Optical Telescope Element

Optical Telescope Element (OTE) is a sub-section of the James Webb Space Telescope, a large infrared space telescope scheduled to be launched early 2021.[1] The OTE consists of some major parts of the telescopes including the main mirror, the secondary mirrors, the framework and controls to support those mirrors, and various thermal and other systems to support the functioning of the telescope.[1] 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 and Sunshield.[2] The OTE collects the light and sends it to the science instruments in the ISIM.[1] The OTE has been compared to being the "eye" of the telescope and the backplane of it to being the "spine".[3]

The mirror assembly from the front with primary mirrors attached, November 2016
The secondary mirror being cleaned with carbon dioxide snow

The primary mirror is a tiled assembly of 18 hexagonal elements, each 1.32 meters from flat to flat. This combination yields an effective aperture of 6.5 meters and a total collecting surface of 27 square meters.[4] Secondary mirrors complete the f/20 anastigmatic imaging optics.[5] The complete system provides an effective f/number of f/16.67 and focal length of 131.4 meters.[5][6] The main three-mirror telescope is a Korsch-type design,[6] 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.

Overview

The OTE combines a large amount of the optics and structural components of the James Webb Space Telescope, including the Main mirror.[7] It also has the fine steering mirror, which, provides that final precise pointing, and it works in conjunction with the fine guidance sensor and other controls systems and sensors in the Spacecraft Bus.[7]

The main mirror segments are aligned roughly using a coarse phasing algorithm.[7] 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 nm.[7] To function as focusing mirror correctly the 18 main mirror segments need to be aligned very closely to perform as one.[7] This needs to be done in outer space, so extensive testing on Earth is required to ensure that it will work properly.[7] To align each mirror segment, it is mounted to six actuators that can adjust that segment in 5 nm steps.[7] 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.[7] Although in the essentially weightless environment of space the mirror will weigh hardly anything, it needs to be very stiff to maintain its shape.[7] 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.[8] There are nine distance alignment process that the telescope goes through to achieve this.[8] Another important aspect to the adjustments is that the primary mirror backplane assembly is steady.[9] The backplane assembly is made of graphite composite, invar, and titanium.[9]

The ADIR, Aft Deployable Infrared Radiator is a radiator behind the main mirror, that helps keep the telescope cool.[10] There are two ADIR's and they are made of high-purity aluminum.[10] There is a special black coating on the radiators that helps them emit heat into space.[11]

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:[1]

  • Primary mirror (18 segments)
  • Secondary mirror (74 cm 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.[1] One of the tasks for the Fine steering mirror is image stabilization.[5]

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.[5] Other infrared telescopes that have used beryllium mirrors include IRAS, COBE, and Spitzer.[5] The Subscale Beryllium Model Demonstrator (SBMD) was successfully tested at cryogenic temperatures, and one of the concerns was surface roughness at low kelvin numbers.[5] The beryllium mirrors are coated with a very fine layer of Gold to reflect infrared light.[6] There are 18 hexagonal segments that are grouped together to create a single mirror with an overall diameter of 6.5 meters (650 cm, ~7.1 yards, ~256 inches).[6]

DTA
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 a critical structural component that connects OTE to the Spacecraft Bus, it is called the Deployable Tower Assembly (DTA).[12] It also must expand to allow the Sunshield (JWST) to spread out to allow the space between its five layers to expand.[12] The Sunshield segment has various structure, including six spreaders at its outer edge to spread the layers out at its six extremities.

During launch its is shrunk down, but at the right time in space the DTA must extend.[12] The extended DTA structure allows the sun-shield layers to be fully spread out.[12] The DTA must also thermally isolate the cold section of the OTE from the hot spacecraft bus.[12] The Sunshield will protect the OTE from direct sunlight and reduce the thermal radiation hitting it, but another aspect is the OTE's physcially connection to the rest of the spacecraft.[12] (see Thermal conduction and Heat transfer) Whereas the sunshield stops the telescope getting hot at a distance from a fire (near not in it), the DTA has to handle the heat flow like how a handle on a pan might get warm when its on the stove if not insulated enough.

