Editing Active optics (section)

=== 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.