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Overview. Hubble Space Telescope was first of NASA's Great ObservatoriesLaunched in 1990Flawed optics only discovered once it was on-orbitInvestigation later revealed mirror flaws should have been apparent early onClassic
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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 Overview Ritchey-Chretien Cassegrain Design 2.4M Aspheric Primary Mirror .3M Aspheric Secondary Mirror
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. Null Corrector Design 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
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. Primary Mirror Testing 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
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) learned Why 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