news center

Technology: Star wars gives a clear view of the sky

Technology: Star wars gives a clear view of the sky

作者:还饲  时间:2019-03-03 03:12:00  人气:

By HELEN GAVAGHAN in WASHINGTON DC Military technology which was made public by the Strategic Defense Initiative Office (SDIO) last week will give a major boost to astronomy in the US. Ground-based optical telescopes equipped with this technology could be able to see many objects in space with the same resolution as NASA’s $1.5 billion Hubble Space Telescope was designed to have. The technology was developed as part of the $100 million SDI project on sophisticated imaging techniques known as adaptive optics which allow astronomers to compensate for the way that the atmosphere distorts light. Light from stars and galaxies has a uniform wavefront. As it travels through the atmosphere, the wavefront encounters variations in temperature and density that refract parts of it to differing extents, the same way water distorts light coming from the bottom of a swimming pool. So, the light that arrives at the ground is no longer an accurate representation of objects in the Universe. For example, from the Earth, stars twinkle, but viewed from above the atmosphere they do not. The practical upshot is that the atmosphere limits the detail ground-based telescopes can discern. Astronomers around the world are developing mirrors that can vary their shape to compensate for atmospheric distortions (adaptive optics). They hope eventually to build large telescopes that have a resolution equal to the theoretical limit which depends only on the wavelength of the light and the diameter of the mirror. The SDI technology was developed to control laser beams travelling up through the atmosphere to shoot down satellites or missiles. Adaptive optics control what goes up in the same way that they compensate for what comes down. The SDIO has agreed to transfer equipment containing adaptive optics that it has developed during the past 10 years to the National Science Foundation, so that scientists can modify the equipment for astronomy. The SDIO will also soon declassify background information on adaptive optics, which will be of equal, if not greater, value to astronomers. Wayne van Sitter, programme manager for instrumentation at the foundation, says that the newly released technology is five years ahead of equivalent technologies in the civilian world. And Samuel Durrance, an astronomer at the Johns Hopkins University, says: ‘We could not afford to do what the SDIO has done.’ The principle governing adaptive optics is relatively straightforward. Light enters the telescope and is focused onto an instrument known as a collimator that projects the image onto the flexible mirror. The telescope must also view a bright source of known intensity near to the astronomical object under observation. That bright source acts as a reference, enabling the telescope’s computer to calculate how the mirror should move to correct for distortion in the light coming from the unknown astronomical object. The flexible mirror is divided into sections, and their movement is electronically controlled. The larger the mirror, the more subdivisions are needed to achieve an image with the same degree of clarity. For example, an 8-metre telescope would need about 3000 movable sections. Producing a mirror with so many subdivisions is extremely difficult. In particular, the computer code controlling their movements becomes increasingly complex, and could easily contain errors that lead to greater distortions of the image rather than corrections. Durrance is a leader in the field of adaptive optics and is currently testing the technique on a 1-metre telescope at Las Campanas in Chile. His equipment has a mirror with 91 subdivisions. The equipment that the SDI office will release has 241 subdivisions, and so can provide corrections for a larger telescope. Tom Meyer, head of the directed energy division of the SDIO, says that his group has also tested a mirror with 2000 connected movable sections. If adaptive optics is to succeed, astronomers must solve another problem. What should they do if there is no bright, well understood, reference star in the vicinity of the object they are observing? One solution is to create artificial beacons, possibly by bouncing laser light off a sodium layer that exists in the atmosphere. In this area, too,