Astrometry, explains Jefferys, 51, is the "measurement of positions and motions of stars and other celestial objects." The classical method of determining the distances of stars is by measuring the parallax, or changes in the stars' apparent position owing to the Earth's motion around the sun. Ground-based telescopes, however, are hampered by the unsteadiness of the Earth's atmosphere, which adversely affects the accuracy of parallax measurements. The Hubble telescope, orbiting above the atmosphere, aimed to improve this accuracy.
While Hubble's much-lamented focusing problems have caused some delay in the astrometry project, Jefferys says the telescope will accomplish its intended goal of improving distance measurements. "A good deal of good science is coming out, despite the problems with the telescope," he says. "Both the people who work with me and the entire Hubble team are doing a lot of engineering observations to understand how the spherical aberration affects [the measurements] and ...
