REPORT OF THE HST STRATEGY PANE: A STRATEGY FOR CM RECOVERY The Results of a Special Study August-October 1990 W*»u^ Barnaul IT^u^^ 4 BMDOTIC R.A. EDITED DY BROWN AND H.C.FORD Support for the preparation of this report was provided by NASA under Contract NAS5-26555 through the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy Space Telescope Science Institute Baltimore, Maryland 1991 Accession Number: 6332 Title: Report Of The HSR Strategy Panel: A Strategy For Recovery Personal Author: Brown, R.A.; Ford, H.C. Contract Number: NAS5-26555 Corporate Author or Publisher: Space Telescope Science Institute, Baltimore, MD Report Prepared For: NASA Publication Date: Jan 01,1991 Pages: 119 Comments on Document: from BMDO/DE Descriptors, Keywords: HST Hubble space telescope strategy recovery optical problem solution aberration COSTAR pointing WFPC mission shuttle risk management aperture masking implementation second generation SI mirror Abstract: The HST Strategy Panel held four meetings between mid-August and mid-October 1990. At these meetings, a wide variety of options for correcting spherical aberration were identified and debated. This report presents the Panel's findings and recommendations. Problems included: the optical problem and solution; COSTAR; pointing; WFPC; servicing mission; recommended strategy; and fresh reasons to commit to the HST. REPORT OF THE HST STRATEGY PANEL: A STRATECY FOR RECOVERY THE RESULTS OF A SPECIAL STUDY äUGUST-0CT0RER1990 EDITED DY R. A. BROWN AND H. C. FORD PANEL MEMBERS Robert Brown, Co-Chairman Space Telescope Science Institute Holland Ford, Co-Chairman Space Telescope Science Institute Roger Angel University of Arizona Jacques Beckers European Southern Observatory Pierre Bely Space Telescope Science Institute Piero Benvenuti ST European Coordinating Facility Murk Bottema Ball Aerospace Christopher Burrows Space Telescope Science Institute James Crocker Space Telescope Science Institute Rodger Doxsey Space Telescope Science Institute Sandra Faber Lick Observatory Edward Groth Princeton University Shrinivas Kulkarni California Institute of Technology Bruce McCandless Johnson Space Center/NASA (Ret.) Francesco Paresce Space Telescope Science Institute Lyman Spitzer, Jr. Princeton University Raymond Wilson European Southern Observatory PANEL CHARTER The Panel will identify and assess strategies for recovering HST capabilities degraded by spherical aberration. It will review the current state of the observatory, the Allen Board findings, the scientific potential of the ideal HST, and the tentative science pro- gram of the unimproved HST. It will develop a comprehensive framework for identi- fying possible improvements, including OTA-, instrument-, spacecraft-, and operations- level changes, and including hybrid combinations. Within this framework, the Panel will develop and debate the technical and scientific merits of particular improvements. On the basis of their findings, the Panel will formulate a set of recommendations and conclusions. The Panel will cast its net widely, especially seeking the ideas and appropriate in- volvement from ST Sei staff. It is expected that the whole Institute will be informed regularly of the Panel's thinking and progress. As necessary in the course of its work, the Panel can request ST Sei support for short studies of specific technical or scientific issues that may arise. They may also request the support of outside experts including but not limited to NASA and NASA contractor personnel. The Panel is appointed by and reports to the ST Sei Director, who will take the panel's findings to NASA. CALENDAR OF MEETINGS August 17 & 18,1990 Space Telescope Science Institute, Baltimore September 3 & 4,1990 European Coordinating Facility, Garching October 1& 2,1990 Space Telescope Science Institute, Baltimore October 17 & 18,1990 Space Telescope Science Institute, Baltimore ACKNOWLEDGEMENTS The HST Stragety Panel thanks the many individuals who assisted and participated in this study. The following participated in one or more meetings: Albert Boggess GSFC/NASA Kenneth Olson GSFC Bernard Delabre ESO Michel Saisse LAS William Fastie JHU Ethan Schreier STScI Chris Haniff Caltech Daniel Schroeder Beloit Richard Ho GSFC Ronald Sheffield LMSC Keith Home STScI Peter Stockman STScI James Kunst GSFC Harold Weaver STScI Robin Laurance ESA Richard White STScI Tadashi Nakajima CfA John Wood GSFC Colin Norman STScI Barbara Eller, Alex Aimette, and Britt Sjöberg for their excellent support of the meet- ings of the HST Strategy Panel. Patrick Seitzer, Zoltan Levay, Kip Kunst, and Vance Haemmerle helped prepare the as- tronomical images. This report was produced with editorial assistance from Jan Ishee, Ray Villard, and Lisa Walter. Carl Schuetz, Foxglove Communications, coordinated the graphics, design, and pro- duction of the report. The Co-Chairmen wish to thank Colin Norman for his early, enthusiastic support of the original concept for this study. CONTENTS PREFACE 1 SYNOPSIS.. : .