View source for Optical Telescope Element

{{Short description|Subsection of the James Webb Space Telescope}}
[[File:JWST Full Mirror.jpg|thumb|300px|The mirror assembly from the front with primary mirrors attached, November 2016]]
[[File:Engineers Clean JWST Secondary Reflector with Carbon Dioxide Snow.jpg|thumb|The secondary mirror being cleaned with [[Carbon dioxide cleaning|carbon dioxide snow]]]]

<!-- [[File:Four types of mirrors on the Webb telescope.jpg|thumb|440px|Examples of the different types of mirrors on the JWST/OTE]]-->
'''Optical Telescope Element''' ('''OTE''') is a sub-section of the [[James Webb Space Telescope]], a large infrared space telescope launched on {{#statements:UTC date of spacecraft launch|from=Q108476255}},<ref name="nasa.gov">{{cite web|url=http://jwst.nasa.gov/ote.html|title=The James Webb Space Telescope|publisher=NASA|access-date=30 July 2018}}</ref> consisting of its [[Primary mirror|main mirror]], [[secondary mirror]]s, the framework and controls to support the mirrors, and various thermal and other systems.<ref name="nasa.gov"/>

The OTE collects the light and sends it to the science instruments in Webb's [[Integrated Science Instrument Module]].<ref name="nasa.gov"/> The OTE has been compared to being the "[[eye]]" of the telescope and the backplane of it to being the "[[Vertebral column|spine]]".<ref>{{cite web|url=http://jwst.nasa.gov/observatory.html|title=The James Webb Space Telescope|access-date=5 December 2016}}</ref>

The primary mirror is a tiled assembly of 18 [[hexagon]]al elements, each {{convert|1.32|m|ft|sp=us}} from flat to flat. This combination yields an effective [[aperture]] of {{convert|6.5|m|ft|sp=us}} and a total collecting surface of {{convert|27|m2|ft2|sp=us}}.<ref>{{cite web|url=https://jwst.nasa.gov/mirrors.html|author=NASA|title=JWST Innovations: The Primary Mirror|quote=Each of the 18 hexagonal-shaped mirror segments is 1.32 meters (4.3 feet) in diameter, flat to flat.|access-date=2 August 2017}}</ref> Secondary mirrors complete [[anastigmat]]ic imaging optics with effective {{f/}}20 [[focal ratio]] and [[focal length]] of {{convert|131.4|m|ft|sp=us}}.<ref name="JWST User Documentation">{{cite web|url=https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-telescope|title=JWST Telescope – JWST User Documentation|access-date=27 January 2022}}</ref> The main three-mirror telescope is a [[Three-mirror anastigmat#Korsch telescope|Korsch]]-type design,<ref name="NTRS - NASA Technical Reports Server">{{cite web|url=https://ntrs.nasa.gov/api/citations/20080040728/downloads/20080040728.pdf|title=Optical Modeling Activities for NASA's James Webb Space Telescope (JWST): V. Operational Alignment Updates |access-date=27 January 2022}}</ref>  and it feeds into the Aft Optics Subsystem (part of OTE), which in turn feeds into the Integrated Science Instrument Module which holds the science instruments and fine guidance sensor.

The other two major sections of the JWST are the Integrated Science Instrument Module (ISIM) and the Spacecraft Element (SE), which includes the [[spacecraft bus (JWST)|spacecraft bus]] and [[sunshield (JWST)|sunshield]].<ref name=isimnasa>{{cite web|url=http://jwst.nasa.gov/isim.html|title=The James Webb Space Telescope - Instruments and ISIM|access-date=November 3, 2022|archive-date=December 3, 2016|archive-url=https://web.archive.org/web/20161203070235/http://jwst.nasa.gov/isim.html|url-status=dead}}</ref> The components of OTE were integrated by [[L3Harris Technologies]] to form the final system.<ref name="l3harris.com">{{cite web|url=https://www.l3harris.com/all-capabilities/james-webb-space-telescope|title=L3Harris James Webb Space Telescope Summary|access-date=November 3, 2022}}</ref>

