Hubble’s troublesome mirror
Stories from Physics for 11-14 14-16
During the construction of the Hubble Space Telescope’s mirror, a device called a null corrector was used to shine a beam of light onto the surface of the mirror and analyse the reflection to find imperfections in the mirror’s surface. To test the surface of the mirror, engineers used both a reflective null corrector, which used mirrors, and a refractive null corrector, which used lenses and was supposedly less accurate than the reflective corrector. The null correctors were so sensitive that they could only be used in the middle of the night when there were no vibrations due to passing traffic.
When the reflective null corrector showed an error that had not been detected by the cruder refractive device, it was assumed that the refractive device was poorly calibrated and the mirror was ground to remove the ‘defects’ spotted by the reflective device. When a final check was done with the refractive corrector, it showed a spherical aberration of a quarter of a wavelength but it was assumed that this was to be expected with a crude measuring device. The quarter wavelength error turned out to be a real error. The project’s chief scientist, C. Robert O’Dell, spoke for the team who worked on the project saying: “All of us feel horrible.”
It is thought that the error in the null corrector arose because the lens of the device wouldn’t descend far enough, and, because the operating engineers were close to a deadline, three household washers were inserted into the million-dollar measuring device.
In 1993, the company responsible for producing the mirror was ordered to pay NASA $25 million. Entirely replacing the defective mirror was not practical, so a shuttle mission fitted corrective optics (consisting of five pairs of mirrors), much like a pair of glasses to remedy the effects of the defect.
Serious damage to the mirror was prevented on another occasion when an optician, Wilhelm R. Geissler, accidentally typed ‘1.0’ instead of ‘0.1’ into the computer controlling an automated polishing tool. Fortunately, a technician with the mindnumbing job of spending hours monitoring the tool for just such an error, managed to hit the kill switch in time to prevent serious damage.