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The article compares closed-loop real-time measurement and correction to alternative active control strategies, contrasts monolithic and segmented-mirror control technologies, and traces conceptual origins from early support methods to 20th-century systematic ideas.",{"@graph":69,"@context":122},[70,84,105],{"@type":71,"itemListElement":72},"BreadcrumbList",[73,77,79,82],{"item":74,"name":75,"@type":76,"position":8},"https://docshare.wps.com","Home","ListItem",{"item":78,"name":9,"@type":76,"position":14},"https://docshare.wps.com/document/",{"item":80,"name":40,"@type":76,"position":81},"https://docshare.wps.com/document/research-report/",3,{"item":83,"name":65,"@type":76,"position":19},"https://docshare.wps.com/document/telescopes-and-instrumentation-the-history-and-development-of-the-eso-active-optics-system-active-optics-with-monoliths/269399/",{"url":83,"name":65,"@type":85,"image":86,"author":91,"headline":65,"publisher":94,"fileFormat":97,"inLanguage":63,"description":67,"dateModified":98,"datePublished":99,"encodingFormat":97,"isAccessibleForFree":100,"interactionStatistic":101},"DigitalDocument",{"url":87,"@type":88,"width":89,"height":90},"https://docshare.wps.com/thumbnails/telescopes-and-instrumentation-the-history-and-development-of-the-eso-active-optics-system-active-optics-with-monoliths/269399.png","ImageObject",300,407,{"name":92,"@type":93},"Đào","Person",{"url":74,"name":95,"@type":96},"DocShare","Organization","application/pdf","2026-10-17","2026-09-15",true,{"@type":102,"interactionType":103,"userInteractionCount":44},"InteractionCounter",{"@type":104},"ViewAction",{"@type":106,"mainEntity":107},"FAQPage",[108,114,118],{"name":109,"@type":110,"acceptedAnswer":111},"Why was the ESO active optics system essential for the VLT?","Question",{"text":112,"@type":113},"The VLT’s very thin, flexible mirror needed active correction to generate a usable optical image. Without the approach, the mirror could not deliver a satisfactory star image.","Answer",{"name":115,"@type":110,"acceptedAnswer":116},"How does ESO active optics differ from the Keck telescopes’ correction method?",{"text":117,"@type":113},"ESO uses a closed-loop process that measures star-image quality in real time to drive corrections. The Keck telescopes rely on an internal active system rather than the same closed-loop star-image measurement approach.",{"name":119,"@type":110,"acceptedAnswer":120},"What major technological difference is emphasized between monolith and segmented mirror active control?",{"text":121,"@type":113},"The control technologies differ because monolithic mirrors have no discontinuities in their flexure behavior, while segmented mirrors have fundamental discontinuities that change how flexure must be handled.","https://schema.org",{"og:url":83,"og:type":124,"og:title":65,"og:site_name":95,"og:description":67},"article",{"robots":126,"canonical":83},"index,follow",{"doc_id":128,"site_id":62},269399,1789451270,{"code":4,"msg":5,"data":131},{"doc_id":128,"user_id":132,"nickname":92,"user_avatar":133,"doc_module":4,"category_id":39,"category_name":40,"doc_title":65,"doc_description":67,"doc_content":134,"file_id":135,"file_url":136,"file_type":137,"file_size":138,"view_count":44,"is_deleted":4,"is_public":8,"is_downloadable":8,"audit_status":8,"page_count":39,"language":139,"language_code":63,"site_id":62,"html_lang":63,"table_of_contents":140,"faqs":141,"seo_title":142,"seo_description":67,"update_tm":129,"read_time":46,"preview_page_count":4},1374402968488,"https://ap-avatar.wpscdn.com/davatar_29158cc5080c5b710cf443261637dec0","Telescopes and Instrumentation  \nTHE HISTORY AND DEVELOPMENT OF THE ESO ACTIVE OPTICS SYSTEM  \nTHE ACTIVE OPTICS SYSTEM IS THE FUNDAMENTAL OPTICAL CHARACTERISTIC OF THE ESO NEW TECHNOLOGY TELESCOPE (NTT) AND VERY LARGE TELESCOPE (VLT). THE NTT PIONEERED THIS SYSTEM. WITHOUT IT, THE VLT, WITH ITS THIN, VERY FLEXIBLE MIRROR, COULD NOT GIVE A USABLE OPTICAL IMAGE AT ALL.  \n ~~ ~~ R. N. WILSON  \nT HE TERM “ACTIVE OPTICS”  \nis normally, and I believe  \ncorrectly, associated with the  \nT  \nESO system developed for  \ntelescopes with monolithic primaries and applied in the ESO NTT and VLT telescopes. Technical systems based on the same principles, also with thin meniscus primaries, are used in the other very large telescopes (8 m) of GEMINI (2x) and SUBARU (1x) . Other important telescopes with monolithic primaries, but using stiffer lightweighted blanks, are also actively controlled, the most notable being the WIYN (3.5 m) telescope, the three 6.5 m telescopes of the MMT upgrade and the two Magellan telescopes, and the two 8.4 m telescopes of the Large Binocular Telescope (LBT) . The other major branch of modern optical telescope development, that using segmented mirrors, was pioneered and exemplified by the two Keck 10 m telescopes. This has its own system of active control of the segmented primary. Although the aim is the same, the technologies involved in the control systems of monoliths and segmented mirrors are essentially different. This is the case because the flexure function of a monolith has no discontinuities, which are fundamental to the nature of segmented mirrors. The ESO active optics system is“closed loop” in the sense that correction is made by measurements in real time of the quality of a star image. This is not the case with the Keck telescopes which correct the primary with an internal active system.“Closed loop”in the ESO sense is also not necessarily used in the other active systems for monolithic primaries: some of them rely on “precalibration” of the flexure effects (see page 9) .  \nIn this article, I shall confine myself to active optics with monoliths. When was the idea of active optics for monoliths first conceived? We must remember that all “classical” telescopes had monolithic  \nmirrors, normally made as thick and stiff as possible to avoid flexure. The idea of segmenting goes back to Lord Rosse in 1828, but was first realised in practice by Horn dA’ rturo in the 1950s using a fixed primary. Keck I, finished in 1992, was the first such telescope with a normal 2-axis mounting. So it was logical that earlier ideas of active optics should have been limited to monoliths. The history of the development of mirror support systems and of active optics for monoliths is given in relatively complete form in my second book, Reflecting Telescope Optics II (Wilson 1999; W99 in the following) . The present article is a simplified and much abbreviated version, with more emphasis on the personalities involved.  \nA friend of mine at ESO, who comes from the rich French tradition in telescope optics, recently suggested to me that active optics (and the Ritchey-Chrétien (RC) aplanatic telescope) might have been invented by one of my great French heroes in optics: Jean Bernard Léon Foucault. Reference is made in this connection to his largest − 80 cm − Newton telescope completed in 1862 (Wilson 1996) . With all respect to the great genius Foucault, I believe that neither of these inventions would have been possible at that time. For active optics, no technology existed for measuring in a systematic way  \nthe errors in a star image, although Foucault's invention of the knife-edge test enabled a very sensitive qualitative assessment. The detector available, the eye, was sensitive but highly non-linear; photography was terribly slow and insensitive, and also non-linear. Third order aberration theory, due to Seidel, was only published in 1856 and existed only for spherical surfaces, not for a Newton parabolic primary. The complete","cbCaijWitgYhmTlK","https://ap.wps.com/l/cbCaijWitgYhmTlK","pdf",1881565,"English","# THE HISTORY AND DEVELOPMENT OF THE ESO ACTIVE OPTICS SYSTEM\n## Active optics terminology and telescope families\n## Origins and limitations of earlier ideas","[{\"question\":\"Why was the ESO active optics system essential for the VLT?\",\"answer\":\"The VLT’s very thin, flexible mirror needed active correction to generate a usable optical image. Without the approach, the mirror could not deliver a satisfactory star image.\"},{\"question\":\"How does ESO active optics differ from the Keck telescopes’ correction method?\",\"answer\":\"ESO uses a closed-loop process that measures star-image quality in real time to drive corrections. The Keck telescopes rely on an internal active system rather than the same closed-loop star-image measurement approach.\"},{\"question\":\"What major technological difference is emphasized between monolith and segmented mirror active control?\",\"answer\":\"The control technologies differ because monolithic mirrors have no discontinuities in their flexure behavior, while segmented mirrors have fundamental discontinuities that change how flexure must be handled.\"}]","Telescopes and Instrumentation - THE HISTORY AND DEVELOPMENT OF THE ESO ACTIVE OPTICS SYSTEM - Active optics with monoliths | PDF"]