The way the DTA extends is that it has two telescoping tubes that can slide between each-other on rollers.[12] There is an inner tube and an outer tube.[12] The DTA is extended by an electric motor that rotates a ball screw nut that pushes the two tubes apart.[12] When the DTA is fully deployed it is 10 feet long (~3 meter).[13] The DTA tubes are made of graphite-composite carbon fiber, and it is intended that it will be able to survive the conditions in space.[14]

Timeline
  • December 2001, final results from the SBMD test published.[15]
  • April 2012, primary mirror backplane support structure completed.[16]
  • November 2015, first main mirror segment installed.[17]
  • December 2015, half of the main mirror segments installed.[18]
  • February 3, 2016, 18th of 18, the final primary mirror segment was installed [19]
  • March 3, 2016, secondary mirror installed[20]
  • March 2016, Aft Optics Subsystem installed.[21]
  • May 2016, OTE and ISIM are merged into the OTIS, which is the combination of these two regions.[22]

Development testbeds
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.[7] Part of the JWST development included validating and testing JWST on various testbeds of different functions and sizes.[23]

Some types of development items include pathfinders, test beds, and engineering test units.[24] 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.[24] The NEXUS space telescope was 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 2.8 meters.[25] It was lighter, so it was envisioned it could be launched as early as 2004 on a Delta 2 launch rocket.[25] The design was cancelled at the end of 2000.[26] At that time NGST/JWST was still a 8-meter design (50 m2), a few years later this was reduced eventually to the 25 m2 (6.5 m) design.[27]

OTE Pathfinder
Main article: OTE Pathfinder

One part of JWST development was the production of the Optical Telescope Element Pathfinder.[28] 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.[28] This supports the GSE being used on the JWST itself later on, and allows testing of mirror integration.[28] OTE pathfinder as 12 rather than 18 cells compared to the full telescope, but it does include a test of the backplane structure.[29]

Additional tests/models

There are many test articles and developmental demonstrators for the creation of JWST.[24] Some important ones were early demonstrators, that showed that many of fundamental technologies of JWST were possible.[24] 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 was a 1/6th scale functioning version of the main mirror and technology, used especially to ensure the many segments can work as one.[8] Another optics testbed is called JOST, which stands for JWST Optical Simulation Testbed.[23]

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).[15] The SBMD was a half-meter diameter mirror made from powered beryllium.[15] 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.[15] It was then mounted to a rigidly with titanium and underwent various tests.[15] This included freezing it down to the low temperatures required and seeing how it behaved optically and physically.[15] The tests were conducted with the Optical Testing System (aka the OTS) which was created specifically to test the SBMD.[15][30] The SBMD had to meet the requirements for a space-based mirror, and these lessons were important to the development of the JWST.[31] The tests were conducted at the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC) in the U.S. State of Alabama.[15][30]

The Optical Testing System (OTS) had to be developed to test the SBMD (the NGST mirror prototype) under cryogenic vacuum conditions.[30] The OTS included a WaveScope Shack-Hartmann sensor and a Leica Disto Pro distance measurement instrument.[30]

Some JWST technology Testbeds, Pathfinders, etc.:

  • OTE Pathfinder.[28]
  • TBT (1/6th scale testbed)[23]
  • JOST (JWST Optical Simulation Testbed)[8][23]
  • SBMD ( Subscale Beryllium Model Demonstrator )[15]
  • OTS (test system for SBMD)[30]
  • ITM (this is a software model)[23]
  • OSIM (OTE Simulator)[32]
  • Beam Image Analyzer[32]

Another related program was the Advanced Mirror System Demonstrator (AMSD) program.[33] The AMSD results were utilized in construction of beryllium mirrors.[33]

OTE pathfinder heads into a thermal vacuum chamber, 2015
Engineering Test Unit (ETU) mirror segment being hauled into a cleanroom in its container, 2013
The Beam Image Analyzer is shown, being prepared to test OSIM under a cryogenic vacuum, 2012.[32]

Diagram 1
Labeled diagram of components of the Optical Telescope Element
Backplane after being tested at Marshall Space Flight, 2013
OTE assembled in April 2016
Backplane with 12 of 18 segments attached, the segments are covered over for protection
Primary mirror almost fully assembled (18/18 segments), with covers, robotic arm holding the last segment, February 2016
Backplane assembly arrives at Goddard, 2015

