Hubble’s Mirror Flaw
NASA has established how a mirror aboard its $1.5 billion Hubble Telescope came to be the wrong shape. The agency said last week that errors in a test instrument apparently led Perkin-Elmer, which fabricated the optics, to finish the 2.4-metre primary mirror of the Hubble Space Telescope incorrectly. Tests by NASA earlier this month showed that a lens in the test instrument, called the ‘reflective null corrector’, is about a millimetre askew. Preliminary analysis indicates that an error of this magnitude could cause the spherical aberration that prevents Hubble from focusing sharply.
Spiral galaxy M100. Left: image taken in 1993 with Hubble’s flawed primary mirror. Right: image taken by the Wide Field Camera-3 after it was installed in 2009. Credit: NASA
The crucial error, the misalignment of a lens by a millimetre, is ‘very large’ by optical standards, says Daniel Schroeder, an astronomer at Beloit College in Wisconsin, and a codesigner of Hubble. In some optical instruments, positions are measured to a fraction of the wavelength of light, less than a thousandth of a millimetre.
Ironically, Perkin-Elmer ignored warnings of the problem detected by a cruder test instrument. NASA says that it trusted the refractive null corrector because it had been ‘certified’ for final tests of the mirror’s shape.
Perkin-Elmer, which built Hubble at its Danbury plant in Connecticut, tested the primary and secondary mirrors separately, but no one tested the complete telescope before launch. An earlier check by NASA absolved the design itself of blame, leading the agency to narrow the inquiry to possible errors in the testing of the mirrors.
The reflective null corrector is a cylinder 76 centimetres high and about half a metre wide. It contains two mirrors and a lens, made specifically to test the Hubble primary. By passing light between two mirrors and through a field lens, the instrument should have generated a wavefront matching the desired shape of the primary mirror. The mirror was finished to match the wavefront, so errors in the corrector meant that the final mirror had the wrong shape (see above diagram).