Monday, September 10, 2012

Perfection is Hard to Come By

“The closer you get to perfection, the easier it is to screw it up.” Ain’t it the truth? This statement has a world of applications, but it was uttered by a researcher interviewed in an NPR story. He was referencing the careful grinding of critical lenses for a telescope located in Tucson, AZ. If you remember back to the launch of the Hubble telescope, Perkin-Elmer of Connecticut (no slouches when it comes to scientific optics), made a miscalculation in the primary lens grinding that left the Hubble producing subpar images initially.

Analysis of the flawed images showed that the cause of the problem was that the primary mirror had been ground to the wrong shape. Although it was probably the most precisely figured mirror ever made, with variations from the prescribed curve of only 10 nanometers; it was too flat at the edges by about 2,200 nanometers. This difference proved catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edge of a mirror focuses on a different point from the light reflecting off its center. It had to be corrected by a later shuttle flight and crew.

A commission established that the main null corrector, a device used to measure the exact shape of the mirror, had been incorrectly assembled—one lens was wrongly spaced by 1.3 mm. During the polishing of the mirror, Perkin-Elmer had analyzed its surface with two other null correctors, both of which correctly indicated that the mirror was suffering from spherical aberration. The company ignored these test results, as it believed that the two null correctors were less accurate than the primary device that was reporting that the mirror was perfectly figured.

Perfection is hard to come by when human beings are involved, apparently.