Editing Active optics

{{Short description|Shaping technology for reflecting telescopes}}
[[Image:GTC Active Optics Acutators.jpg|thumb|right|Actuators of the active optics of the ''[[Gran Telescopio Canarias]]''.]]
'''Active optics''' is a [[technology]] used with [[reflecting telescope]]s developed in the 1980s,<ref>{{cite journal|last=Hardy|first=John W.|title=Active optics: A new technology for the control of light|date=June 1977|series=Proceedings of the IEEE|pages=110|url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA339170|bibcode=1978IEEEP..66..651H|journal=IEEE Proceedings|volume=66|access-date=2011-06-01|archive-url=https://web.archive.org/web/20151222112555/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA339170|archive-date=2015-12-22|url-status=dead}}</ref> which actively shapes a telescope's [[mirror]]s to prevent deformation due to external influences such as wind, temperature, and mechanical stress. Without active optics, the construction of 8 metre class telescopes is not possible, nor would telescopes with segmented mirrors be feasible.

This method is used by, among others, the [[Nordic Optical Telescope]],<ref>{{cite conference|last=Andersen|first=T.|author2=Andersen, T.|author3= Larsen, O. B.|author4= Owner-Petersen, M.|author5= Steenberg, K.|title=Active Optics on the Nordic Optical Telescope|conference=ESO Conference and Workshop Proceedings|date=April 1992|series=Progress in Telescope and Instrumentation Technologies|pages=311–314|editor1-first=Marie-Helene|editor1-last=Ulrich|bibcode=1992ESOC...42..311A}}</ref> the [[New Technology Telescope]], the [[Telescopio Nazionale Galileo]] and the [[Keck telescope]]s, as well as all of the largest telescopes built since the mid-1990s.

Active optics is not to be confused with [[adaptive optics]], which operates at a shorter timescale and corrects atmospheric distortions.

== In astronomy ==
[[File:Prototype of part of the adaptive support system of the E-ELT.jpg|thumb|Prototype of part of the adaptive support system of the [[E-ELT]].<ref>{{cite news|title=ESO Awards Contract for E-ELT Adaptive Mirror Design Study|url=http://www.eso.org/public/announcements/ann12032/|accessdate=25 May 2012|newspaper=ESO Announcements}}</ref> ]]

Most modern telescopes are reflectors, with the [[Primary mirror|primary element]] being a very large [[mirror]].  Historically, primary mirrors were quite thick in order to maintain the correct surface figure in spite of forces tending to deform it, like wind and the mirror's own weight.  This limited their maximum diameter to 5 or 6 metres (200 or 230&nbsp;inches), such as [[Palomar Observatory]]'s [[Hale Telescope]].

A new generation of telescopes built since the 1980s uses thin, lighter weight mirrors instead. They are too thin to maintain themselves rigidly in the correct shape, so an array of [[actuator]]s is attached to the rear side of the mirror. The actuators apply variable forces to the mirror body to keep the reflecting surface in the correct shape over repositioning. The telescope may also be segmented into multiple smaller mirrors, which reduce the sagging due to weight that occurs for large, monolithic mirrors.

The combination of actuators, an image quality [[detector]], and a computer to control the actuators to obtain the best possible image, is called ''active optics''.

The name ''active'' optics means that the system keeps a mirror (usually the primary) in its optimal shape against environmental forces such as wind, sag, thermal expansion, and telescope axis deformation. Active optics compensate for distorting forces that change relatively slowly, roughly on timescales of seconds. The telescope is therefore ''actively'' still, in its optimal shape.