See also

References
  1. "The James Webb Space Telescope". Retrieved 30 July 2018.
  2. "The James Webb Space Telescope". Retrieved 5 December 2016.
  3. "The James Webb Space Telescope". Retrieved 5 December 2016.
  4. NASA. "JWST Innovations: The Primary Mirror". Retrieved 2 August 2017. Each of the 18 hexagonal-shaped mirror segments is 1.32 meters (4.3 feet) in diameter, flat to flat.
  5. "JWST – eoPortal Directory – Satellite Missions". Retrieved 5 December 2016.
  6. "JWST: A Giant Cyclops to Reveal the Depths of Space". Retrieved 5 December 2016.
  7. Daukantas, Patricia (November 2011). "Optical Innovations in the James Webb Space Telescope". Optics & Photonics News. 22 (11): 22. doi:10.1364/OPN.22.11.000022. Retrieved May 19, 2017.
  8. "The James Webb Space Telescope". Retrieved 5 December 2016.
  9. "The James Webb Space Telescope". Retrieved 5 December 2016.
  13. Jenner, Lynn (2015-09-09). "The Secrets of NASA's Webb Telescope's "Deployable Tower Assembly"". NASA. Retrieved 2017-01-21.
  14. Jenner, Lynn (9 September 2015). "The Secrets of NASA's Webb Telescope's "Deployable Tower Assembly"".
  15. Reed, Timothy; Kendrick, Stephen E.; Brown, Robert J.; Hadaway, James B.; Byrd, Donald A. (1 December 2001). "Final results of the Subscale Beryllium Mirror Demonstrator (SBMD) program". Proc. SPIE. Optical Manufacturing and Testing IV. 4451: 5–14. Bibcode:2001SPIE.4451....5R. doi:10.1117/12.453614. S2CID 120007487 – via NASA ADS.
  16. "NASA –

    NASA'S Webb Telescope Flight Backplane Section Completed

    "    . www.nasa.gov.
  17. "First of 18 Mirrors Installed as Final Assembly Phase for NASA's James Webb Space Telescope Begins". 26 November 2015.
  18. News, Headlines & Global. "NASA's Optical Telescope Element Manager : Headlines & Global News". Retrieved 5 December 2016.
  19. "NASA's James Webb Space Telescope Primary Mirror Fully Assembled". NASA press release. Space Daily. February 5, 2016. Retrieved 2016-02-05.
  20. Jenner, Lynn (2016-03-07). "NASA's James Webb Space Telescope Secondary Mirror Installed". NASA. Retrieved 2017-01-24.
  21. "GMS: JWST Aft-Optics System (AOS) Installed at GSFC". Retrieved 5 December 2016.
  22. 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); https://doi.org/10.1117/12.2312651
  23. Perrin, Marshall D.; et al. (2014). Oschmann, Jacobus M; Clampin, Mark; Fazio, Giovanni G; MacEwen, Howard A (eds.). "James Webb Space Telescope Optical Simulation Testbed I: Overview and First Results". Proceedings of SPIE. Space Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wave. 9143: 914309. arXiv:1407.0591. Bibcode:2014SPIE.9143E..09P. doi:10.1117/12.2056936. S2CID 118347299.
  26. "MIT Strategic Engineering Research Group: Olivier L. de Weck". strategic.mit.edu. Retrieved 2017-02-03.
  28. Feinberg, Lee D.; Keski-Kuha, Ritva; Atkinson, Charlie; Booth, Andrew; Whitman, Tony (2014). "James Webb Space Telescope (JWST) Optical Telescope Element (OTE) Pathfinder status and plans". In Oschmann, Jacobus M; Clampin, Mark; Fazio, Giovanni G; MacEwen, Howard A (eds.). Space Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wave. 9143. pp. 91430E. doi:10.1117/12.2054782. S2CID 121581750.
  30. Hadaway, James B.; Geary, Joseph M.; Reardon, Patrick J.; Peters, Bruce R.; Stahl, H. Philip; Eng, Ron; Keidel, John W.; Kegley, Jeffrey R.; Reed, Timothy; Byrd, Donald A. (1 January 2001). "Cryogenic optical testing results for the Subscale Beryllium Mirror Demonstrator (SBMD)". Optical Manufacturing and Testing IV. 4451: 15–26. Bibcode:2001SPIE.4451...15H. doi:10.1117/12.453625. S2CID 110914244. Cite journal requires |journal= (help)
  31. "Subscale Beryllium Mirrors Demonstrator (SBMD) Program Summary and Ball Modeling". January 2001.
  32. "NASA – Scanning Webb's Surrogate Eye". www.nasa.gov. Retrieved 2017-01-21.
  33. Thronson, Harley A.; Stiavelli, Massimo; Tielens, Alexander (2009). Astrophysics in the Next Decade: The James Webb Space Telescope and Concurrent Facilities. Springer Science & Business Media. p. 8. ISBN 978-1-4020-9457-6.
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