Editing Hubble Space Telescope (section)

== Flawed mirror ==
[[File:Hubble PSF with flawed optics.jpg|thumb|An extract from a WF/PC image shows the light from a star spread over a wide area instead of being concentrated on a few pixels.]]

Within weeks of the launch of the telescope, the returned images indicated a serious problem with the optical system. Although the first images appeared to be sharper than those of ground-based telescopes, Hubble failed to achieve a final sharp focus and the best image quality obtained was drastically lower than expected. Images of [[point source]]s spread out over a radius of more than one arcsecond, instead of having a [[point spread function]] (PSF) concentrated within a circle 0.1&nbsp;[[arcsecond]]s (485&nbsp;n[[radian|rad]]) in diameter, as had been specified in the design criteria.<ref>{{cite journal |title=The imaging performance of the Hubble Space Telescope |journal=Astrophysical Journal Letters |first1=Christopher J. |last1=Burrows |first2=Jon A. |display-authors=4 |last2=Holtzman |first3=S. M. |last3=Faber |first4=Pierre Y. |last4=Bely |first5=Hashima |last5=Hasan |first6=C. R. |last6=Lynds |first7=Daniel |last7=Schroeder |volume=369 |pages=L21–L25 |date=March 10, 1991 |doi=10.1086/185950 |bibcode=1991ApJ...369L..21B}}</ref><ref>{{cite web |url=https://www.stsci.edu/files/live/sites/www/files/home/hst/documentation/_documents/wfpc2/wfpc2_ihb_cycle17.pdf |title=WFPC2 Instrument Handbook |at=Chapter 5.1 |publisher=STScI |date=2008 |last1=McMaster |first1=Matt |last2=Biretta |first2=John |location=Baltimore |version=10.0 |access-date=April 7, 2022 |archive-date=July 15, 2022 |archive-url=https://web.archive.org/web/20220715150041/https://www.stsci.edu/files/live/sites/www/files/home/hst/documentation/_documents/wfpc2/wfpc2_ihb_cycle17.pdf |url-status=live }}</ref>

Analysis of the flawed images revealed that the primary mirror had been polished to the wrong shape. Although it was believed to be one of the most precisely [[figuring|figured]] optical mirrors ever made, smooth to about 10 nanometers,<ref name="ScienceSPF" /> the outer perimeter was too flat by about 2200 nanometers (about {{frac|450}} mm or {{frac|11000}} inch).<ref name="Servicing Mission 1">{{cite web |url=http://asd.gsfc.nasa.gov/archive/hubble/missions/sm1.html |title=Servicing Mission 1 |publisher=NASA |access-date=March 28, 2016 |archive-url=https://web.archive.org/web/20080420202154/http://hubble.nasa.gov/missions/sm1.php |archive-date=April 20, 2008}}</ref> This difference was catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edge of a mirror [[focus (optics)|focuses]] on a different point from the light reflecting off its center.{{sfn|Tatarewicz|1998|p=375}}