==Overview==
The OTE combines a large amount of the optics and structural components of the James Webb Space Telescope, including the Main mirror.<ref name="osa-opn.org"/> It also has the fine steering mirror, which provides that final precise pointing, and it works in conjunction with the [[Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph|fine guidance sensor]] and other controls systems and sensors in the [[spacecraft bus (James Webb Space Telescope)|spacecraft bus]].<ref name="osa-opn.org"/>

The main mirror segments are aligned roughly using a coarse phasing [[algorithm]].<ref name="osa-opn.org"/> Then for finer alignment, special optical devices inside [[NIRCam]] are used to conduct a phase retrieval technique, to achieve designed [[wavefront]] error of less than 150&nbsp;nm.<ref name="osa-opn.org"/> To function as focusing mirror correctly the 18 main mirror segments need to be aligned very closely to perform as one.<ref name="osa-opn.org"/> This needs to be done in outer space, so extensive testing on [[Earth]] is required to ensure that it will work properly.<ref name="osa-opn.org"/> To align each mirror segment, it is mounted to six actuators that can adjust that segment in 5&nbsp;nm steps.<ref name="osa-opn.org"/> One reason the mirror was divided into segments is that it cuts down on weight, because a mirror's weight is related to its size, which is also one of the reasons beryllium was chosen as the mirror material because of its low weight.<ref name="osa-opn.org"/> Although in the essentially weightless environment of space the mirror will weigh hardly anything, it needs to be very stiff to maintain its shape.<ref name="osa-opn.org"/> The ''Wavefront sensing and control'' sub-system is designed to make the 18 segment primary mirror behave as a monolithic (single-piece) mirror, and it does this in part by actively sensing and correcting for errors.<ref name="nasa.gov1"/> There are nine distance alignment processes that the telescope goes through to achieve this.<ref name="nasa.gov1"/> Another important aspect to the adjustments is that the primary mirror backplane assembly is steady.<ref name="nasa.gov2">{{cite web|url=http://jwst.nasa.gov/backplane.html|title=The James Webb Space Telescope Innovations - Backplane|access-date=November 3, 2022}}</ref> The backplane assembly is made of graphite composite, [[invar]], and [[titanium]].<ref name="nasa.gov2"/>

The ADIR, ''Aft Deployable Infrared Radiator'' is a [[radiator]] behind the main mirror, that helps keep the telescope cool.<ref name="tdl.org">{{cite journal|title=High Performance Cryogenic Radiators for James Webb Space Telescope |url=https://ttu-ir.tdl.org/ttu-ir/bitstream/handle/2346/67554/ICES_2016_141.v2.pdf?sequence=1|journal=46th International Conference on Environmental Systems|date=July 10, 2016}}</ref> There are two ADIR's and they are made of high-purity aluminum.<ref name="tdl.org"/> There is a special black coating on the radiators that helps them emit heat into space.<ref name="tdl.org"/>

[[File:ASA'S Webb Telescope Team Completes Optical Milestone (8411148241).jpg|thumb|Testing of the ''Aft Optic Subsytem'' in 2011, which contains the Tertiary (3rd) mirror and ''Fine Steering Mirror'']]
Some major parts of the OTE according to NASA:<ref name="nasa.gov"/>
* [[Primary mirror]] (18 segments)
* [[Secondary mirror]] ({{cvt|74|cm|in}} diameter)
* Tertiary mirror (3rd) (in ''Aft Optics Subsystem'')
* Fine Steering Mirror (in ''Aft Optics Subsystem'')
* Telescope structure
** primary mirror backplane assembly
** main backplane support fixture (BSF)
** secondary mirror support structure
** deployable tower array
* Thermal Management Subsystem
* Aft Deployable ISIM Radiator (ADIR)
* Wavefront sensing and control

