CCD scanning for comets and asteroids

Some populations of objects in the solar system are still poorly known, and the long-range goal of this program is to improve that situation. For instance, the statistics of Trojan asteroids are uncertain, while previous serveying indicates there is an appreciable systematic difference between the L-4 and L-5 regions, which is hard to explain. Researchers are developing for this goal a new technique of sky surveillance, namely scanning with a charge coupled device (CCD). With its quantum efficiency and wavelength range greater than that of photographic plates, the CCD should be especially valuable for fast-moving objects such as near-Earth asteroids. A 320 x 512 pixel CCD has been in operation since 1983 on a telescope that is dedicated during the dark half of each month to sky surveillance, that is the Spacewatch Telescope which is the 91-cm Newtonian reflector of the Steward Observatory on Kitt Peak. The system was found to be of special value for astrometry. The telescope drive is turned off at a selected distance west of the object and the scan is continued such that the number of astrometric standards is optimized. Because the drive is off, the effects of refraction practically vanish.