The effect of the mirror flaw on scientific observations depended on the particular observation—the core of the aberrated PSF was sharp enough to permit high-resolution observations of bright objects, and spectroscopy of point sources was affected only through a sensitivity loss. However, the loss of light to the large, out-of-focus halo severely reduced the usefulness of the telescope for faint objects or high-contrast imaging. This meant nearly all the cosmological programs were essentially impossible, since they required observation of exceptionally faint objects.{{sfn|Tatarewicz|1998|p=375}} This led politicians to question NASA's competence, scientists to rue the cost which could have gone to more productive endeavors, and comedians to make jokes about NASA and the telescope. In the 1991 comedy ''[[The Naked Gun 2½: The Smell of Fear]]'', in a scene where historical disasters are displayed, Hubble is pictured with [[Titanic|RMS ''Titanic'']] and [[LZ 129 Hindenburg|LZ 129 ''Hindenburg'']].<ref>{{cite web |author=Powell |first=Corey S. |date=April 24, 2015 |title=The Many Resurrections of the Hubble Space Telescope |url=https://www.discovermagazine.com/the-sciences/the-many-resurrections-of-the-hubble-space-telescope |url-status=live |archive-url=https://web.archive.org/web/20220715150043/https://www.discovermagazine.com/the-sciences/the-many-resurrections-of-the-hubble-space-telescope |archive-date=July 15, 2022 |access-date=December 16, 2020 |work=Discover Magazine}}</ref>{{sfn|Tatarewicz|1998|p=373}} Nonetheless, during the first three years of the Hubble mission, before the optical corrections, the telescope carried out a large number of productive observations of less demanding targets.<ref>{{cite journal |author=Goodwin, Irwin |date=1994 |title=Hubble repair improves vision and helps restore NASA's image |journal=Physics Today |volume=47 |issue=3 |page=42 |author2=Cioffi, Denis F. |doi=10.1063/1.2808434 |bibcode=1994PhT....47c..42G}}</ref> The error was well characterized and stable, enabling astronomers to partially compensate for the defective mirror by using sophisticated [[image processing]] techniques such as [[deconvolution]].{{sfn|Dunar|Waring|1999|pp=514–515}}

=== Origin of the problem ===
[[File:PIA22913-HubbleSpaceTelescope-ComparisonOfCameraImages-20181204.jpg|thumb|upright=2.5|Optical evolution of Hubble's primary camera system: these images show spiral galaxy [[Messier 100|M100]] as seen with WFPC1 in 1993 before corrective optics (left), with WFPC2 in 1994 after correction (center), and with WFC3 in 2018 (right).]]

A commission headed by [[Lew Allen]], director of the [[Jet Propulsion Laboratory]], was established to determine how the error could have arisen. The Allen Commission found that a reflective [[null corrector]], a testing device used to achieve a properly shaped non-spherical mirror, had been incorrectly assembled—one lens was out of position by {{convert|1.3|mm|in|abbr=on}}.{{sfn|Allen|Angel|Mangus|Rodney|1990|p=7-1|ps=: The spacing of the field lens in the corrector was to have been done by laser measurements off the end of an invar bar. Instead of illuminating the end of the bar, however, the laser in fact was reflected from a worn spot on a black-anodized metal cap placed over the end of the bar to isolate its center (visible through a hole in the cap). The technician who performed the test noted an unexpected gap between the field lens and its supporting structure in the corrector and filled it in with an ordinary metal washer.}} During the initial grinding and polishing of the mirror, [[Perkin-Elmer]] analyzed its surface with two conventional refractive null correctors. However, for the final manufacturing step ([[figuring]]), they switched to the custom-built reflective null corrector, designed explicitly to meet very strict tolerances. The incorrect assembly of this device resulted in the mirror being ground very precisely but to the wrong shape. During fabrication, a few tests using conventional null correctors correctly reported [[spherical aberration]]. But these results were dismissed, thus missing the opportunity to catch the error, because the reflective null corrector was considered more accurate.{{sfn|Dunar|Waring|1999|p=512|ps=: "the firm's optical operations personnel dismissed the evidence as itself flawed. They believed the other two null correctors were less accurate than the reflective null corrector and so could not verify its reliability. Since they assumed the perfection of the mirror and reflective null corrector, they rejected falsifying information from independent tests, believed no problems existed, and reported only good news."}}

The commission blamed the failings primarily on Perkin-Elmer. Relations between NASA and the optics company had been severely strained during the telescope construction, due to frequent schedule slippage and cost overruns. NASA found that Perkin-Elmer did not review or supervise the mirror construction adequately, did not assign its best optical scientists to the project (as it had for the prototype), and in particular did not involve the optical designers in the construction and verification of the mirror. While the commission heavily criticized Perkin-Elmer for these managerial failings, NASA was also criticized for not picking up on the quality control shortcomings, such as relying totally on test results from a single instrument.{{sfn|Allen|Angel|Mangus|Rodney|1990|p=10-1}}

=== 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>