these three instruments used photon-counting digicons as their detectors. FOC was constructed by ESA, while the Martin Marietta corporation built the FOS.
The final instrument was the HSP, designed and built at the University of Wisconsin. It was optimised for visible and ultraviolet light observations of variable stars and other astronomical objects varying in brightness. It could take up to 100,000 measurements per second with a photometric accuracy of about 2% or better .
The "sixth instrument" used for scientific observations on the Hubble are its three Fine Guidance Sensors (FGS), which have a dual purpose. The first is to keep the telescope accurately pointed during an observation; and the second is to perform astrometric science on selected stars. Since there are three FGS, and only two are needed to point the telescope, the third can be used to acquire stars and measure their positions to within a millisecond of arc. The original specifications required that measurements on a single star have a precision of 0.002 arcseconds; with repeated measurements, parallax and proper motion precisions down to about 0.0002 arcseconds have been achieved. The FGS is the most complex optical instrument flown to date. In the presence of primary mirror spherical aberration, the precision achieved was non-trivial. Extensive characterization, combined with secondary mirror compensation, has provided enough optical recompense to achieve this precision.
Important discoveries
Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required whole 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 Hubble, 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 the age of the universe, it also threw doubt on its future. Astronomers using the telescope to observe distant supernovae uncovered evidence that far from decelerating under the influence of gravity, the universe may in fact be accelerating. This acceleration was later measured more accurately by other ground-based and space-based telescopes which confirmed Hubble's finding, but the cause of this acceleration is currently very poorly understood.
The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 was very 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; and the optical counterparts of the still-mysterious gamma-ray bursts.
A unique legacy of Hubble is the set of Deep Field Images, which utilized Hubble's unmatched sensitivity at visible wavelengths. These are the most sensitive observations ever made at visible wavelengths, and have generated a wealth of scientific papers, providing a new window on the early Universe.
Many objective measures show the enormous 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 very 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 cost about 100 times as much to build and maintain . Even before Hubble was launched, speckle imaging and aperture synthesis could provide higher resolution than Hubble would ever achieve, but these techniques did not have the sensitivity of Hubble to faint objects. The development of adaptive optics in recent years now means that ground-based telescopes can take images of relatively faint objects with resolution similar to that of Hubble images, at much lower cost, and this has been a key consideration in the debate about the future of space telescopes . However, ground-based telescopes are still not capable of produce visible images which are as sensitive as the Hubble Ultra Deep Field.
Images of Hubble
Left :A WFPC2 image of a small region of the Tarantula Nebula in the Large Magellanic Cloud
Right : The Horsehead Nebula
Above : Pillars of creation where stars are forming in the Eagle Nebula
(50%)
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