Editing Hubble Space Telescope (section)

== Conception, design and aim ==
=== Proposals and precursors ===
[[File:Astronaut Owen Garriott Performs EVA During Skylab 3 - GPN-2002-000065.jpg|thumb|upright|left|Astronaut [[Owen Garriott]] working next to Skylab's crewed solar space observatory, 1973]]

In 1923, [[Hermann Oberth]]—considered a father of modern rocketry, along with [[Robert H. Goddard]] and [[Konstantin Tsiolkovsky]]—published ''{{lang|de|[[Die Rakete zu den Planetenräumen]]}}'' ("The Rocket into Planetary Space"), which mentioned how a telescope could be propelled into [[Earth]] orbit by a rocket.<ref>{{cite book |last=Oberth |first=Hermann |title=Die Rakete zu den Planetenräumen |date=1923 |publisher=R. Oldenbourg-Verlay |page=85 |language=de}}</ref>

[[File:1946- Lyman Spitzer (4526166350).jpg|thumb|right|upright|[[Lyman Spitzer]] played a major role in the birth of the Hubble Space Telescope project.|alt=[[Lyman Spitzer]] played a major role in the birth of the Hubble Space Telescope project.]]
The history of the Hubble Space Telescope can be traced to 1946, to [[astronomer]] [[Lyman Spitzer]]'s paper "Astronomical advantages of an extraterrestrial observatory".<ref name=":0">Spitzer, Lyman Jr., "Report to Project Rand: Astronomical Advantages of an Extra-Terrestrial Observatory", reprinted in [https://history.nasa.gov/SP-4407/vol5/ExploreUnknown.pdf ''NASA SP-2001-4407: Exploring the Unknown''] {{Webarchive|url=https://web.archive.org/web/20170120024958/https://history.nasa.gov/SP-4407/vol5/ExploreUnknown.pdf |date=January 20, 2017 }}, Chapter 3, Document III-1, p. 546.</ref> In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes. First, the [[angular resolution]] (the smallest separation at which objects can be clearly distinguished) would be limited only by [[Diffraction-limited system|diffraction]], rather than by the turbulence in the atmosphere, which causes stars to twinkle, known to astronomers as [[astronomical seeing|seeing]]. At that time ground-based telescopes were limited to resolutions of 0.5–1.0 [[arcsecond]]s, compared to a theoretical diffraction-limited resolution of about 0.05 arcsec for an optical telescope with a [[mirror]] {{cvt|2.5|m}} in diameter. Second, a space-based telescope could observe [[infrared]] and ultraviolet light, which are strongly absorbed by the [[atmosphere of Earth]].<ref name=":0" />

Spitzer devoted much of his career to pushing for the development of a space telescope.<ref>{{Cite web|title=Celebrating Lyman Spitzer, the father of PPPL and the Hubble Space Telescope|url=https://research.princeton.edu/news/celebrating-lyman-spitzer-father-pppl-and-hubble-space-telescope|access-date=December 4, 2021|website=Office of the Dean for Research|archive-date=December 7, 2021|archive-url=https://web.archive.org/web/20211207155008/https://research.princeton.edu/news/celebrating-lyman-spitzer-father-pppl-and-hubble-space-telescope|url-status=live}}</ref> In 1962, a report by the U.S. [[National Academy of Sciences]] recommended development of a [[space telescope]] as part of the [[Human spaceflight|space program]], and in 1965, Spitzer was appointed as head of a committee given the task of defining scientific objectives for a large space telescope.<ref>{{cite web |url=http://www.spitzer.caltech.edu/about/spitzer.shtml|title=About Lyman Spitzer, Jr|publisher=Caltech|access-date=April 26, 2008|archive-url=https://web.archive.org/web/20080327091202/http://www.spitzer.caltech.edu/about/spitzer.shtml|archive-date=March 27, 2008}}</ref>

[[File:Nancy Grace Roman with Space Telescope Model in 1966 (27154772837).jpg|thumb|upright|[[Nancy Grace Roman]] with a model of the Large Space Telescope that was eventually developed as the Hubble Space Telescope. While listed as a 1966 photo, this design was not the standard until the mid-1970s.]]
Also crucial was the work of [[Nancy Roman|Nancy Grace Roman]], the "Mother of Hubble".<ref>{{Cite web|last=Smith|first=Yvette|date=May 15, 2020|title=Nancy Grace Roman: The Mother of Hubble|url=http://www.nasa.gov/image-feature/nancy-grace-roman-the-mother-of-hubble-2|access-date=December 4, 2021|website=NASA|archive-date=December 7, 2021|archive-url=https://web.archive.org/web/20211207160733/https://www.nasa.gov/image-feature/nancy-grace-roman-the-mother-of-hubble-2/|url-status=live}}</ref> Well before it became an official [[NASA]] project, she gave public lectures touting the scientific value of the telescope. After it was approved, she became the program scientist, setting up the steering committee in charge of making astronomer needs feasible to implement<ref>{{Cite web|title=Explorer 1 {{!}} Stories {{!}} Nancy Grace Roman|url=https://explorer1.jpl.nasa.gov/stories/nancy-grace-roman/|access-date=December 4, 2021|website=explorer1.jpl.nasa.gov|archive-date=May 31, 2022|archive-url=https://web.archive.org/web/20220531015937/https://explorer1.jpl.nasa.gov/stories/nancy-grace-roman/|url-status=live}}</ref> and writing testimony to [[United States Congress|Congress]] throughout the 1970s to advocate continued funding of the telescope.<ref name=AnnRev>{{Cite journal |author= Roman, Nancy Grace |title= Nancy Grace Roman and the Dawn of Space Astronomy |journal= Annual Review of Astronomy and Astrophysics |volume= 57 |pages= 1–34 |year= 2019 |doi= 10.1146/annurev-astro-091918-104446 |bibcode= 2019ARA&A..57....1R |doi-access= free}}</ref> Her work as project scientist helped set the standards for NASA's operation of large scientific projects.<ref name=WilliamsBook>{{cite book |last= Williams |first= Robert |date= October 1, 2018 |title= Hubble Deep Field and the Distant Universe |url= https://iopscience.iop.org/book/978-0-7503-1756-6 |location= Bristol, UK |publisher= IOP Publishing |pages= 2–9 |isbn= 978-0-7503-1756-6 |archive-date= June 5, 2020 |archive-url= https://web.archive.org/web/20200605012705/https://iopscience.iop.org/book/978-0-7503-1756-6 |url-status= live}}</ref>