The  Aft Optics Subsystem includes the Tertiary mirror and the Fine Steering Mirror.<ref name="nasa.gov"/> One of the tasks for the Fine steering mirror is image stabilization.<ref name="eoportal.org1">{{cite web|url=https://directory.eoportal.org/web/eoportal/satellite-missions/j/jwst|title=JWST – eoPortal Directory – Satellite Missions|access-date=5 December 2016}}</ref>

The metal [[beryllium]] was chosen for a number of reasons including weight, but also for its low-temperature [[coefficient of thermal expansion]] compared to glass.<ref name="eoportal.org1"/> Furthermore beryllium is not magnetic and a good conductor of electricity and heat.<ref>{{cite web|url=https://webb.nasa.gov/content/observatory/ote/mirrors/index.html|title=James Webb Space Telescope - Webb's Mirrors|publisher=NASA.gov|access-date=November 3, 2022}}</ref> Other infrared telescopes that have used beryllium mirrors include [[IRAS]], [[Cosmic Background Explorer|COBE]], and [[Spitzer Space Telescope|Spitzer]].<ref name="eoportal.org1"/> The  Subscale Beryllium Model Demonstrator (SBMD) was successfully tested at cryogenic temperatures, and one of the concerns was surface roughness at low kelvin numbers.<ref name="eoportal.org1"/> The beryllium mirrors are coated with a very fine layer of [[gold]] to reflect infrared light.<ref name="eoportal.org1"/><ref name="armaghplanet.com">{{cite web|url=https://armaghplanet.com/jwst-a-giant-cyclops-to-reveal-the-depths-of-space.html |archive-url=https://web.archive.org/web/20170701032108/www.armaghplanet.com/blog/jwst-a-giant-cyclops-to-reveal-the-depths-of-space.html |archive-date=2017-07-01 |title=JWST: A Giant Cyclops to Reveal the Depths of Space|date=10 June 2014 |access-date=1 July 2017}}</ref> There are 18 hexagonal segments that are grouped together to create a single mirror with an overall diameter  of {{convert|6.5|m|ft}}.<ref name="armaghplanet.com"/>

==DTA==
[[File:The Secrets of NASA's Webb Telescope’s "Deployable Tower Assembly" (21274602675).jpg|thumb|The Deployable Tower Assembly (DTA)  is where OTE connects with the rest of telescope such as the spacecraft bus. During stowage there is another attachment point for the folded sunshield higher up on the OTE]]
At the base of the OTE is the critical Deployable Tower Assembly (DTA).<ref name="esmats.eu">{{cite web|last=Tran|first=Anh|title=James Webb Space Telescope Deployment Tower Assembly Deploying Anomaly and Lessons Learned|url=http://esmats.eu/esmatspapers/pastpapers/pdfs/2015/tran.pdf|access-date=November 3, 2022}}</ref> component which connects the OTE to the spacecraft bus. It must expand to allow the [[Sunshield (JWST)]] to spread out, allowing the space between the five layers to expand.<ref name="esmats.eu"/> The sunshield segment has multiple components, including six spreaders at the outer edge to spread the layers out at the six extremities.

During launch the DTA is shrunk down, but must extend at the right moment.<ref name="esmats.eu"/> The extended DTA structure allows the sun shield layers to be fully spread-out.<ref name="esmats.eu"/> The DTA must also thermally isolate the cold section of the OTE from the hot spacecraft bus.<ref name="esmats.eu"/> The Sunshield will protect the OTE from direct sunlight and reduce the thermal radiation hitting it, but another aspect is the OTE's physical connection to the rest of the spacecraft.<ref name="esmats.eu"/> (see [[Thermal conduction]] and [[Heat transfer]]) Whereas the sunshield stops the telescope getting hot due to radiated heat from the Sun, the DTA must insulate the telescope from the heat of the rest of the structure, similar to the way an insulated pan handle protects from the heat of a stove.

