Hubble Space Telescope

The Hubble Space Telescope is a telescope in orbit around the Earth. Its position outside the Earth's atmosphere allows it to take extremely sharp images, and since its launch in 1990, it has become one of the most important telescopes in the history of astronomy. It has been responsible for many ground-breaking observations and has helped astronomers achieve a better understanding of many fundamental problems in astrophysics.

Conception, design and aims

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. Ground-based telescopes are typically 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.

Related Topics:
Astronomer - Lyman Spitzer - Angular resolution - Diffraction - Seeing - Arcsecond - Mirror - Infrared - Ultraviolet

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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.

Related Topics:
National Academy of Sciences - Space program

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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.

Related Topics:
World War II - Rocket - Spectrum - Sun - UK - Ariel - NASA - Orbiting Astronomical Observatory - Star - Galaxies

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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 {{ref|Spitzer}}.

Related Topics:
Reflecting telescope - Space Shuttle

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The quest for funding

The continuing success of the OAO program encouraged increasingly strong consensus within the astronomical community that the LST should be a major goal. In 1970 NASA established two committees, one to plan the engineering side of the space telescope project, and the other to determine the science goals of the mission. Once these had been established, the next hurdle for NASA was to obtain funding for the instrument, which would be far more costly than any Earth-based telescope. The US Congress questioned many aspects of the proposed budget for the telescope and forced cuts in the budget for the planning stages, which at the time consisted of very detailed studies of potential instruments and hardware for the telescope. In 1974, public spending cuts instigated by Gerald Ford led to Congress cutting all funding for the telescope project.

Related Topics:
Consensus - Congress - Public spending - Gerald Ford

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In response to this, a nationwide lobbying effort was co-ordinated among astronomers. Many astronomers met congressmen and senators in person, and large scale letter-writing campaigns were organised. The National Academy of Sciences published a report emphasising the need for a space telescope, and eventually the Senate agreed to a budget half that originally refused by Congress.

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The funding issues led to something of a reduction in the scale of the project, with the proposed mirror diameter reduced from 3 m to 2.4 m, both to cut costs and to allow a more compact and effective configuration for the telescope hardware. A proposed precursor 1.5m space telescope to test the systems to be used on the main satellite was dropped, and budgetary concerns also prompted collaboration with the European Space Agency. ESA agreed to supply some of the instruments for the telescope as well as the solar cells which would power it and contribute approximately 15% of the costs, in return for European astronomers being guaranteed at least 15% of observing time on the telescope. Congress eventually approved funding of US$36,000,000 for 1978, and the design of the LST began in earnest, aiming for a launch date of 1983. During the early 1980s, the telescope was named after Edwin Hubble, who made one of the greatest scientific breakthroughs of the 20th century when he discovered that the universe was expanding.

Related Topics:
European Space Agency - Solar cell - Europe - Edwin Hubble - Universe

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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 were commissioned to construct the spacecraft in which the telescope would be housed. {{ref|Dunar}}

Related Topics:
Marshall Space Flight Center - Goddard Space Flight Center - Perkin-Elmer - Lockheed

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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.

Related Topics:
Accuracy - Wavelength - Visible light - Ultraviolet - Infrared - Nanometre

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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 were 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.

Related Topics:
Kodak - Glass - Honeycomb

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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.

Related Topics:
May - 1981 - October - 1984 - Aluminum - Magnesium fluoride - Reflectivity

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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 {{ref|Dunar}}.

Related Topics:
April - 1985 - March - September - 1986 - $

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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.

Related Topics:
Shadow - Insulation - Aluminium - Graphite - Epoxy

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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 {{ref|Dunar}}.

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Ground support

In 1983, the Space Telescope Science Institute (STScI) was established after something of a power struggle between NASA and the scientific community at large. STScI is operated by the Association of Universities for Research in Astronomy (AURA) and is physically located on the Homewood campus of Johns Hopkins University in Baltimore, which is one of the 32 U.S. universities and 7 international affiliates that comprise the AURA consortium.

Related Topics:
Space Telescope Science Institute - Association of Universities for Research in Astronomy - Johns Hopkins University - Baltimore

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STScI is responsible for the scientific operation of the telescope and delivery of data products to astronomers, a function which NASA had wanted to keep 'in-house', but which scientists were keen to see based in an academic establishment. Engineering support is provided by NASA and contractor personnel at the Goddard Space Flight Center in Greenbelt, Maryland, 30 miles south of the STScI. Hubble's operation is monitored 24 hours per day by four teams of flight controllers who make up Hubble's Flight Operations Team.

Related Topics:
Academic - Goddard Space Flight Center

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The Space Telescope European Coordinating Facility was established at Garching bei München near Munich in 1984 to provide similar support primarily for European astronomers.

Related Topics:
Space Telescope European Coordinating Facility - Garching bei München - Munich

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Challenger disaster

In early 1986, the planned launch date of October that year looked feasible, but the Challenger disaster brought the US space program to a halt, grounding the Space Shuttle fleet and forcing the launch of Hubble to be postponed for several years. All telescope parts had to be kept in clean rooms until a launch could be rescheduled, a costly situation which pushed the overall costs of the project still higher.

Related Topics:
October - ''Challenger'' disaster - Space Shuttle - Clean room

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Eventually, following the resumption of Shuttle flights in 1988, the launch of the telescope was scheduled for 1990. In preparation for its final launch, dust which had accumulated on the mirror since its completion had to be removed with jets of nitrogen, and all systems were tested extensively to ensure they were fully functional. Finally, on 24 April 1990, shuttle mission STS-31 saw Atlantis launch the telescope successfully into its planned orbit.

Related Topics:
Nitrogen - 24 April - STS-31 - ''Atlantis''

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From its original total cost estimate of 435 million dollars (in FY77 funds), the telescope had by now cost over US$2.5 billion to construct. Hubble's cumulative costs to-date are approximately 14 billion dollars (inflation adjusted to the buying power of FY2005).

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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.

Related Topics:
Wide Field and Planetary Camera - Goddard High Resolution Spectrograph - High Speed Photometer - Faint Object Camera - Faint Object Spectrograph - Jet Propulsion Laboratory - Filter - Spectral line - Focal length

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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 {{ref|Brandt}}. 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.

Related Topics:
Spectrograph - Ultraviolet - Goddard Space Flight Center - Ball Aerospace - Spectral resolution - Photon - Digicon - Martin Marietta

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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 {{ref|Bless}}.

Related Topics:
University of Wisconsin - Variable star - Photometric

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