=== Comparison with adaptive optics ===

Active optics should not be confused with [[adaptive optics]], which operates on a much shorter timescale to compensate for atmospheric effects, rather than for mirror deformation. The influences that active optics compensate (temperature, gravity) are intrinsically slower (1&nbsp;Hz) and have a larger amplitude in aberration. Adaptive optics on the other hand corrects for [[Earth's atmosphere|atmospheric]] distortions that affect the image at 100–1000&nbsp;Hz (the [[Greenwood frequency]],<ref>
{{cite journal
|last=Greenwood
|first=Darryl P.
|title=Bandwidth specification for adaptive optics systems
|journal=Journal of the Optical Society of America
|date=March 1977
|volume=67
|issue=3
|pages=390–393
|doi=10.1364/JOSA.67.000390
|url=http://www.astro.uu.nl/~werkhvn/study/Y5_07_08/master/papers/1977JOSA...67..390G.pdf
|bibcode=1977JOSA...67..390G}}</ref>
depending on wavelength and weather conditions). These corrections need to be much faster, but also have smaller amplitude. Because of this, adaptive optics uses smaller [[Deformable mirror|corrective mirrors]]. This used to be a separate mirror not integrated in the telescope's light path, but nowadays this can be the [[Secondary mirror|second]],<ref>
{{cite journal
 |last=Riccardi 
 |first=Armando 
 |author2=Brusa, Guido 
 |author3=Salinari, Piero 
 |author4=Gallieni, Daniele 
 |author5=Biasi, Roberto 
 |author6=Andrighettoni, Mario 
 |author7=Martin, Hubert M 
 |editor-first1=Peter L 
 |editor-first2=Domenico 
 |editor-last1=Wizinowich 
 |editor-last2=Bonaccini 
 |title=Adaptive secondary mirrors for the Large Binocular Telescope 
 |journal=Proceedings of the SPIE 
 |date=February 2003 
 |volume=4839 
 |series=Adaptive Optical System Technologies II 
 |pages=721–732 
 |doi=10.1117/12.458961 
 |bibcode=2003SPIE.4839..721R 
 |url=http://obelix.arcetri.astro.it/tech/4839-85.pdf
|archive-url=https://web.archive.org/web/20110823131420/http://obelix.arcetri.astro.it/tech/4839-85.pdf
|url-status=dead
|archive-date=2011-08-23
|citeseerx=10.1.1.70.8438 
 |s2cid=124041896 
 }}</ref><ref>{{cite conference
|last=Salinari
|first=P.
|author2=Del Vecchio, C.|author3= Biliotti, V.
|title=A Study of an Adaptive Secondary Mirror
|series=Active and adaptive optics
|conference=ESO Conference and Workshop Proceedings
|date=August 1994
|pages=247–253
|publisher=ESO
|location=Garching, Germany
|bibcode=1994ESOC...48..247S}}</ref> third or fourth<ref>{{cite conference
|last=Crépy
|first=B.
|title=The M4 adaptive unit for the E-ELT
|conference=1st AO4ELT conference – Adaptative Optics for Extremely Large Telescopes Proceedings
|date=June 2009
|doi=10.1051/ao4elt/201006001
|publisher=EDP Sciences
|location=Paris, France
|bibcode=2010aoel.confE6001C|display-authors=etal|doi-access=free
}}</ref> mirror in a telescope.

== Other applications ==
Complicated laser set-ups and interferometers can also be actively stabilized.

A small part of the beam leaks through beam steering mirrors and a four-quadrant-diode is used to measure the position of a laser beam and another in the focal plane behind a lens is used to measure the direction.  The system can be sped up or made more noise-immune by using a [[PID controller]]. For pulsed lasers the controller should be locked to the repetition rate. A continuous (non-pulsed) pilot beam can be used to allow for up to 10&nbsp;kHz bandwidth of stabilization (against vibrations, air turbulence, and acoustic noise) for low repetition rate lasers.

Sometimes [[Fabry–Pérot interferometer]]s have to be adjusted in length to pass a given wavelength. Therefore, the reflected light is extracted by means of a [[Faraday rotator]] and a [[polarizer]]. Small changes of the incident wavelength generated by an [[acousto-optic modulator]] or [[Interference (wave propagation)|interference]] with a fraction of the incoming radiation delivers the information whether the Fabry Perot is too long or too short.
 
Long [[optical cavity|optical cavities]] are very sensitive to the mirror alignment. A control circuit can be used to peak power. One possibility is to perform small rotations with one end mirror. If this rotation is about the optimum position, no power oscillation occurs. Any beam pointing oscillation can be removed using the beam steering mechanism mentioned above.

[[X-ray]] active optics, using actively deformable grazing incidence mirrors, are also being investigated.<ref>{{cite web|title=Research Partnership Advances X-ray Active Optics|url=http://www.adaptiveoptics.org/News_0305_1.html|work=adaptiveoptics.org|accessdate=2 June 2011|archiveurl=https://web.archive.org/web/20070311072126/http://www.adaptiveoptics.org/News_0305_1.html|date=March 2005|archivedate=March 11, 2007}} [http://www.mssl.ucl.ac.uk/smartoptics/Newsletter/SO_Newsletter_7.pdf Alt URL]</ref>

==See also==
* [[Adaptive optics]] – faster technology for smaller aberrations.
* [[Telescope]]
* [[Active surface]] – similar technology for radio telescopes.
* [[List of telescope parts and construction]]

==References==
{{Reflist}}

==External links==
* [https://web.archive.org/web/19980215175128/http://www.eso.org/aot/introduction.html An introduction to active & adaptive optics] ([[European Southern Observatory]] web-site)
* [http://www.eso.org/sci/facilities/eelt/owl/FAQs.html#Active_optics Active optics] on ESO's [[New Technology Telescope|NTT]].
* [http://www.gtcdigital.net/todogtc.php?op1=2&op2=13&lang=en Active optics] at the [[Gran Telescopio Canarias]].

{{Authority control}}

[[Category:Telescopes]]
[[Category:Astronomical imaging]]

[[sv:Teleskop#Aktiv optik]]