Space-based astronomy had begun on a very small scale following [[World War II]], as scientists made use of developments that had taken place in [[rocket]] technology. The first ultraviolet [[electromagnetic spectrum|spectrum]] of the [[Sun]] was obtained in 1946,<ref>{{cite journal|title=Solar Ultraviolet Spectrum to 88 Kilometers|last1=Baum |first1=W. A.|display-authors=4|last2=Johnson |first2=F. S.|last3=Oberly |first3=J. J. |last4=Rockwood |first4=C. C.|last5=Strain |first5=C. V.|last6=Tousey |first6=R.|journal=Physical Review|volume=70|pages=781–782|date=November 1946|doi=10.1103/PhysRev.70.781|issue=9–10 |bibcode=1946PhRv...70..781B}}</ref> and NASA launched the [[Orbiting Solar Observatory]] (OSO) to obtain UV, X-ray, and gamma-ray spectra in 1962.<ref>{{cite web|url=https://heasarc.gsfc.nasa.gov/docs/heasarc/missions/oso1.html|title=The First Orbiting Solar Observatory|date=June 26, 2003|work=heasarc.gsfc.nasa.gov|publisher=NASA Goddard Space Flight Center|access-date=September 25, 2011|archive-date=May 3, 2019|archive-url=https://web.archive.org/web/20190503001707/https://heasarc.gsfc.nasa.gov/docs/heasarc/missions/oso1.html|url-status=live}} {{PD-notice}}</ref> An [[Ariel 1|orbiting solar telescope]] was launched in 1962 by the United Kingdom as part of the [[Ariel programme]], and in 1966 [[NASA]] launched the first [[Orbiting Astronomical Observatory]] (OAO) mission. OAO-1's battery failed after three days, terminating the mission. It was followed by [[Orbiting Astronomical Observatory 2]] (OAO-2), which carried out ultraviolet observations of [[star]]s and [[galaxy|galaxies]] from its launch in 1968 until 1972, well beyond its original planned lifetime of one year.<ref>{{cite web |url=http://nasascience.nasa.gov/missions/oao|title=OAO|publisher=NASA|access-date=April 26, 2008|url-status=dead|archive-url=https://web.archive.org/web/20080916121848/http://nasascience.nasa.gov/missions/oao |archive-date=September 16, 2008}} {{PD-notice}}</ref>

The OSO and OAO missions demonstrated the important role space-based observations could play in astronomy. In 1968, NASA developed firm plans for a space-based [[reflecting telescope]] with a mirror {{cvt|3|m}} in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope (LST), with a launch slated for 1979. These plans emphasized the need for crewed 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.{{sfn|Spitzer|1979|p=32}}

=== 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 scientific 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 [[United States Congress|U.S. 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 led to Congress deleting all funding for the telescope project.{{sfn|Spitzer|1979|pp=33–34}}

In 1977, then NASA Administrator [[James C. Fletcher]] proposed a token $5 million for Hubble in NASA's budget. Then NASA Associate Administrator for Space Science, [[Noel Hinners]], instead cut all funding for Hubble, gambling that this would galvanize the scientific community into fighting for full funding. As Hinners recalls:<ref>{{cite web
|url=https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/NASA_HQ/Administrators/HinnersNW/HinnersNW_8-19-10.htm
|title=NASA Headquarters Oral History Project – Noel W. Hinners
|date=August 19, 2010
|website=Johnson Space Center History Portal
|publisher=NASA
|access-date=July 14, 2022
|archive-date=July 15, 2022
|archive-url=https://web.archive.org/web/20220715150041/https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/NASA_HQ/Administrators/HinnersNW/HinnersNW_8-19-10.htm
|url-status=live
}}</ref>

{{blockquote
|text=It was clear that year that we weren't going to be able to get a full-up start. There was some opposition on [Capitol] Hill to getting a new start on [Hubble]. It was driven, in large part as I recall, by the budget situation. Jim Fletcher proposed that we put in $5 million as a placeholder. I didn't like that idea. It was, in today's vernacular, a "sop" to the astronomy community. "There's something in there, so all is well".

I figured in my own little head that to get that community energized we'd be better off zeroing it out. Then they would say, "Whoa, we're in deep trouble", and it would marshal the troops. So I advocated that we not put anything in. I don't remember any of the detailed discussions or whether there were any, but Jim went along with that so we zeroed it out. It had, from my perspective, the desired impact of stimulating the astronomy community to renew their efforts on the lobbying front. While I like to think in hindsight it was a brilliant political move, I'm not sure I thought it through all that well. It was something that was spur of the moment.