- 3 OPTICAL PROBLEM , • 5 OPTICAL SOLUTION 7 COSTAR .' 13 POINTING '. : 15 WFPCII - • 17 1993 SERVICING MISSION 19 RECOMMENDED STRATEGY 23 FRESH REASONS i -25 APPENDICES GUIDE TO THE APPENDICES 33 APPENDIX A: OPTIONS ...:... ■■ -35 APPENDIX B: OPTICAL ANALYSIS , 73 APPENDIX C: APERTURE MASKING 89 APPENDIX D: POINTING ISSUES 93 APPENDIX E: SHUTTLE SERVICING OF HST , 99 APPENDIX F: RISK MANAGEMENT : 109 APPENDIX G: IMPLEMENTATION FACTORS Ill APPENDIX H: SECOND GENERATION SIS 113 APPENDIX J: GLOSSARY : 117 PREFACE ASTRONOMERS AND ENGINEERS REALIZED that Secondary there was a problem with the images of /v : Mirror Hubble Space Telescope (HST) shortly ii after it was launched in April 1990. The i; I j quality of the images failed to improve t : f ; I despite attempts to adjust the alignment f of the optics. NASA concluded in June r : : : c 1990 that the HST primary mirror had ' ■ - i- : : i - been manufactured with the wrong ' : ! i: ■ i : i : 1 1 : ■ : shape. Compared with the desired pro- i' :. :: . :: file, the mirror surface is too low by an amount that from the center to the edge 42.4 m 1 grows from zero to 0.002 mm or four Primary wavelengths of optical light. NASA Mirror convened an investigatory board in July 1990 under Dr. Lew Allen, which re- ported in November 1990 how the error 1 ' ; 1 Paraxial ■ :■ Focus '■ probably occurred. In late 1980 or early TV- 5= 1981, a technician had improperly as- 5= E •', ro E •: sembled a measuring device used to <*• cO ■• I to ;■ figure the primary mirror. Though tests I i MM arginal. Focus ••' at the time indicated a problem, the warning was not heeded, and the HST was assembled and launched with the Figure 1. Spherical aberration means that flawed mirror. light rays from different radii on the The deformity of the HST mirror primary mirror come to focus at different causes spherical aberration in the im- distances. The marginal focus is 35 mm ages. This means light rays come to a below the focus of the innermost rays, focus at different distances depending which graze the secondary mirror. The on the radius at which the rays strike the paraxial focus is obscured. Currently, the mirror, as shown in Figure 1. Light from adopted focus (not shown) is 12 mm below the edge of the primary mirror comes to the paraxial focus. a focus about 38 mm beyond where the innermost rays converge. No positions, orientations, or other adjustments of the primary and secondary mir- ror can produce the diffraction-limited images required by much of the HST science program. The center of a star image in visible light has a core of radius 0.1 arcsec con- taining about 15% of the light; 70% was expected. The rest is spread about in a complex halo of radius 3 arcsec. Since aperture diffraction sets the size of the image core, the size is smaller at shorter wavelengths. The size of the halo, on the other hand, is set by geo- metrical optics and is constant. (The pattern of the halo varies with wavelength because it is an interference pattern.) Spherical aberration degrades the science capacity of HST. Good science is being accomplished with HST as it is, but many crucial investigations—including many of the original justifications for HST—are on hold until the problem is solved. When the optical problem was announced, NASA began to seek solutions and de- velop a recovery plan. In the first phase, NASA focused on how to modify the scientific instruments already under development. These instruments are the Space Telescope Imaging Spectrograph (STB), the Near-Infrared Camera and Multi-Object Spectrom- eter (NICMOS), and the second Wide Field and Planetary Camera (WFPC II), which NASA began to build in 1985 as a "clone" of the WFPC now in HST. NASA found it is feasible to correct these future instruments to compensate for spherical aberration. Based on this finding, NASA adopted an initial baseline plan to install the corrected WFPC II in place of WFPC on the first servicing mission in 1993, and later, on a second mission in 1996, to install STIS or NICMOS either to recover spectroscopic capabilities (in the case of STIS) or to add new infrared capabilities (with NICMOS). This initial recovery plan of NASA restored faint source detection, one of the most critical capabilities crippled by spherical aberration. However, the plan delayed im- proving spectroscopy until the second half of