Editing Navy Precision Optical Interferometer

{{Short description|US Navy astronomical interferometer}}
{{Infobox telescope}}

The '''Navy Precision Optical Interferometer''' ('''NPOI''') is an American [[astronomical interferometer]], with the world's largest baselines, operated by the [[Naval Research Laboratory]] (NRL).
Until the end of 2022, it was operated by a consortium that included NRL with the [[United States Naval Observatory Flagstaff Station|Naval Observatory Flagstaff Station]] (NOFS) and [[Lowell Observatory]].  The NPOI primarily produces space imagery and astrometry, the latter a major component required for the safe position and navigation of all manner of vehicles for the DoD. The facility is located at Lowell's [[Anderson Mesa Station]] on [[Anderson Mesa]] about {{Convert|25|km|mi|sp=us}} southeast of [[Flagstaff, Arizona]] (US).  Until November 2011, the facility was known as the Navy Prototype Optical Interferometer (NPOI). Subsequently, the instrument was temporarily renamed the Navy Optical Interferometer, and now permanently, the Kenneth J. Johnston Navy Precision Optical Interferometer (NPOI) – reflecting both the operational maturity of the facility, and paying tribute to its principal driver and retired founder, Kenneth J. Johnston.<ref name=NOI-ANNC /><ref>{{Cite web |url=http://www.public.navy.mil/fltfor/cnmoc/Pages/Johnston.aspx |title=Archived copy |access-date=2013-01-25 |archive-url=https://web.archive.org/web/20131004212912/http://www.public.navy.mil/fltfor/cnmoc/Pages/Johnston.aspx |archive-date=2013-10-04 |url-status=dead }}</ref>

The NPOI project was initiated by the [[United States Naval Observatory]] (USNO) in 1987.<ref name=1998WHIT />  Lowell joined the project the following year when the USNO decided to build the NPOI at Anderson Mesa.<ref name=1997HUTT />  The first phase of construction was completed in 1994, which allowed the interferometer to see its first fringes, or light combined from multiple sources, that year.<ref name=1995HUTT />  The Navy began regular science operations in 1997.<ref name=1997ARM />  The NPOI has been continuously upgraded and expanded since then. The workings of NPOI as a classic interferometer, are described at Scholarpedia,<ref>{{Cite journal|doi = 10.4249/scholarpedia.10586|title = Ground-based optical interferometry|year = 2012|last1 = Hutter|first1 = Donald|journal = Scholarpedia|volume = 7|issue = 6|page = 10586|bibcode = 2012SchpJ...710586H| doi-access=free }}</ref> and at the NPOI site.<ref>{{cite web |url=http://www2.lowell.edu/npoi/what_is_it/default.php |url-status=dead |archive-url=https://web.archive.org/web/20100527113331/http://www.lowell.edu/npoi/what_is_it/default.php |archive-date=2010-05-27 |title=NPOI - Navy Prototype Optical Interferometer}}</ref>

==Description==

The NPOI is an [[astronomical interferometer]] laid out in a three-arm "Y" configuration, with each equally-spaced arm measuring {{Convert|250|m|ft|sp=us}} long.  There are two types of stations that can be used in the NPOI.  [[Astrometry|Astrometric stations]], used to measure the positions of celestial objects very accurately, are fixed units placed {{Convert|21|m|ft|sp=us}} apart, with one on each arm and one at the center.  [[Optical imaging|Imaging stations]] can be moved to one of nine positions on each arm, and up to six can be used at one time to perform standard observing.  Light from either type of station is first directed into the feed system, which consists of long pipes which have been evacuated of all air.  They lead to a switchyard of mirrors, where the light is directed into the six Long Delay Lines, which is another set of long pipes that compensate for the different distances to each station.  The light is then sent into the Beam Combining Facility, where it enters the Fast Delay Lines.  This third set of evacuated pipes contains mechanisms that move mirrors back and forth with a very high degree of accuracy.  These compensate for the movement of the mirrors as they track an object across the sky, and for other effects.  Finally, the light leaves the pipes inside the BCF and goes to the Beam Combining Table, where the light is combined in a way that allows images to be formed.<ref name=1998WHIT />

[[File:NOFS npoimesa-art.jpg|thumb|left|250px|alt=Navy Precision Optical Interferometer layout|Navy Precision Optical Interferometer layout.]]
Both types of station have three elements: a [[siderostat]], a Wide Angle Star Acquisition (WASA) camera, and a Narrow Angle Tracking (NAT) mirror.  The first is a precisely-ground flat mirror {{Convert|50|cm|in|abbr=on}} in diameter.  The WASA cameras control the aiming of the mirror at the celestial target.  The reflected light from the siderostat is directed through a telescope which narrows the beam down to the diameter of the pipes, which is {{Convert|12|cm|in|abbr=on}}.  The light then hits the mirror of the NAT, which compensates for atmospheric effects and directs the light into the feed system.<ref name=1998WHIT />