[...] $5 million would let them think that all is well anyway, but it's not. So let's give them a message. My own thinking, get them stimulated to get into action. Zeroing it out would certainly give that message. I think it was as simple as that. Didn't talk to anybody else about doing it first, just, "Let's go do that". Voila, it worked. Don't know whether I'd do that again.
}}

The political ploy worked. In response to Hubble being zeroed out of NASA's budget, a nationwide lobbying effort was coordinated among astronomers. Many astronomers met [[United States House of Representatives|congressmen]] and [[United States Senate|senators]] in person, and large-scale letter-writing campaigns were organized. The [[National Academy of Sciences]] published a report emphasizing the need for a space telescope, and eventually, the Senate agreed to half the budget that had originally been approved by Congress.{{sfn|Spitzer|1979|p=34}}

The funding issues led to 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<ref name="gander">{{cite book|last=Andersen |first=Geoff|title=The telescope: its history, technology, and future|date=2007|publisher=Princeton University Press|isbn=978-0-691-12979-2|page=[https://archive.org/details/telescopeitshist00ande/page/116 116] |url=https://archive.org/details/telescopeitshist00ande/page/116}}</ref> and to allow a more compact and effective configuration for the telescope hardware. A proposed precursor {{cvt|1.5|m}} 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). ESA agreed to provide funding and supply one of the first generation instruments for the telescope, as well as the [[solar cell]]s that would power it, and staff to work on the telescope in the United States, in return for European astronomers being guaranteed at least 15% of the observing time on the telescope.<ref>"Memorandum of Understanding Between The European Space Agency and The United States National Aeronautics and Space Administration", reprinted in [https://history.nasa.gov/SP-4407/vol5/ExploreUnknown.pdf ''NASA SP-2001-4407: Exploring the Unknown''] {{Webarchive|url=https://web.archive.org/web/20170120024958/https://history.nasa.gov/SP-4407/vol5/ExploreUnknown.pdf|date=January 20, 2017}} Chapter 3, Document III-29, p. 671.</ref> Congress eventually approved funding of US$36 million for 1978, and the design of the LST began in earnest, aiming for a launch date of 1983.{{sfn|Spitzer|1979|p=34}} In 1983, the telescope was named after [[Edwin Hubble]],<ref>{{cite web|url=https://history.nasa.gov/hubble/chron.html|last=Okolski|first=Gabriel|title=A Chronology of the Hubble Space Telescope|publisher=NASA|access-date=April 26, 2008|archive-date=June 27, 2008|archive-url=https://web.archive.org/web/20080627010420/http://history.nasa.gov/hubble/chron.html|url-status=live}} {{PD-notice}}</ref> who confirmed one of the greatest scientific discoveries of the 20th century, made by [[Georges Lemaître]], that the [[universe]] is [[expanding universe|expanding]].<ref>{{cite web|url=http://hubble.nasa.gov/overview/conception-part1.php|title=The Path to Hubble Space Telescope|publisher=NASA|access-date=April 26, 2008|url-status=dead|archive-url=https://web.archive.org/web/20080524211736/http://hubble.nasa.gov/overview/conception-part1.php|archive-date=May 24, 2008}} {{PD-notice}}</ref>

=== Construction and engineering ===
[[File:Hubble mirror polishing.jpg|thumb|upright=1.0|right|Grinding of Hubble's primary mirror at Perkin-Elmer, March 1979]]

Once the Space Telescope project had been given the go-ahead, work on the program was divided among many institutions. [[Marshall Space Flight Center]] (MSFC) was given responsibility for the design, development, and construction of the telescope, while [[Goddard Space Flight Center]] was given overall control of the scientific instruments and ground-control center for the mission.{{sfn|Dunar|Waring|1999|pp=487–488}} MSFC commissioned the optics company [[PerkinElmer|Perkin-Elmer]] to design and build the [[Optical train|optical telescope assembly]] (OTA) and Fine Guidance Sensors for the space telescope. [[Lockheed Corporation|Lockheed]] was commissioned to construct and integrate the spacecraft in which the telescope would be housed.{{sfn|Dunar|Waring|1999|p=489}}

=== Optical telescope assembly ===
Optically, the HST is a [[Cassegrain reflector]] of [[Ritchey–Chrétien telescope|Ritchey–Chrétien design]], as are most large professional telescopes. This design, with two hyperbolic mirrors, is known for good imaging performance over a wide field of view, with the disadvantage that the mirrors have shapes that are hard to fabricate and test. The mirror and optical systems of the telescope determine the final performance, and they were designed to exacting specifications. Optical telescopes typically have mirrors polished to an [[accuracy]] of about a tenth of the [[wavelength]] of [[visible light]], but the Space Telescope was to be used for observations from the visible through the ultraviolet (shorter wavelengths) and was specified to be [[diffraction-limited system|diffraction limited]] to take full advantage of the space environment. Therefore, its mirror needed to be polished to an accuracy of 10 nanometers, or about 1/65 of the wavelength of red light.<ref name="ScienceSPF">{{cite journal |last=Waldrop |first=M. M. |date=August 17, 1990 |title=Hubble: The Case of the Single-Point Failure |journal=Science Magazine |volume=249 |issue=4970 |pages=735–736 |bibcode=1990Sci...249..735W |doi=10.1126/science.249.4970.735 |pmid=17756776}}</ref> On the long wavelength end, the OTA was not designed with optimum infrared performance in mind—for example, the mirrors are kept at stable (and warm, about 15&nbsp;°C) temperatures by heaters. This limits Hubble's performance as an infrared telescope.<ref name="IR" />