The DTA extends by means of two telescoping tubes that can slide between each other on rollers.<ref name="esmats.eu"/> There is an inner tube and an outer tube.<ref name="esmats.eu"/> The DTA is extended by an electric motor that rotates a [[ball screw]] nut which pushes the two tubes apart.<ref name="esmats.eu"/> When the DTA is fully deployed it is {{convert|10|ft|m|order=flip|sp=us|sigfig=1}} long.<ref>{{Cite news|url=https://www.nasa.gov/feature/goddard/the-secrets-of-nasas-webb-telescope-s-deployable-tower-assembly/|title=The Secrets of NASA's Webb Telescope's "Deployable Tower Assembly"|last=Jenner|first=Lynn|date=2015-09-09|newspaper=NASA|language=en|access-date=2017-01-21}}</ref> The DTA tubes are made of graphite-composite carbon fiber, and it is intended that they will be able to survive the conditions in space.<ref>{{cite web|url=https://www.nasa.gov/feature/goddard/the-secrets-of-nasas-webb-telescope-s-deployable-tower-assembly/|title=The Secrets of NASA's Webb Telescope's "Deployable Tower Assembly"|first=Lynn|last=Jenner|date=9 September 2015}}</ref>

==Timeline==
{{Main|Timeline of the James Webb Space Telescope}}
* December 2001: final results from the SBMD test published.<ref name="auto1"/>
* April 2012: primary mirror backplane support structure completed.<ref>{{cite web|url=https://www.nasa.gov/home/hqnews/2012/apr/HQ_12-129_Webb_Backplane_Completed.html|title=NASA – NASA'S Webb Telescope Flight Backplane Section Completed|website=www.nasa.gov}}</ref>
* November 2015: first primary mirror segment installed.<ref>{{cite web|url=http://www.americaspace.com/?p=88944|title=First of 18 Mirrors Installed as Final Assembly Phase for NASA's James Webb Space Telescope Begins|date=26 November 2015}}</ref>
* December 2015: half of the primary mirror segments installed.<ref>{{cite web|url=http://www.hngn.com/tags/nasas-optical-telescope-element-manager|title=NASA's James Webb Space Telescope Closer To Completion, Reaches Halfway Point For Segmented Primary Mirror|publisher=Headlines & Global News|date=December 28, 2015|access-date=November 3, 2022}}</ref>
* February 3, 2016: last of the 18 primary mirror segments installed.<ref>{{cite news |url=http://www.spacedaily.com/reports/NASAs_James_Webb_Space_Telescope_Primary_Mirror_Fully_Assembled_999.html |title=NASA's James Webb Space Telescope Primary Mirror Fully Assembled |work=NASA press release |publisher=Space Daily |date=February 5, 2016 |access-date=2016-02-05 }}</ref>
* March 3, 2016: secondary mirror installed<ref>{{Cite news|url=https://www.nasa.gov/feature/goddard/2016/nasas-james-webb-space-telescope-secondary-mirror-installed|title=NASA's James Webb Space Telescope Secondary Mirror Installed|last=Jenner|first=Lynn|date=2016-03-07|newspaper=NASA|language=en|access-date=2017-01-24}}</ref>
* March 2016: Aft Optics Subsystem installed.<ref>{{cite web|url=https://svs.gsfc.nasa.gov/12213|title=GMS: JWST Aft-Optics System (AOS) Installed at GSFC|date=14 April 2016|access-date=5 December 2016}}</ref>
* May 2016: OTE and ISIM are merged into the OTIS, which is the combination of these two regions.<ref>Maurice Te Plate,                    Stephan Birkmann,                    Marco Sirianni,                    Timothy Rawle,                    Catarina Alves de Oliveira,                    Torsten Böker,                    Elena Puga,                    Nora Lützgendorf,                    Anthony Marston,                    Peter Rumler,                    Peter Jensen,                    Giovanna Giardino,                    Pierre Ferruit,                    Ralf Ehrenwinkler,                    Peter Mosner,                    Hermann Karl,                    Martin Altenburg,                    Marc Maschmann,                    Robert Rapp,                    Corbett Smith,                    Patrick Ogle,                    Maria Pena Guerrero,                    Charles Proffitt,                    Rai Wu,                    Graham Kanarek, and                    James Muzerolle "JWST’s near infrared spectrograph status and first OTIS test results", Proc. SPIE 10698, Space Telescopes and Instrumentation 2018: Optical, Infrared, and Millimeter Wave, 1069807 (6 July 2018); {{doi|10.1117/12.2312651}}</ref>