the HST mission, and did not address full- resolution imaging at all. For these reasons, the HST Strategy Panel was formed in mid- August 1990 with a charter to search briskly for additional or alternative solutions. In this second phase of NASA study, the HST Strategy Panel sought the best overall strategy to recover all primary HST science capabilities at an early time. The Panel did not adopt the WFPC II fix as a groundrule, but started "with a clean sheet of paper," and tried to identify and review all potential options to alleviate the negative effects of spherical aberration on the HST science program. However, the Panel's recommenda- tions and deliberations were firmly rooted in the assumption that the schedule for the two second generation instruments, STIS and NICMOS, would be adhered to by NASA. The HST Strategy Panel's findings and recommendations were presented to Dr. Riccardo Giacconi, Director of the Space Telescope Science Institute, on October 18,1990. The Panel proposed a new program component as part of an augmented recovery strategy. The new component is the Corrective Optics Space Telescope Axial Replace- ment (COSTAR), a device to deploy corrective optics in front of the Faint Object Cam- era (FOC), High Resolution Spectrograph (HRS), and Faint Object Spectrograph (FOS). The strategy is to install both COSTAR and WFPC II into HST on the first servicing mission in 1993, and to fix the HST pointing problems. This strategy recovers essentially all the science capabilities expected at launch. Dr. Giacconi endorsed the oral recommendations of the Panel and took the findings to NASA management. The Panel made a presentation at NASA Headquarters on Oc- tober 26,1990. In the following weeks, NASA conducted an intensive study of the feasibility and costs of COSTAR. In December 1990, NASA Headquarters authorized the implementation of the COSTAR program to proceed. SYNOPSIS SS THE HST STRATEGY PANEL HELD FOUR MEETINGS between mid-August and mid-October 1990. ^^ At these meetings, a wide variety of options for correcting spherical aberration were identified and debated. This report, as outlined below, presents the Panel's findings and recommendations. The OPTICAL PROBLEM is now understood well enough to design and install a highly ef- fective optical correction. The OPTICAL SOLUTION is a pair of mirrors for each Science Instrument (SI) field of view. The first corrective mirror forms an image of the HST primary mirror on the second corrective mirror; the second corrective mirror has spherical aberration in precisely the same amount as the primary mirror—but with the opposite mathematical sign, thus cancelling the effect. The COSTAR is the proposed device to carry and deploy the corrective optics for three scientific instruments, the FOC, HRS, and FOS. COSTAR would replace the High Speed Photometer (HSP). The POINTING of HST must be improved to gain full value from the restored HST optical performance. The solar array "snap" that causes HST to lose pointing lock at day/night transitions must be fixed. The Panel further recommends that the operational parameters of the guidance system be adjusted to reduce jitter in the coarse tracking mode. The WFPCII is being corrected with the same optical solution used in COSTAR. The Panel found that the alignment of the corrective optics is critical, which COSTAR can assure by special mechanisms. No comparable mechanisms exist in the original design for WFPC, and because WFPC II is a close copy of the original, the Panel recommends that the issue of WFPC II align- ment be addressed with critical attention. The 1993 SERVICING MISSION can install the WFPC LI and COSTAR. This currently planned mission can solve the spherical aberration problem for the Sis, fix the so- lar array disturbances, and replace other subsystems, as necessary. The RECOMMENDED STRATEGY is to develop COSTAR on an urgent basis, continue WFPC II development with special attention to the alignment concerns, and improve the coarse track pointing performance by operational measures. Then, the 1993 HST servicing mission restores the scientific functionality expected at launch. The FRESH REASONS to commit new resources to fix HST are abundant in the science program awaiting sharp images and precise pointing. This science pro- gram is the culmination of decades, even centuries, of maturing questions about the universe. It is also a program proposed largely by young as- tronomers, who need a restored HST to make the discoveries that will propel astronomical exploration into the twenty-first century. The APPENDICES document the approach, options, background findings, and analyses of the HST Strategy Panel.