[[File:A20010288000 NASM2017-10014 (cropped).jpg|thumb|upright=1.0|right|The backup mirror, by Kodak; its inner support structure can be seen because it is not coated with a reflective surface.]]
Perkin-Elmer (PE) intended to use custom-built and extremely sophisticated [[Computer-aided manufacturing|computer-controlled polishing machines]] to grind the mirror to the required shape.{{sfn|Dunar|Waring|1999|p=489}} However, in case their cutting-edge technology ran into difficulties, NASA demanded that PE sub-contract to [[Kodak]] to construct a back-up mirror using traditional mirror-polishing techniques.{{sfn|Allen|Angel|Mangus|Rodney|1990|pp=3–4}} (The team of Kodak and [[Itek]] also bid on the original mirror polishing work. Their bid called for the two companies to double-check each other's work, which would have almost certainly caught the polishing error that later caused [[#Flawed mirror|problems]].)<ref>{{cite news|url=https://query.nytimes.com/gst/fullpage.html?res=9C0CEEDF1731F93BA15754C0A966958260|title=Losing Bid Offered Two Tests on Hubble|agency=Associated Press|date=July 28, 1990|newspaper=The New York Times|access-date=April 26, 2008|archive-date=February 4, 2009|archive-url=https://web.archive.org/web/20090204215644/http://query.nytimes.com/gst/fullpage.html?res=9C0CEEDF1731F93BA15754C0A966958260|url-status=live}}</ref> The Kodak mirror is now on permanent display at the [[National Air and Space Museum]].<ref>{{cite press release|title=Hubble Space Telescope Stand-in Gets Starring Role|date=September 21, 2001|url=http://www.gsfc.nasa.gov/news-release/releases/2001/h01-185.htm|author=Goddard Space Flight Center|publisher=NASA|access-date=April 26, 2008|url-status=dead|archive-url=https://web.archive.org/web/20080226075115/http://www.gsfc.nasa.gov/news-release/releases/2001/h01-185.htm|archive-date=February 26, 2008}} {{PD-notice}}</ref><ref>{{cite web|title=Backup Mirror, Hubble Space Telescope|url=http://www.nasm.si.edu/collections/artifact.cfm?id=A20010288000|publisher=National Air and Space Museum|access-date=November 4, 2012|archive-url=https://web.archive.org/web/20121102124612/http://airandspace.si.edu/collections/artifact.cfm?id=A20010288000 |archive-date=November 2, 2012}}</ref> An Itek mirror built as part of the effort is now used in the 2.4 m telescope at the [[Magdalena Ridge Observatory]].<ref>{{cite tech report|author=Magdalena Ridge Observatory|title=2.4m Observatory Technical Note|date=January 1, 2008|url=http://www.mro.nmt.edu/data/2.4m/doc-public/OTN-Overview.pdf|access-date=January 21, 2013|page=2|version=1.6|archive-date=March 4, 2016|archive-url=https://web.archive.org/web/20160304103937/http://www.mro.nmt.edu/data/2.4m/doc-public/OTN-Overview.pdf|url-status=live}}</ref>

Construction of the Perkin-Elmer mirror began in 1979, starting with a blank manufactured by [[Corning Incorporated|Corning]] from their ultra-low expansion glass. To keep the mirror's weight to a minimum it consisted of top and bottom plates, each {{cvt|25|mm}} thick, sandwiching a [[honeycomb]] lattice. Perkin-Elmer simulated [[microgravity]] by supporting the mirror from the back with 130 rods that exerted varying amounts of force.<ref>{{cite conference|title=Design and fabrication of the NASA 2.4-meter space telescope|first1=Daniel J.|last1=McCarthy|first2=Terence A.|last2=Facey|editor-first1=Paul R. |editor-last1=Yoder, Jr. |work=Proc. SPIE 0330, Optical Systems Engineering II|series=Optical Systems Engineering II |pages=139–143|date=1982|volume=0330 |publisher=International Society for Optics and Photonics|doi=10.1117/12.934268}}</ref> This ensured the mirror's final shape would be correct and to specification when deployed. Mirror polishing 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 moved the launch date of the telescope to October 1984.{{sfn|Dunar|Waring|1999|p=496}} The mirror was completed by the end of 1981; it was washed using {{cvt|9100|L}} of hot, [[deionized water]] and then received a reflective coating of 65&nbsp;nm-thick [[aluminium|aluminum]] and a protective coating of 25&nbsp;nm-thick [[magnesium fluoride]].<ref name="IR">{{cite journal|title=The Performance of HST as an Infrared Telescope|first1=M.|last1=Robberto|first2=A.|last2=Sivaramakrishnan|first3=J. J.|last3=Bacinski|first4=Daniele|last4=Calzetti|first5=J. E.|last5=Krist|first6=J. W.|last6=MacKenty|first7=J.|last7=Piquero|first8=M.|last8=Stiavelli|journal=Proc. SPIE|volume=4013|pages=386–393|date=2000|doi=10.1117/12.394037|series=UV, Optical, and IR Space Telescopes and Instruments|editor1-last=Breckinridge|editor1-first=James B.|editor2-last=Jakobsen|editor2-first=Peter|bibcode=2000SPIE.4013..386R|citeseerx=10.1.1.358.1298 |s2cid=14992130}} {{PD-notice}}</ref><ref>{{cite book|title=The Space Telescope|publisher=Michael Friedman|location=New York|first=David|last=Ghitelman|page=[https://archive.org/details/spacetelescope00ghit/page/32 32]|date=1987 |isbn=978-0-8317-7971-9|url=https://archive.org/details/spacetelescope00ghit/page/32}}</ref>

[[File:Early stages of Hubble construction.jpg|thumb|upright=1.0|right|The OTA, metering truss, and secondary baffle are visible in this image of Hubble during early construction.]]
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 March and then September 1986. By this time, the total project budget had risen to US$1.175 billion.{{sfn|Dunar|Waring|1999|p=504}}