==Development testbeds==
[[File:James Webb Space Telescope (4370444559).jpg|thumb|A one sixth scale test version of the primary mirror]]
Achieving a working main mirror was considered one of the greatest challenges of JWST development.<ref name="osa-opn.org">
{{cite journal
|url=http://www.osa-opn.org/home/articles/volume_22/issue_11/features/optical_innovations_in_the_james_webb_space_telesc/
|title=Optical Innovations in the James Webb Space Telescope
|journal=Optics & Photonics News
|date= November 2011
|last=Daukantas
|first=Patricia
|volume=22
|issue=11
|page=22
|doi=10.1364/OPN.22.11.000022
|access-date=May 19, 2017
}}
</ref> Part of the JWST development included validating and testing JWST on various [[testbed]]s of different functions and sizes.<ref name="arxiv.org">{{Cite conference |editor1-last=Oschmann |editor1-first=Jacobus M. |editor2-last=Clampin |editor2-first=Mark |editor3-last=Fazio |editor3-first=Giovanni G. |editor4-last=MacEwen |editor4-first=Howard A. |last1=Perrin |first1=Marshall D. |last2=Soummer |first2=Rémi |last3=Choquet |first3=Élodie |last4=N'Diaye |first4=Mamadou |last5=Levecq |first5=Olivier |last6=Lajoie |first6=Charles-Phillipe |last7=Ygouf |first7=Marie |last8=Leboulleux |first8=Lucie |last9=Egron |first9=Sylvain |last10=Anderson |first10=Rachel |last11=Long |first11=Chris |last12=Elliott |first12=Erin |last13=Hartig |first13=George |last14=Pueyo |first14=Laurent |last15=van der Marel |first15=Roeland |last16=Mountain |first16=Matt |date=June 2014 |title=James Webb Space Telescope Optical Simulation Testbed I: Overview and first results |conference=Space Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wave |location=Montréal, Quebec, Canada |publisher=[[SPIE]] |publication-date=2 August 2014 |volume=9143 |arxiv=1407.0591 |bibcode=2014SPIE.9143E..09P |doi=10.1117/12.2056936 |issn=0277-786X |conference-url=https://www.spiedigitallibrary.org/conference-proceedings-of-SPIE/9143 |s2cid=118347299 }}</ref>

