Open main menu
Home
Random
Recent changes
Special pages
Community portal
Preferences
About Wikipedia
Disclaimers
Incubator escapee wiki
Search
User menu
Talk
Dark mode
Contributions
Create account
Log in
Editing
Active optics
(section)
Warning:
You are not logged in. Your IP address will be publicly visible if you make any edits. If you
log in
or
create an account
, your edits will be attributed to your username, along with other benefits.
Anti-spam check. Do
not
fill this in!
== 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 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 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 100β1000 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=390β393 |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=721β732 |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=247β253 |publisher=ESO |location=Garching, Germany |bibcode=1994ESOC...48..247S}}</ref> third or fourth<ref>{{cite conference |last=CrΓ©py |first=B. |title=The M4 adaptive unit for the E-ELT |conference=1st AO4ELT conference β 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.
Edit summary
(Briefly describe your changes)
By publishing changes, you agree to the
Terms of Use
, and you irrevocably agree to release your contribution under the
CC BY-SA 4.0 License
and the
GFDL
. You agree that a hyperlink or URL is sufficient attribution under the Creative Commons license.
Cancel
Editing help
(opens in new window)