=== Spacecraft systems ===
The spacecraft in which the telescope and instruments were to be housed was another major engineering challenge. It would have to withstand frequent passages from direct sunlight into the darkness of Earth's [[shadow]], which would cause major changes in temperature, while being stable enough to allow extremely accurate pointing of the telescope. A shroud of [[multi-layer insulation]] keeps the temperature within the telescope stable and surrounds a light aluminum shell in which the telescope and instruments sit. Within the shell, a [[carbon fiber reinforced polymer|graphite-epoxy]] frame keeps the working parts of the telescope firmly aligned.<ref>{{cite web|url=http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01/hubble_space_telescope_systems.htm|title=Hubble Space Telescope Systems|publisher=Goddard Space Flight Center|access-date=April 26, 2008|url-status=dead|archive-url=https://web.archive.org/web/20030317035553/http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01/hubble_space_telescope_systems.htm|archive-date=March 17, 2003}} {{PD-notice}}</ref> Because graphite composites are [[hygroscopic]], there was a risk that water vapor absorbed by the truss while in Lockheed's clean room would later be expressed in the vacuum of space; resulting in the telescope's instruments being covered by ice. To reduce that risk, a nitrogen gas purge was performed before launching the telescope into space.<ref>Ghitelman, David (1987) ''The Space Telescope,'' New York: Michael Friedman Publishing, p. 50.</ref>

As well as electrical power systems, the '''Pointing Control System''' controls HST orientation using five types of sensors (magnetic sensors, optical sensors, and six gyroscopes) and two types of [[actuator]]s ([[reaction wheel]]s and [[magnetic torquer]]s).<ref name=PCS/>

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 experienced some budget and schedule slippage, and by the summer 1985, construction of the spacecraft was 30% over budget and three months behind schedule. An MSFC report said Lockheed tended to rely on NASA directions rather than take their own initiative in the construction.{{sfn|Dunar|Waring|1999|p=508}}

=== Computer systems and data processing ===
[[File:DF-224.jpg|thumb|upright=1.2|right|DF-224 in Hubble, before it was replaced in 1999]]

The two initial, primary computers on the HST were the 1.25 [[Hertz|MHz]] [[DF-224]] system, built by Rockwell Autonetics, which contained three redundant CPUs, and two redundant [[NSSC-1]] (NASA Standard Spacecraft Computer, Model 1) systems, developed by [[Westinghouse Electric Corporation|Westinghouse]] and GSFC using [[diode–transistor logic]] (DTL). A co-processor for the DF-224 was added during Servicing Mission 1 in 1993, which consisted of two redundant strings of an Intel-based 80386 processor with an 80387 math co-processor.<ref>{{cite web|url=http://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/facts/CoProcessor.pdf|title=Co-Processor|series=NASA Facts|publisher=NASA|date=June 1993|id=NF-193|access-date=May 16, 2016|archive-date=July 23, 2012|archive-url=https://web.archive.org/web/20120723055334/http://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/facts/CoProcessor.pdf|url-status=live}} {{PD-notice}}</ref> The DF-224 and its 386 co-processor were replaced by a 25&nbsp;MHz Intel-based 80486 processor system during [[#Servicing Mission 3A|Servicing Mission 3A]] in 1999.<ref>{{cite web|url=http://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/facts/FS09.pdf|title=Hubble Space Telescope Servicing Mission 3A: New Advanced Computer|series=NASA Facts|publisher=NASA|date=1999|id=FS-1999-06-009-GSFC|access-date=May 16, 2016|archive-date=May 9, 2016|archive-url=https://web.archive.org/web/20160509132748/http://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/facts/FS09.pdf|url-status=live}}</ref> The new computer is 20 times faster, with six times more memory, than the [[DF-224]] it replaced. It increases throughput by moving some computing tasks from the ground to the spacecraft and saves money by allowing the use of modern programming languages.<ref>{{cite tech report|url=https://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/SM3A-MediaGuide.pdf|title=Hubble Space Telescope Servicing Mission 3A Media Reference Guide|publisher=NASA|author=Lockheed Martin Missiles and Space|access-date=April 7, 2022|pages=5–9 and Section 7.1.1|archive-date=November 25, 2011|archive-url=https://web.archive.org/web/20111125161422/http://hubble.nasa.gov/a_pdf/news/SM3A-MediaGuide.pdf|url-status=live}} {{PD-notice}}</ref>

Additionally, some of the science instruments and components had their own embedded microprocessor-based control systems. The MATs (Multiple Access Transponder) components, MAT-1 and MAT-2, use Hughes Aircraft CDP1802CD microprocessors.<ref>{{cite journal|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160005759.pdf|title=How Long Can the Hubble Space Telescope Operate Reliably? A Total Dose Perspective|journal=IEEE Transactions on Nuclear Science|first1=M. A.|last1=Xapsos|first2=C.|last2=Stauffer|first3=T.|last3=Jordan|first4=C.|last4=Poivey|first5=D. N.|last5=Haskins|first6=G.|last6=Lum|first7=A. M.|last7=Pergosky|first8=D. C.|last8=Smith|first9=K. A.|last9=LaBel|volume=61|issue=6|pages=3356–3362|date=December 2014|bibcode=2014ITNS...61.3356X|doi=10.1109/TNS.2014.2360827|hdl=2060/20160005759|s2cid=1792941|hdl-access=free|access-date=July 7, 2017|archive-date=February 27, 2017|archive-url=https://web.archive.org/web/20170227173247/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160005759.pdf|url-status=live}} {{PD-notice}}</ref> The [[Wide Field and Planetary Camera]] (WFPC) also used an [[RCA 1802|RCA 1802 microprocessor]] (or possibly the older 1801 version).<ref>{{cite magazine|url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/35164/1/93-0731.pdf|title=Hubble Space Telescope's Wide Field/Planetary Camera|magazine=Shutterbug|first=A.|last=Afshari|date=January 1993|url-status=dead|archive-url=https://web.archive.org/web/20161006205644/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/35164/1/93-0731.pdf|archive-date=October 6, 2016}} {{PD-notice}}</ref> The WFPC-1 was replaced by the [[Wide Field and Planetary Camera 2|WFPC-2]] during Servicing Mission 1 in 1993, which was then replaced by the [[Wide Field Camera 3]] (WFC3) during Servicing Mission 4 in 2009. The upgrade extended Hubble's capability of seeing deeper into the universe and providing images in three broad regions of the spectrum.<ref>{{Cite web|title=The 'Camera That Saved Hubble'|url=https://www.jpl.nasa.gov/news/the-camera-that-saved-hubble|access-date=November 27, 2021|website=NASA Jet Propulsion Laboratory (JPL)|archive-date=November 27, 2021|archive-url=https://web.archive.org/web/20211127133956/https://www.jpl.nasa.gov/news/the-camera-that-saved-hubble|url-status=live}}</ref><ref>{{Cite web|last=Garner|first=Rob|date=August 22, 2016|title=Hubble Space Telescope – Wide Field Camera 3|url=http://www.nasa.gov/content/hubble-space-telescope-wide-field-camera-3|access-date=November 27, 2021|website=NASA|archive-date=November 13, 2021|archive-url=https://web.archive.org/web/20211113213252/https://www.nasa.gov/content/hubble-space-telescope-wide-field-camera-3/|url-status=live}}</ref>

