The Hubble Space Telescope Spherical Aberration - It All Started With A Scratch…

1. The Hubble Space Telescope Spherical Aberration- It All Started With A Scratch… Opti 521 Paul Ward-Dolkas

2. Overview • Hubble Space Telescope was first of NASA’s Great Observatories • Launched in 1990 • Flawed optics only discovered once it was on-orbit • Investigation later revealed mirror flaws should have been apparent early on • Classic “How not to do things” story

3. Telescope OverviewRitchey-Chretien Cassegrain Design 2.4M Aspheric PrimaryMirror .3M Aspheric SecondaryMirror

4. Hubble Programmatics • Two NASA Centers vying for lead • Marshall (MSFC) vs. Goddard (GSFC) • Goddard nominal choice – on paper • Normal center for space science missions • Overcommitted in early 70’s • Marshall primarily rocket center – however: • Apollo Telescope Mount (ATM) on Skylab • Too much staff, too few programs • Need to retain staff for Shuttle program

5. Hubble Programmatics (con’t) • Project Lead given to Marshall • Goddard in charge of Scientific Instruments (SIs) • Decision did not bode well for project • Most astronomy expertise @ Goddard • Can’t force cooperation between autonomous centers • Lack of cooperation between staff

6. Contractors • Lockheed won contract to build surrounding spacecraft & systems • Perkin-Elmer (vs. Kodak) built OTA • NASA/Marshall (initially) did systems integration • Later turned role over to Lockheed • Program culture (vis-a-vie PE) established and unable to change

7. Perkin-Elmer Culture • Not traditionally a NASA contractor • Closed shop mentality • Classified project in-house @PE initially gave NASA staff limited access • Few NASA staff had extensive optics background • PE became unresponsive to NASA, then Lockheed oversight

8. Mirror Fabrication • Perkin-Elmer planned on using new, automated mirror fabrication process • Innovative reflective null corrector (RNC) to guide polishing • RNC design easy to inspect & verify • Polished to .014 Wrms @ 632.8nm • Better than spec

9. Reflective Null Corrector used to fabricate primary mirror. Null correctors produce an “optical template” of the correct mirror shape In a perfect mirror, wavefront reflects back exactly to origin Difference between source & image used to measure error in lens shape. Much more accurate than conventional refractive null corrector Designed to be easily verifiable All three elements accessible to verify spacing Spacing required to be +/- 10 mm Spacing checked with interferometer against reference bar Same physical unit used to test mirror Judge & jury Null Corrector Design

10. Null Corrector Assembly • Interferometers used to check element spacing • Compared to spacer bars of correct length • Painted caps placed over ends of spacers to cover all but small spot for measurement • One cap had a slight chip in the dark paint… • Interferometer measured distance to scratch on cap vs. distance to end of bar • Technician thought element spacing was 1.3 mm shorter than it was • “Fixed” problem by adding washers to RNC • Didn’t flag problem or bother to verify measurement

11. Inverse Null Corrector (INC) showed something was amiss Used to check alignment of setup by simulating primary mirror Showed RNC had spherical aberration Not thought to be as accurate as RNC, so concerns dismissed Primary Mirror Testing

12. Primary Mirror Testing (cont’d) • Conventional Refractive Null Corrector (RvNC) used as check for final mirror • Initially used for mirror polishing, since RNC wasn’t ready yet • Clearly showed spherical aberration when used on final mirror • Wasn’t as accurate as RNC, so concerns dismissed.

13. On-Orbit Test Results

14. Analysis of Problem • Identical results were obtained with 2 independent SIs – so problem must be with OTA • Flaws in primary mirror would be symmetrical in nature • Flaws in secondary mirror would produce coma aberration as well • Lack of significant coma indicated primary mirror was flawed

15. Commission Findings • Allen Commission established to determine cause and (hopefully) remedy • Investigation quickly found discrepancies in test data • Old tests with RvNC found in engineer’s notebook didn’t match RNC data • RvNC tests matched tests with INC • Independent testing of RvNC found no flaws • Focus shifted to RNC

16. Null Corrector Testing • Testing with INC verified RNC as source of primary mirror flaw. • Tests showed same aberration in the RNC in 1990 as in 1982

17. Remedy • Primary mirror had been precisely polished to wrong prescription • NASA was able to duplicate on-orbit results with ray trace analysis once actual shape was known • New corrective lenses designed for installation, initially replacing Axial SI

18. Lessons (hopefully) learnedWhy bother with the Allen Report? • Relying on the same device to both fabricate and verify is like using the same lawyer for judge & jury • Oversight can be a pain, but it serves as a valuable sanity check • Overseers must have proper skill set, and ask the right questions • Staff should relay problems & concerns upward and not just “fix the problem” • Suspicious test data should be investigated and not dismissed out of hand