Editing Hubble Space Telescope (section)

=== Design of a solution ===
[[File:Feustel moving COSTAR.jpg|thumb|left|COSTAR being removed by astronaut [[Andrew J. Feustel]] during SM4 in 2009]]

Many feared that Hubble would be abandoned.{{sfn|Tatarewicz|1998|p=374}} The design of the telescope had always incorporated servicing missions, and astronomers immediately began to seek potential solutions to the problem that could be applied at the first servicing mission, scheduled for 1993. While Kodak had ground a back-up mirror for Hubble, it would have been impossible to replace the mirror in orbit, and too expensive and time-consuming to bring the telescope back to Earth for a refit. Instead, the fact that the mirror had been ground so precisely to the wrong shape led to the design of new optical components with exactly the same error but in the opposite sense, to be added to the telescope at the servicing mission, effectively acting as "[[Glasses|spectacles]]" to correct the spherical aberration.<ref>{{Cite book |last=Chaisson |first=Eric |url=http://archive.org/details/hubblewarsastrop00chai |title=The Hubble wars: astrophysics meets astropolitics in the two-billion-dollar struggle over the Hubble Space Telescope |date=1994 |publisher=New York : HarperCollins Publishers |others=Internet Archive |isbn=978-0-06-017114-8 |page=184 |language=en-us}}</ref><ref>{{cite journal |journal=Popular Science |url=https://books.google.com/books?id=lQEAAAAAMBAJ&pg=PA72 |date=October 1990 |title=The Trouble with Hubble |first=Arthur |last=Fisher |page=100 |access-date=November 8, 2012 |archive-date=January 8, 2022 |archive-url=https://web.archive.org/web/20220108230555/https://books.google.com/books?id=lQEAAAAAMBAJ&pg=PA72 |url-status=live }}</ref>

The first step was a precise characterization of the error in the main mirror. Working backwards from images of point sources, astronomers determined that the [[conic constant]] of the mirror as built was {{val|−1.01390|0.0002|fmt=none}}, instead of the intended {{val|−1.00230|fmt=none}}.<ref>{{cite tech report |title=Image inversion analysis of the HST OTA (Hubble Space Telescope Optical Telescope Assembly), phase A |publisher=TRW, Inc. Space and Technology Group |first=M. M. |last=Litvac |date=1991 |bibcode=1991trw..rept.....L}}</ref><ref>{{cite journal |title=Optical Prescription of the HST |journal=Calibrating Hubble Space Telescope. Post Servicing Mission |pages=132 |url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/31621/1/95-1205_A1b.pdf |hdl=2014/31621 |date=July 1995 |publisher=NASA JPL |last1=Redding |first1=David C. |last2=Sirlin |first2=S. |last3=Boden |first3=A. |last4=Mo |first4=J. |last5=Hanisch |first5=B. |last6=Furey |first6=L. |url-status=dead|archive-url=https://web.archive.org/web/20150501140016/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/31621/1/95-1205_A1b.pdf |archive-date=May 1, 2015|bibcode=1995chst.conf..132R}}</ref> The same number was also derived by analyzing the null corrector used by Perkin-Elmer to figure the mirror, as well as by analyzing [[interferogram]]s obtained during ground testing of the mirror.{{sfn|Allen|Angel|Mangus|Rodney|1990|pp=E-1}}

Because of the way the HST's instruments were designed, two different sets of correctors were required. The design of the Wide Field and Planetary Camera 2, already planned to replace the existing WF/PC, included relay mirrors to direct light onto the four separate charge-coupled device (CCD) chips making up its two cameras. An inverse error built into their surfaces could completely cancel the aberration of the primary. However, the other instruments lacked any intermediate surfaces that could be configured in this way, and so required an external correction device.{{sfn|Tatarewicz|1998|p=376}}

The [[Corrective Optics Space Telescope Axial Replacement]] (COSTAR) system was designed to correct the spherical aberration for light focused at the FOC, FOS, and GHRS. It consists of two mirrors in the light path with one ground to correct the aberration.<ref>{{cite journal |author=Jedrzejewski |first1=R. I. |last2=Hartig |first2=G. |last3=Jakobsen |first3=P. |last4=Ford |first4=H. C. |date=1994 |title=In-orbit performance of the COSTAR-corrected Faint Object Camera |journal=Astrophysical Journal Letters |volume=435 |pages=L7–L10 |bibcode=1994ApJ...435L...7J |doi=10.1086/187581}}</ref> To fit the COSTAR system onto the telescope, one of the other instruments had to be removed, and astronomers selected the High Speed Photometer to be sacrificed.{{sfn|Tatarewicz|1998|p=376}} By 2002, all the original instruments requiring COSTAR had been replaced by instruments with their own corrective optics.<ref name="COSTARNotNeeded" /> COSTAR was then removed and returned to Earth in 2009 where it is exhibited at the National Air and Space Museum in Washington, D.C.<ref>{{Cite news|title=Camera That Saved Hubble Now On Display|work=NPR|url=https://www.npr.org/templates/story/story.php?storyId=120539846|access-date=December 30, 2021|archive-date=December 30, 2021|archive-url=https://web.archive.org/web/20211230042239/https://www.npr.org/templates/story/story.php?storyId=120539846|url-status=live}}</ref> The area previously used by COSTAR is now occupied by the [[Cosmic Origins Spectrograph]].<ref name="SM4">{{cite web |title=Hubble Essentials |url=http://hubblesite.org/the_telescope/hubble_essentials/ |url-status=dead |archive-url=https://web.archive.org/web/20121028200607/http://hubblesite.org/the_telescope/hubble_essentials/ |archive-date=October 28, 2012 |access-date=November 8, 2012 |website=HubbleSite.org |publisher=[[Space Telescope Science Institute]]}}</ref>