=== Initial instruments ===
{{Main|Wide Field and Planetary Camera|Goddard High Resolution Spectrograph|High Speed Photometer|Faint Object Camera|Faint Object Spectrograph}}
[[File:HubbleExploded edit 1.svg|thumb|upright=1.8|right|[[Exploded view]] of the Hubble Space Telescope]]

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 used a radial instrument bay, and the other four instruments were each installed in an axial instrument bay.<ref name="CP-2244"/>

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 [[filter (optics)|filters]] isolating [[spectral line]]s of particular astrophysical interest. The instrument contained eight [[charge-coupled device]] (CCD) chips divided between two cameras, each using four CCDs. Each CCD has a resolution of 0.64 megapixels.<ref name=wfpc2>{{cite web|url=https://esahubble.org/about/general/instruments/wfpc2/|title=Hubble's Instruments: WFPC2 Wide Field Planetary Camera 2|website=esahubble.org|publisher=[[European Space Agency]]|access-date=April 7, 2022|archive-date=April 7, 2022|archive-url=https://web.archive.org/web/20220407073748/https://esahubble.org/about/general/instruments/wfpc2/|url-status=live}}</ref> The wide field camera (WFC) covered a large angular field at the expense of resolution, while the planetary camera (PC) took images at a longer effective [[focal length]] than the WF chips, giving it a greater magnification.<ref name="CP-2244">{{cite tech report|title=The Space Telescope Observatory|number=CP-2244|publisher=NASA|editor-first=Donald N. B.|editor-last=Hall|url=https://ntrs.nasa.gov/citations/19820025420|date=1982|access-date=April 7, 2022|archive-date=April 7, 2022|archive-url=https://web.archive.org/web/20220407073748/https://ntrs.nasa.gov/citations/19820025420|url-status=live}}</ref>

The [[Goddard High Resolution Spectrograph]] (GHRS) was a [[spectrograph]] designed to operate in the ultraviolet. It was built by the Goddard Space Flight Center and could achieve a [[spectral resolution]] of 90,000.<ref>{{cite journal |author=Brandt |first1=J. C. |last2=Heap |first2=S. R. |last3=Beaver |first3=E. A. |last4=Boggess |first4=A. |last5=Carpenter |first5=K. G. |last6=Ebbets |first6=D. C. |last7=Hutchings |first7=J. B. |last8=Jura |first8=M. |last9=Leckrone |first9=D. S. |date=1994 |title=The Goddard High Resolution Spectrograph: Instrument, goals, and science results |journal=Publications of the Astronomical Society of the Pacific |volume=106 |pages=890–908 |bibcode=1994PASP..106..890B |doi=10.1086/133457 |doi-access=|s2cid=120181145 }}</ref> Also optimized for ultraviolet observations were the FOC and FOS, which were capable of the highest spatial resolution of any instruments on Hubble. Rather than CCDs, these three instruments used [[photon]]-counting [[digicon]]s as their detectors. The FOC was constructed by ESA, while the [[University of California, San Diego]], and [[Martin Marietta Corporation]] built the FOS.<ref name="CP-2244"/>

The final instrument was the HSP, designed and built at the [[University of Wisconsin–Madison]]. It was optimized for visible and ultraviolet light observations of [[variable star]]s and other astronomical objects varying in brightness. It could take up to 100,000 measurements per second with a [[photometry (astronomy)|photometric]] accuracy of about 2% or better.<ref>Bless, R. C.; Walter, L. E.; White R. L. (1992) ''High Speed Photometer Instrument Handbook'' v 3.0 STSci.</ref>

HST's guidance system can also be used as a scientific instrument. Its three [[Fine guidance sensor|Fine Guidance Sensor]]s (FGS) are primarily used to keep the telescope accurately pointed during an observation, but can also be used to carry out extremely accurate [[astrometry]]; measurements accurate to within 0.0003 arcseconds have been achieved.<ref>{{cite conference |author=Benedict |first1=G. Fritz |last2=McArthur |first2=Barbara E. |date=2005 |editor-last=Kurtz |editor-first=D. W. |title=High-precision stellar parallaxes from Hubble Space Telescope fine guidance sensors |url=https://pdfs.semanticscholar.org/ce07/0e358a8f3ed7a8bb7b470fc2986c8833d3f0.pdf |conference=IAU Colloquium #196 |publisher=Cambridge University Press |pages=333–346 |bibcode=2005tvnv.conf..333B |doi=10.1017/S1743921305001511 |archive-url=https://web.archive.org/web/20200227131024/https://pdfs.semanticscholar.org/ce07/0e358a8f3ed7a8bb7b470fc2986c8833d3f0.pdf |archive-date=February 27, 2020 |work=Transits of Venus: New Views of the Solar System and Galaxy |s2cid=123078909 |url-status=dead}}</ref>