In 2009 NOFS began final plans for NPOI to incorporate four {{Convert|1.8|m|in|abbr=on}} aperture optical-infrared telescopes into the array, which were accepted by the Navy in 2010,<ref>{{Cite web |url=http://www.nofs.navy.mil/about_NOFS/telescopes/LTRs_1.8m_UNSECNAV_Accept.pdf |title=Archived copy |access-date=2012-02-05 |archive-url=https://web.archive.org/web/20161227202608/http://www.nofs.navy.mil/about_NOFS/telescopes/LTRs_1.8m_UNSECNAV_Accept.pdf |archive-date=2016-12-27 |url-status=dead }}</ref><ref>{{Cite web |url=http://www.chara.gsu.edu/CHARA/Papers/Pasadena/12.Hutter.pdf |title=Archived copy |access-date=2012-04-25 |archive-date=2012-03-12 |archive-url=https://web.archive.org/web/20120312023304/http://www.chara.gsu.edu/CHARA/Papers/Pasadena/12.Hutter.pdf |url-status=dead }}</ref> and assigned to the [[United States Naval Observatory Flagstaff Station|Naval Observatory Flagstaff Station]].<ref name=2008VITT />  They were originally intended to be "outrigger" telescopes for the [[W. M. Keck Observatory]] in Hawaii, but were never installed and incorporated into Keck's interferometer. Three telescopes were planned for initial installation,<ref name=TEL-ACCPT /><ref name=NPOI-0311 /> though these plans have been shelved.  The fourth is currently at [[Mount Stromlo Observatory]] in Australia and will be incorporated at some point in the future.<ref name=2008VITT />  The new telescopes will help with faint object imaging and improved absolute astrometry, due to their greater [[Optical telescope#Light-gathering power|light-gathering]] abilities than the existing siderostats.<ref name=2008VITT />

NRL operates and leads the science for the NPOI. 

With the cancellation of the four 1.8m "outrigger" telescope project, installation plans for an array of three 1 meter telescopes were developed by NRL and Lowell Observatory, based on the funded science performed.  Three PlaneWave CDK1000 telescopes were purchased for this project, with initial installation for two of the telescopes completed in 2020.

NPOI is an example of the  [[Michelson Interferometer]] design, with the principal science managed by NRL. NPOI is one of the few major instruments globally which can conduct [[Astronomical optical interferometry|''optical'' interferometry]].<ref>{{Cite web |url=http://usic.wikispaces.com/file/view/Armstrong_OpticalInterferometry_TEC_OIR.pdf |title=Archived copy |access-date=2012-04-25 |archive-date=2012-03-23 |archive-url=https://web.archive.org/web/20120323170104/http://usic.wikispaces.com/file/view/Armstrong_OpticalInterferometry_TEC_OIR.pdf |url-status=dead }}</ref><ref>{{cite web |url=http://frank.harvard.edu/~howard/papers/optical_interferometry_review.pdf |title=Archived copy |website=frank.harvard.edu |access-date=30 June 2022 |archive-url=https://web.archive.org/web/20110720200519/http://frank.harvard.edu/~howard/papers/optical_interferometry_review.pdf |archive-date=20 July 2011 |url-status=dead}}</ref> See an illustration of its layout, at bottom. NOFS has used NPOI to conduct a wide and diverse series of scientific studies, beyond just the study of absolute astrometric positions of stars,;<ref>[http://adsabs.harvard.edu/abs/2009AAS...21441102H Large-Angle Astrometry at the Navy Prototype Optical Interferometer (NPOI)]. Adsabs.harvard.edu. Retrieved on 18 October 2011.</ref> additional NOFS science at NPOI includes the study of [[binary stars]], [[Be star|Be Stars]], [[Gravity darkening|Oblate stars]], [[Stellar rotation|rapidly rotating stars]], those with [[starspot]]s, and the imaging of [[Limb darkening|stellar disks]] (the first in history) and [[flare stars]].<ref>{{Cite web |url=http://www2.lowell.edu/npoi/publications/bibliography.php |title=NPOI - Navy Prototype Optical Interferometer |access-date=2012-07-01 |archive-date=2010-05-27 |archive-url=https://web.archive.org/web/20100527162627/http://www.lowell.edu/npoi/publications/bibliography.php |url-status=dead }}</ref> In 2007–2008, NRL with NOFS used NPOI to obtain first-ever [[closure phase]] image precursors of satellites orbiting in [[geostationary orbit]].<ref>[http://www.amostech.com/TechnicalPapers/2010/Systems/Jorgensen.pdf Satellite Imaging and Characterization with Optical Interferometry] amostech.com</ref><ref>[http://apps.dtic.mil/dtic/tr/fulltext/u2/a531729.pdf Observations of a Geosynchronous Satellite with Optical Interferometry] {{Webarchive|url=https://web.archive.org/web/20221129133407/https://apps.dtic.mil/dtic/tr/fulltext/u2/a531729.pdf |date=2022-11-29 }} dtic.mil</ref>