Some types of development items include ''pathfinders'', ''test beds'', and ''engineering test units''.<ref name="jwst.nasa.gov">{{cite web | url=https://jwst.nasa.gov/resources/caa_2006/sabelhaus_projectstatus.pdf | date=2006-05-20 | author=Phil Sabelhaus | title=JWST project status for the CAA}}</ref> Sometimes a single item can be used for different functions, or it may not be a physically created item at all, but rather a software simulation.<ref name="jwst.nasa.gov"/> The NEXUS space telescope was a complete space telescope, but essentially a scaled down JWST but with a number of changes including only three mirror segments with one folding out for a main mirror diameter of {{convert|2.8|m|ft|sp=us}}.<ref name="strategic.mit.edu">{{cite web | url=http://strategic.mit.edu/docs/3_13_SPIE-4849-39.pdf | title=Multidisciplinary Analysis of the NEXUS Precursor Space Telescope | date=2002-12-18 | access-date=2021-11-07}}</ref> It was lighter, so it was envisioned it could be launched as early as 2004 on a Delta 2 launch rocket.<ref name="strategic.mit.edu"/> The design was cancelled at the end of 2000.<ref>{{Cite web|url=http://strategic.mit.edu/downloads.php?page=nexus_telescope|title=MIT Strategic Engineering Research Group: Olivier L. de Weck|website=strategic.mit.edu|access-date=2017-02-03}}</ref> At that time NGST/JWST was still a {{convert|8|m|ft|sp=us|adj=on}} design, with an area of {{cvt|50|m2|ft2}}, a few years later this was reduced eventually to the {{convert|6.5|m|ft|sp=us|adj=on}} design, with an area of {{cvt|25|m2|ft2}}.<ref>{{cite web|last=Mather|first=John|title=JWST Science Summary for SSB|url=https://jwst.nasa.gov/resources/ssb_2006/mather_sciencesummary.pdf|date=March 2022|access-date=November 3, 2022}}</ref>

===OTE Pathfinder===
{{main|OTE Pathfinder}}
One part of JWST development was the production of the Optical Telescope Element Pathfinder.<ref name="auto">{{cite book|chapter=James Webb Space Telescope (JWST) Optical Telescope Element (OTE) Pathfinder status and plans|first1=Lee D.|title = Space Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wave|volume=9143|pages=91430E|last1=Feinberg|first2=Ritva|last2=Keski-Kuha|first3=Charlie|last3=Atkinson|first4=Andrew|last4=Booth|first5=Tony|last5=Whitman|s2cid=121581750|editor4-first=Howard A|editor4-last=MacEwen|editor3-first=Giovanni G|editor3-last=Fazio|editor2-first=Mark|editor2-last=Clampin|editor1-first=Jacobus M|editor1-last=Oschmann|doi=10.1117/12.2054782|year=2014|url=https://zenodo.org/record/1235628}}</ref> The OTE pathfinder uses two additional mirror segments, and additional secondary mirror, and puts together various structures to allow testing of various aspects of the section, including Ground Support Equipment.<ref name="auto"/> This supports the GSE being used on the JWST itself later on, and allows testing of mirror integration.<ref name="auto"/> OTE pathfinder as 12 rather than 18 cells compared to the full telescope, but it does include a test of the backplane structure.<ref>{{cite web|first=Scott P. |last=Willoughby|title=PRIME: The Untold Story Of NASA's James Webb Space Telescope|url=http://www.satmagazine.com/story.php?number=443229204|publisher=Sat Magazine|date=February 2012|access-date=November 3, 2022}}</ref>

===Additional tests/models===
[[File:Primary Mirror Size Comparison Between Webb and Hubble.webm|thumb|upright=1.0|right|Primary mirror size comparison between the [[James Webb Space Telescope]] (JWST) and [[Hubble Space Telescope]] (HST)]]
There are many test articles and developmental demonstrators for the creation of JWST.<ref name="jwst.nasa.gov"/> Some important ones were early demonstrators, that showed that many of fundamental technologies of JWST were possible.<ref name="jwst.nasa.gov"/> Other test articles are important for risk mitigation, essentially reducing the overall risk of the program by practicing on something other than the actual flight spacecraft.

