Class Note for OPTI 521 at UA
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Date Created: 02/06/15
0PTi 521 synopsis Tianduan Su How Hubble Space Telescope failed Synupsis ufHubee Space Telesmpe Optical Systems Failure Repurt OpTi 521 synopsis by Tianduan Su introduction The Hubble Space Telescope HST was launched on April 24 1990 During checkout on orbit it was discovered that the primary mirror was misshaped leading to 047wave rms wavefront error at 6328nm This fabrication flaw was coursed by an assembling mistake on thetesting instrument Re ective Null Corrector RNC which was used to test the surface of the mirror Through the investigation ofthe course forthe flaw engineers and researchers should learn how to avoid similar situations happening again HST OPTICAL DESIGN The Optical Telescope Assembly OTA in The Hubble Space Telescope is a wormirror reflecting telescope with a focal ratio of f24 This kind oftelescope is generally referred to as Cassegrain telescope but the mirrors in the OTA are slightly more aspheric than in the normal Cassegrain type The primary mirror in the OTA the one in which the error exists is a 247m diameter concave hyperboloid The 037m diameter secondary mirror is a convex hyperboloid nus rawM Veuomce mtsnwinc Avwwkam mum sansmsm insmwiwsw DENSML 51am um room nmi unis mm mm We Fowsu w 1 minim riguie 1 DplicalTelescnpe Assembly REFLECTIVE NUll CORRECTOR USED IN HST TESTING Aspheric mirrors produce better dualityimagesthan spherical mirrors but the shape makes them more dif cult to test when testing sphere surface the center of curvature ofthe test part is putted coincident with thefocus position ofthetransmission sphere thetest wave ofthe interferometer makes a normal incidence angle with the surface under test and is retroreflected This test configuration known as a null test shows the deviations ofthe measured surface from the reference sphere The standard wayapplyingthis method on a general asphere is to introduce some sort of correction optics in order to adaptthe incomingtest wavefront to the Page 1 of4 OPTI 521 synopsis Tianquan Su surface under test This null corrector or null compensator generates from the incoming spherical wave a wavefront that impinges normally to the test surface and is thus retroreflected forming a null interferogram Null correctors may be implemented with refractive reflective or diffractive optics The primary mirror of OTA was measured with a coaxial reference interferometer in conjunction with a novel reflective null corrector RNC designed by Perkin Elmer Corporation PE The company planned to certify the RNC with great care and not to do any independent testing of the mirror As shown in Figure 2 and Figure 3 the RNC consists of two spherical mirrors the larger of which has a clear aperture of lt 10 in 254 cm and a small refraction field lens whose clear aperture is lt1in 254 cm munu vwmnu u rumn passing lhmugh olner 09mm X ugm lmm rasm alter mm svmnr smm rmqu 2 This spacing was 39x39 millimeus askzw nul cavreclor 39 and led in ths llsw 1 1 on Hubble r 2 a 3 m i r ncil Twe 39 mm kmlm mm 1 was was l This wavnlmnl should have been me desired shape a primary mrrar P mary mimn mu smmm mm mum m Hum um mm mm s mmmn mm sumu nums Figure 2 Reflective Null Corrector Figure 3 Structure of RNC ERRORS IN PRIMARY MIRROR AND RNC After the HST was put into orbit and the first light images were taken computer simulation of these images indicated that 05wave rms wavefront spherical aberration at 547 nm existed in the OTA Through analyzing the aberrations of the image the doubt was centered on the primary mirrors The testing results of the primary mirror differed between RNC and a refractive null corrector the former one showed no error while the latter one give the opposite result The refractive null corrector was used to measure the curvature of the primary ts result was right to explain the image error Thus the RNC was doubted And after other possible courses were excluded the spacing of the RNC elements became the suspect The optical element spacing of the RNC was measured by shining collimated light up through the field lens using an interferometer as the source and by placing a flat mirror at the focus of the Page 2 of4 0PTl 521 synopsis Tianquan Su field lens This measurement showed that the field lens was about 13 mm too farfrom the lower mirror When the field lens position error is taken into account and applied in correcting the data taken with the RNC it results in a mirror shape that would account for most of the error observed in the HST images HOW THIS ERROR OCCURRED As the necessary records to reproduce what actually happened were missed the following analysis is a conjecture The RNC used to measure the HST primary mirror was modified from the one used on a smaller mirror prototype This modification required adding a new field lens and respacing the optical elements Figure 4 shows the positions ofthe metering rods used to set the optics The metering rods were made of lnvar a metal with a small temperature expansion coefficient The ends of the metering rods were rounded and polished for using an interferometerto measure the position This procedure involved autoreflecting a focused beam of light offthe end of a rod and observing an interference pattern from the beam that came back on itself Centering the light beam on the rod end was essential forthe measurement To prevent the metering rod from being misaligned laterally with respect to the interferometer axis PE decided to attach field capsquot to one end ofthe rod Figure 5 The field caps were fitted over the rod ends and had a small aperture in the centerto ensure centering of the rod on the beam a nun JESS J 7L 77777 7 iwmw E3 Figure 4 Position of maturing rod uszdto spacing optical EIEmEMS in RNC Page 3 of4 0le 521 synopsis Tianduan Su leis rrru LAU Figure 5 Displacement due m the imerfemmelerfntusing entire field cap instead nflhe merering rat The top surface olthe field cap was covered with nonreflecting material however some of this material had broken away from a small area around the field cap aperture it appears that the operator obtained reflection from the field cap where the nonreflecting material was absent ratherthan the rod end causing the 13 mm misspacing A test with the equipment showed that it was quite easy even probable to make this error with the configuration used The linal location of the field lens wasthen set with the addition of the spacersAs a result the field lens was about 13 mm too far from its correct position relative to the lower mirror CONCLUSION Large proiects usually have considerable quantities of procedure steps Errors in early step can course huge damage to the whole proiect The investigation clearly showed that the method used to spacing the RNC could be improved Reliance on a single test method is a process which is clearly vulnerable to simple error Errors were shown during the labncation and testing procedure but were ridiculously ignored by PE This indicates that ellicient communication within the group is needed Moreover tight budget both timely and economically prevented the system from being tested fully and independently Alter all competitive organizational cost and schedule pressures were all lactors in limiting full exposure of all the test inlormation to qualified reviewers Reference 1 quotThe Hubble Space Telescope 0ptical Systems Failure Reportquot NASA November 1990 2 L Furey T Dubos D HansenandiSamuelsSchwartz Hubble Space Telescope primarymirror characterization by measurement of the reflective null Correctorquot Applied Optics Vol 32 issue 10 pp 17031714 3 Christof Pruss Eugenio Garbusi and Wolfgang Osten TestingAspheres Optics and Photonics News Vol 19 lssue4 pp 2429 Page 4 of4