==Discussion==
[[Optical interferometer]]s are extremely complex, unfilled aperture photon-collecting telescopes in the visual (sometimes the near [[infrared]], too), which produce synthesized images and [[Young's slit|fringe data]] "on the fly" (unlike [[radio interferometer]]s which record the data for ''later'' synthesis), essentially by taking an [[inverse Fourier transform]] of the incoming data. Astrometry is understood by precisely measuring delay line additions while fringing, to match the light-path differences from baseline ends. Using essentially [[trigonometry]], the angle and position of where the array is "pointed" can be determined, thus inferring a precise position on the sphere of the sky.

Only a [[List of astronomical interferometers at visible and infrared wavelengths|few exist]] that can be considered operational. To date NPOI has produced the highest-resolution optical images of any astronomical instrument, though this may change when the [[CHARA array]] and [[Magdalena Ridge Observatory]] Interferometer begin optical-band operations.<ref name=2004ARMS /> The first [[astronomical object]] imaged (resolved) by NPOI was [[Mizar (star)|Mizar]], and since, a significant amount of [[astrometry]], reference tie frame, rapid rotator star, and Be stellar disk study has been performed.<ref name=USNO-PRE /> NPOI is capable of determining positions of celestial objects to a few milli-arcseconds, in part due to the optical anchoring of its components using a complex [[metrology]] array of lasers that connect main optical elements to each other and to bedrock.

Many specialized [[lasers]] are also used to align the long train of optics. The NPOI [[siderostat]] array and the CHARA Array are the world's only long-baseline optical interferometers that can simultaneously co-phase six elements.<ref name=2008HUTT /> 

==See also==
* [[List of astronomical interferometers at visible and infrared wavelengths]]
* [[List of telescope types]]

==References==
{{Reflist|refs=

<ref name=NOI-ANNC>{{cite web |title=NPOI renamed to reflect its evolving role in research |url=http://www.lowell.edu/news/2011/11/npoi-renamed-to-reflect-its-evolving-role-in-research/ |publisher=Lowell Observatory |accessdate=2012-01-04 |url-status=dead |archiveurl=https://web.archive.org/web/20120131193220/http://www.lowell.edu/news/2011/11/npoi-renamed-to-reflect-its-evolving-role-in-research/ |archivedate=2012-01-31 }}</ref>

<ref name=1995HUTT>{{cite journal|bibcode=1995AAS...18712102H|title=Current Status of the Navy Prototype Optical Interferometer|last1=Hutter|first1=Donald J.|volume=27|date=1995|pages=1452|journal=Bulletin of the American Astronomical Society}}</ref>

<ref name=1997HUTT>{{cite journal|bibcode=1997AJ....114.2822H|title=Seeing Tests at Four Sites in Support of the NPOI Project|last1=Hutter|first1=D. J.|last2=Elias|first2=N. M.|last3=Peterson|first3=E. R.|last4=Weaver|first4=W. B.|last5=Weaver|first5=G.|last6=Mozurkewich|first6=D.|last7=Vrba|first7=F. J.|last8=Simon|first8=R. S.|last9=Buscher|first9=D. F.|last10=Hummel|first10=C. A.|volume=114|date=1997|pages=2822|journal=Astronomical Journal|doi=10.1086/118690|doi-access=free}}</ref>

<ref name=1997ARM>{{cite journal|bibcode=1997AAS...191.1603A|title=The Navy Prototype Optical Interferometer (NPOI) is Operational|last1=Armstrong|first1=J. T.|last2=Mozurkewich|first2=D.|last3=Pauls|first3=T. A.|last4=Rickard|first4=L. J.|last5=Benson|first5=J. A.|last6=Dyck|first6=H. M.|last7=Elias|first7=N. M.|last8=Hajian|first8=A. R.|last9=Hummel|first9=C. A.|last10=Hutter|first10=D. J.|last11=Johnston|first11=K. J.|last12=White|first12=N. M.|volume=29|date=1997|pages=1234|journal=Bulletin of the American Astronomical Society|display-authors=9}}</ref>