Another testbed, the Test Bed Telescope, was a 1/6th scale model of the main mirror, with polished segments and working actuators, operating at room temperature, and used to test all the processes for aligning the segments of JWST.<ref name="nasa.gov1">{{cite web|url=http://jwst.nasa.gov/wavefront.html|title=The James Webb Space Telescope Innovations - Wavefront Sensing and Control|access-date=November 3, 2022}}</ref>  Another optics testbed is called JOST, which stands for JWST Optical Simulation Testbed, and uses an MEMS with hexagonal segments to simulate the degrees of freedom of the primary mirror alignment and phasing.<ref name="arxiv.org"/>

The Subscale Beryllium Model Demonstrator (SBMD) was fabricated and tested by 2001 and demonstrated enabling technologies for what was soon Christened the James Webb Space Telescope, previously the ''Next Generation Space Telescope'' (NGST).<ref name="auto1">{{Cite journal|bibcode=2001SPIE.4451....5R|title=Final results of the Subscale Beryllium Mirror Demonstrator (SBMD) program|journal=Proc. SPIE |first1=Timothy|last1=Reed|first2=Stephen E.|last2=Kendrick|first3=Robert J.|last3=Brown|first4=James B.|last4=Hadaway|first5=Donald A.|last5=Byrd|editor-first1=H. Philip |editor-last1=Stahl |s2cid=120007487|date=1 December 2001|volume=4451|pages=5–14|via=NASA ADS|doi=10.1117/12.453614|series=Optical Manufacturing and Testing IV}}</ref> The SBMD was a half-meter diameter mirror made from powdered beryllium.<ref name="auto1"/> The weight of the mirror was then reduced through a mirror-making process called "light-weighting", where material is removed without disrupting its reflecting ability, and in this case 90% of the SBMD mass was removed.<ref name="auto1"/> It was then mounted to a rigid backplane with titanium bipod flexures and underwent various tests.<ref name="auto1"/> This included freezing it down to the low temperatures required and seeing how it behaved optically and physically.<ref name="auto1"/> The tests were conducted with the Optical Testing System (aka the OTS) which was created specifically to test the SBMD.<ref name="auto1"/><ref name="auto2">{{Cite journal|title=Cryogenic optical testing results for the Subscale Beryllium Mirror Demonstrator (SBMD)|first1=James B.|last1=Hadaway|first2=Joseph M.|last2=Geary|first3=Patrick J.|last3=Reardon|first4=Bruce R.|last4=Peters|first5=H. Philip|last5=Stahl|first6=Ron|last6=Eng|first7=John W.|last7=Keidel|first8=Jeffrey R.|last8=Kegley|first9=Timothy|last9=Reed|first10=Donald A.|last10=Byrd|editor-first1=H. Philip |editor-last1=Stahl |s2cid=110914244|date=1 January 2001|volume=4451|pages=15–26|doi=10.1117/12.453625|journal = Optical Manufacturing and Testing IV|bibcode=2001SPIE.4451...15H}}</ref> The SBMD had to meet the requirements for a space-based mirror, and these lessons were important to the development of the JWST.<ref>{{Cite web | url=https://ntrs.nasa.gov/search.jsp?R=20010069321 | title=Subscale Beryllium Mirrors Demonstrator (SBMD) Program Summary and Ball Modeling| date=January 2001}}</ref> The tests were conducted at the X-Ray Calibration Facility (XRCF) at [[Marshall Space Flight Center]] (MSFC) in the U.S. State of Alabama.<ref name="auto1"/><ref name="auto2"/>

The Optical Testing System (OTS) had to be developed to test the SBMD (the NGST mirror prototype) under cryogenic vacuum conditions.<ref name="auto2"/> The OTS included a  WaveScope [[Shack–Hartmann wavefront sensor|Shack-Hartmann sensor]] and a Leica Disto Pro distance measurement instrument.<ref name="auto2"/>

Some JWST technology Testbeds, Pathfinders, etc.:
* [[OTE Pathfinder]].<ref name="auto"/>
* TBT (1/6th scale testbed)<ref name="arxiv.org"/>
* JOST ('''J'''WST '''O'''ptical '''S'''imulation '''T'''estbed)<ref name="nasa.gov1"/><ref name="arxiv.org"/>
* SBMD ('''S'''ubscale '''B'''eryllium '''M'''odel '''D'''emonstrator)<ref name="auto1"/>
* OTS (test system for SBMD)<ref name="auto2"/>
* ITM (this is a software model)<ref name="arxiv.org"/>
* OSIM ('''O'''TE '''Sim'''ulator)<ref name="ReferenceA"/>
* Beam Image Analyzer<ref name="ReferenceA"/>