=== Ground support ===
{{Main|Space Telescope Science Institute}}
[[File:Hubble Control Centre.jpg|thumb|upright=1.0|right|Hubble Control Center at Goddard Space Flight Center, 1999]]

The Space Telescope Science Institute (STScI) is responsible for the scientific operation of the telescope and the delivery of data products to astronomers. STScI is operated by the [[Association of Universities for Research in Astronomy]] (AURA) and is physically located in [[Baltimore, Maryland]] on the Homewood campus of [[Johns Hopkins University]], one of the 39 U.S. universities and seven international affiliates that make up the AURA consortium. STScI was established in 1981<ref name="Edmondson1997">{{cite book |url=https://books.google.com/books?id=jEurac1jvsAC&pg=PA244 |title=AURA and Its US National Observatories |publisher=Cambridge University Press |first=Frank K. |last=Edmondson |page=244 |date=1997 |isbn=978-0-521-55345-2 |access-date=January 7, 2022 |archive-date=July 15, 2022 |archive-url=https://web.archive.org/web/20220715150041/https://books.google.com/books?id=jEurac1jvsAC&pg=PA244 |url-status=live }}</ref><ref>{{cite web |url=http://www.aura-astronomy.org/about.asp |title=About AURA |publisher=AURA |access-date=November 6, 2012 |archive-date=September 29, 2018 |archive-url=https://web.archive.org/web/20180929155829/http://www.aura-astronomy.org/about.asp |url-status=live }}</ref> after something of a power struggle between NASA and the scientific community at large. NASA had wanted to keep this function in-house, but scientists wanted it to be based in an [[academia|academic]] establishment.{{sfn|Dunar|Waring|1999|pp=486–487}}<ref>Roman, Nancy Grace. "Exploring the Universe: Space-Based Astronomy and Astrophysics", in [https://history.nasa.gov/SP-4407/vol5/ExploreUnknown.pdf ''NASA SP-2001-4407: Exploring the Unknown''] {{Webarchive|url=https://web.archive.org/web/20170120024958/https://history.nasa.gov/SP-4407/vol5/ExploreUnknown.pdf |date=January 20, 2017 }} (PDF). NASA. Chapter 3, p. 536.</ref> The [[Space Telescope European Coordinating Facility]] (ST-ECF), established at [[Garching bei München]] near [[Munich]] in 1984, provided similar support for European astronomers until 2011, when these activities were moved to the European Space Astronomy Centre.<ref>{{Cite web |title=Closure of ST-ECF |url=http://www.stecf.org/ECFclosure.php |access-date=April 7, 2022 |website=www.stecf.org}}</ref>

One complex task that falls to STScI is scheduling observations for the telescope.<ref name="Team Hubble">{{cite web |url=https://www.stsci.edu/hst/observing/scheduling |title=Scheduling |publisher=[[Space Telescope Science Institute]] |website=stsci.edu |access-date=April 7, 2022 |archive-date=July 15, 2022 |archive-url=https://web.archive.org/web/20220715144805/https://www.stsci.edu/hst/observing/scheduling |url-status=live }}</ref> Hubble is in a low-Earth orbit to enable servicing missions, which results in most astronomical targets being [[occultation|occulted]] by the Earth for slightly less than half of each orbit. Observations cannot take place when the telescope passes through the [[South Atlantic Anomaly]] due to elevated [[radiation]] levels, and there are also sizable exclusion zones around the Sun (precluding observations of [[Mercury (planet)|Mercury]]), Moon and Earth. The solar avoidance angle is about 50°, to keep sunlight from illuminating any part of the OTA. Earth and Moon avoidance keeps bright light out of the FGSs, and keeps scattered light from entering the instruments. If the FGSs are turned off, the Moon and Earth can be observed. Earth observations were used very early in the program to generate flat-fields for the WFPC1 instrument. There is a so-called continuous viewing zone (CVZ), within roughly 24° of Hubble's [[orbital pole]]s, in which targets are not [[occultation|occulted]] for long periods.<ref>{{Cite web |date=January 15, 1996 |title=Hubble's Deepest View of the Universe Unveils Bewildering Galaxies across Billions of Years |url=http://hubblesite.org/contents/news-releases/1996/news-1996-01 |access-date=April 7, 2022 |website=HubbleSite.org |publisher=[[Space Telescope Science Institute]] |language=en |archive-date=July 15, 2022 |archive-url=https://web.archive.org/web/20220715150044/https://hubblesite.org/contents/news-releases/1996/news-1996-01.html |url-status=live }}</ref><ref>{{Cite journal |last1=Adler |first1=David S. |last2=Kinzel |first2=Wayne |last3=Jordan |first3=Ian |date=August 6, 2014 |editor-last=Peck |editor-first=Alison B. |editor2-last=Benn |editor2-first=Chris R. |editor3-last=Seaman |editor3-first=Robert L. |title=Planning and scheduling at STScI: from Hubble to the James Webb Space Telescope |url=http://proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.2054932 |journal=Proc. SPIE 9149, Observatory Operations: Strategies, Processes, and Systems V |series=Observatory Operations: Strategies, Processes, and Systems V |volume=9149 |location=Montréal, Quebec, Canada |pages=91490D |doi=10.1117/12.2054932 |bibcode=2014SPIE.9149E..0DA |s2cid=122694163 |access-date=July 15, 2022 |archive-date=July 15, 2022 |archive-url=https://web.archive.org/web/20220715150044/https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9149/1/Planning-and-scheduling-at-STScI--from-Hubble-to-the/10.1117/12.2054932.short?SSO=1 |url-status=live }}</ref><ref>{{Cite web |title=HST Cycle 26 Primer Orbital Constraints – HST User Documentation |url=https://hst-docs.stsci.edu/hsp/past-hst-proposal-opportunities/the-hubble-space-telescope-primer-for-cycle-26/hst-cycle-26-primer-orbital-constraints |access-date=July 16, 2022 |website=hst-docs.stsci.edu |archive-date=July 16, 2022 |archive-url=https://web.archive.org/web/20220716221122/https://hst-docs.stsci.edu/hsp/past-hst-proposal-opportunities/the-hubble-space-telescope-primer-for-cycle-26/hst-cycle-26-primer-orbital-constraints |url-status=live }}</ref>

