The Hubble Space Telescope Spherical Aberration

- It All Started With A Scratch…

Opti 521

Paul Ward-Dolkas

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

Telescope OverviewRitchey-Chretien Cassegrain Design

2.4M Aspheric Primary Mirror.3M Aspheric Secondary Mirror

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

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

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

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

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

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 m– Spacing checked with interferometer

against reference bar

• Same physical unit used to test mirror– Judge & jury

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

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

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.

On-Orbit Test Results

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

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

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

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

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