Editing Hubble Space Telescope (section)

=== 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}}