{{multiple image
| align             = right
| direction         = horizontal
| total_width       = 400
| image1            = Diagram of Hubble's orbit.jpg
| caption1          = Hubble's low orbit means many targets are visible for slightly more than half of an orbit's elapsed time, since they are blocked from view by the [[Earth]] for almost one-half of each orbit.
| image2            = Animation of Hubble Space Telescope trajectory.gif
| caption2          = Animation of Hubble's orbit from October 31, 2018, to December 25, 2018; Earth is not shown.
}}
Due to the [[precession]] of the orbit, the location of the CVZ moves slowly over a period of eight weeks. Because the [[wikt:limb#Etymology 2|limb]] of the Earth is always within about 30° of regions within the CVZ, the brightness of scattered [[earthshine]] may be elevated for long periods during CVZ observations. Hubble orbits in low Earth orbit at an altitude of approximately {{convert|540|km|sp=us}} and an inclination of 28.5°.<ref name="heavens-above" /> The position along its orbit changes over time in a way that is not accurately predictable. The density of the upper atmosphere varies according to many factors, and this means Hubble's predicted position for six weeks' time could be in error by up to {{convert|4000|km|abbr=on}}. Observation schedules are typically finalized only a few days in advance, as a longer lead time would mean there was a chance the target would be unobservable by the time it was due to be observed.{{sfn|Strolger|Rose|2017|p=46}} Engineering support for HST is provided by NASA and contractor personnel at the Goddard Space Flight Center in [[Greenbelt, Maryland]], {{convert|48|km|abbr=on}} south of the STScI. Hubble's operation is monitored 24&nbsp;hours per day by four teams of flight controllers who make up Hubble's Flight Operations Team.<ref name="Team Hubble" />

=== ''Challenger'' disaster, delays, and eventual launch ===
[[File:STS-31 Hubble launch roll and pitch.jpg|thumb|upright|[[STS-31]] lifting off, carrying Hubble into orbit]]
[[File:1990 s31 IMAX view of HST release.jpg|thumb|Hubble being deployed from ''Discovery'' in 1990]]

By January 1986, the planned launch date for Hubble that October looked feasible, but the [[Space Shuttle Challenger disaster|''Challenger'' disaster]] brought the U.S. space program to a halt, grounded the Shuttle fleet, and forced the launch to be postponed for several years. During this delay the telescope was kept in a clean room, powered up and purged with nitrogen, until a launch could be rescheduled. This costly situation (about {{US$|6 million}} per month) pushed the overall costs of the project higher. However, this delay allowed time for engineers to perform extensive tests, swap out a possibly failure-prone battery, and make other improvements.{{sfn|Tatarewicz|1998|p=371}} Furthermore, the ground software needed to control Hubble was not ready in 1986, and was barely ready by the 1990 launch.<ref>{{cite news |title=Telescope Is Set to Peer at Space and Time |first=John |last=Wilford |url=https://query.nytimes.com/gst/fullpage.html?res=9C0CE3D6153AF93AA35757C0A966958260&sec=&spon=&pagewanted=all |work=The New York Times |date=April 9, 1990 |access-date=January 19, 2009 |archive-date=November 11, 2012 |archive-url=https://web.archive.org/web/20121111141710/http://www.nytimes.com/1990/04/09/us/telescope-is-set-to-peer-at-space-and-time.html |url-status=live }}</ref> Following the resumption of shuttle flights, {{OV|Discovery}} successfully launched the Hubble on April 24, 1990, as part of the STS-31 mission.<ref>{{cite web |url=http://science.ksc.nasa.gov/shuttle/missions/sts-31/mission-sts-31.html |title=STS-31 |publisher=NASA |access-date=April 26, 2008 |archive-date=August 15, 2011 |archive-url=https://web.archive.org/web/20110815191242/http://science.ksc.nasa.gov/shuttle/missions/sts-31/mission-sts-31.html |url-status=live }}</ref>

At launch, NASA had spent approximately {{US$|4.7 billion}} in inflation-adjusted 2010 dollars on the project.<ref>{{cite web |url=http://www.nasa.gov/pdf/499224main_JWST-ICRP_Report-FINAL.pdf |title=James Webb Space Telescope (JWST) Independent Comprehensive Review Panel (ICRP) Final Report |page=32 |publisher=NASA |access-date=April 7, 2022 |archive-date=November 17, 2021 |archive-url=https://web.archive.org/web/20211117185346/https://www.nasa.gov/pdf/499224main_JWST-ICRP_Report-FINAL.pdf |url-status=live }}</ref> Hubble's cumulative costs are estimated to be about {{US$|11.3 billion}} in 2015 dollars, which include all subsequent servicing costs, but not ongoing operations, making it the most expensive science mission in NASA history.<ref>{{cite book |title=Powering Science: NASA's Large Strategic Science Missions |url=https://nap.nationalacademies.org/catalog/24857/powering-science-nasas-large-strategic-science-missions |publisher=The National Academies of Sciences, Engineering, and Medicine |page=11, footnote 4 |doi=10.17226/24857 |date=2017 |isbn=978-0-309-46383-6 |access-date=April 7, 2022 |archive-date=April 21, 2022 |archive-url=https://web.archive.org/web/20220421203447/https://nap.nationalacademies.org/catalog/24857/powering-science-nasas-large-strategic-science-missions |url-status=live }}</ref>