Another related program was the Advanced Mirror System Demonstrator (AMSD) program.<ref name="Thronson">{{Cite book|url=https://books.google.com/books?id=byopCpRNCSsC&pg=PA8|title=Astrophysics in the Next Decade: The James Webb Space Telescope and Concurrent Facilities|last1=Thronson|first1=Harley A.|last2=Stiavelli|first2=Massimo|last3=Tielens|first3=Alexander|date=2009|publisher=Springer Science & Business Media|isbn=978-1-4020-9457-6|page=8}}</ref> The AMSD results were utilized in construction of beryllium mirrors.<ref name="Thronson"/>

<gallery mode=packed heights='250px'>
File:James Webb Space Telescope in NASA's giant thermal vacuum chamber (17006918727).jpg|OTE pathfinder heads into a thermal vacuum chamber, 2015
File:A James Webb Space Telescope Mirror Takes a Short Trip (11355071886).jpg|Engineering Test Unit (ETU) mirror segment being hauled into a cleanroom in its container, 2013
File:Scanning Webb’s Surrogate Eye (7514480594).jpg|The Beam Image Analyzer is shown, being prepared to test OSIM under a cryogenic vacuum, 2012.<ref name="ReferenceA">{{Cite web|url=https://www.nasa.gov/topics/technology/features/webb-eye-scan.html|title=NASA – Scanning Webb's Surrogate Eye|website=www.nasa.gov|language=en|access-date=2017-01-21}}</ref>
</gallery>

==Diagram 1==
[[File:Ote1jwst.jpg|690px|thumb|left|Labeled diagram of components of the Optical Telescope Element]]
{{clear}}

==Gallery==
<gallery mode=packed heights='200px'>
File:The Backplane of James Webb Space Telescope Completes Testing at Marshall.jpg|Backplane after being tested at Marshall Space Flight, 2013
File:James Webb Space Telescope's Golden Mirror Unveiled (26076364723).jpg|OTE assembled in April 2016
File:By the Dozen- NASA's James Webb Space Telescope Mirrors (23938941370).jpg|Backplane with 12 of 18 segments attached, the segments are covered over for protection
File:NASA's James Webb Space Telescope Primary Mirror Fully Assembled (24722672281).jpg|Primary mirror almost fully assembled (18/18 segments), with covers, robotic arm holding the last segment, February 2016
File:James Webb Space Telescope Backplane Arrives at NASA Goddard for Mirror Assembly (20901832245).jpg|Backplane assembly arrives at Goddard, 2015
File:Jwmirrors2.jpg|All components of the Optical Telescope Element
</gallery>

==See also==
* [[Integrated Science Instrument Module]] (ISIM) (another major JWST section)
* [[Primary mirror]]
* [[Secondary mirror]]
* [[Segmented mirror]]
* [[James Webb Space Telescope timeline]]
* [[Cryogenics]]

==References==
{{reflist|3}}

==External links==
{{commons category|Optical Telescope Element (JWST)}}
* [http://jwst.nasa.gov/images/ote1.jpg hotlink to diagram]
* [http://www.osa-opn.org/home/articles/volume_22/issue_11/features/optical_innovations_in_the_james_webb_space_telesc/ Optics & Photonics : Optical Innovations in the James Webb Space Telescope by P. Daukantas] (November 2011)
* [https://www.reddit.com/r/askscience/comments/3xp92g/askscience_ama_series_im_lee_feinberg_optical/ Reddit AMA with OTE manager]

{{Jwstnav}}

[[Category:James Webb Space Telescope]]
[[Category:Gold objects]]
[[Category:Mirrors]]