<ref name=1998WHIT>{{cite journal|bibcode=1998ApJ...496..550A|title=The Navy Prototype Optical Interferometer|last1=Armstrong|first1=J. T.|last2=Mozurkewich|first2=D.|last3=Rickard|first3=L. J.|last4=Hutter|first4=D. J.|last5=Benson|first5=J. A.|last6=Bowers|first6=P. F.|last7=Elias|first7=N. M.|last8=Hummel|first8=C. A.|last9=Johnston|first9=K. J.|last10=Buscher|first10=D. F.|last11=Clark Iii|first11=J. H.|last12=Ha|first12=L.|last13=Ling|first13=L.-C.|last14=White|first14=N. M.|last15=Simon|first15=R. S.|volume=496|issue=1|date=1998|pages=550–571|journal=Astrophysical Journal |doi=10.1086/305365|s2cid=120515684 |doi-access=}}</ref>

<ref name=2004ARMS>{{cite journal |doi=10.1117/12.553062 |title=Proceedings of SPIE – Precision narrow-angle astrometry of binary stars with the Navy Prototype Optical Interferometer |date=2004 |last1=Armstrong |first1=J. T. |last2=Clark Iii |first2=James H. |last3=Gilbreath |first3=G. C. |last4=Hindsley |first4=Robert B. |last5=Hutter |first5=Donald J. |last6=Mozurkewich |first6=David |last7=Pauls |first7=Thomas A. |volume=5491 |pages=1700|journal=New Frontiers in Stellar Interferometry|s2cid=42997726 }}</ref>

<ref name=2008VITT>{{cite journal|bibcode=2008SPIE.7013E..2UD|title=Plans for utilizing the Keck Outrigger Telescopes at NPOI|last1=Divittorio|first1=Michael|last2=Hutter|first2=Donald J.|last3=Kelley|first3=Michael|volume=7013|date=2008|pages=87|journal=Optical and Infrared Interferometry |editor=Schöller|doi=10.1117/12.787635|s2cid=122637531|url=https://zenodo.org/record/1235612|doi-access=free}}</ref>

<ref name=2008HUTT>{{cite journal |doi=10.1117/12.787486 |title=Proceedings of SPIE – NPOI: recent progress and future prospects |date=2008 |last1=Hutter |first1=Donald J. |last2=Benson |first2=James A. |last3=Buschmann |first3=Tim |last4=Divittorio |first4=Michael |last5=Zavala |first5=Robert T. |last6=Johnston |first6=Kenneth J. |last7=Armstrong |first7=J. Thomas |last8=Hindsley |first8=Robert B. |last9=Schmitt |first9=Henrique R. |last10=Clark Iii |first10=James H. |last11=Restaino |first11=Sergio R. |last12=Tycner |first12=Christopher |last13=Jorgensen |first13=Anders M. |last14=Davis |first14=Sam |volume=7013 |pages=701306|journal=Optical and Infrared Interferometry|s2cid=122096561 }}</ref>



<ref name=USNO-PRE>{{cite web |title=The U.S. Naval Observatory Preprint Library (2011) |url=http://ad.usno.navy.mil/edboard/preprints.html |archive-url=https://web.archive.org/web/20050309075103/http://ad.usno.navy.mil/edboard/preprints.html |url-status=dead |archive-date=March 9, 2005 |publisher=United States Naval Observatory |date=2011-03-01 |accessdate=2012-01-05}}</ref>

<ref name=TEL-ACCPT>{{cite web |title=Acceptance of Gift of Telescopes |url=http://www.nofs.navy.mil/about_NOFS/telescopes/LTRs_1.8m_UNSECNAV_Accept.pdf |publisher=United States Navy |date=2010-11-03 |accessdate=2012-01-04 |archive-url=https://web.archive.org/web/20161227202608/http://www.nofs.navy.mil/about_NOFS/telescopes/LTRs_1.8m_UNSECNAV_Accept.pdf |archive-date=2016-12-27 |url-status=dead }}</ref>

<ref name=NPOI-0311>{{cite web|last=Hutter|first=Don|title=NPOI Update|url=http://www.chara.gsu.edu/CHARA/Papers/Atlanta/Hutter.pdf|publisher=United States Naval Observatory|date=2011-03-01|accessdate=2012-01-05}}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
}}

== External links ==
* [https://web.archive.org/web/20060717184651/http://www.nofs.navy.mil/ United States Naval Observatory Flagstaff Station]
* [http://www.usno.navy.mil/USNO/tours-events/tour-information/visit-usnofs  Additional USNOFS Information] {{Webarchive|url=https://archive.today/20120805213639/http://www.usno.navy.mil/USNO/tours-events/tour-information/visit-usnofs |date=2012-08-05 }}
* [http://www2.lowell.edu/npoi/ Lowell Observatory NPOI site] {{Webarchive|url=https://web.archive.org/web/20120406152511/http://www.lowell.edu/npoi/ |date=2012-04-06 }}

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[[Category:Astrometry]]
[[Category:Optical telescopes]]
[[Category:Interferometric telescopes]]
[[Category:Military installations in Arizona]]
[[Category:United States Naval Observatory]]