Scanning with charge-coupled devices

Abstract

The 0.91-m ‘Spacewatch Telescope’ of the Steward Observatory of the University of Arizona on Kitt Peak is dedicated to scanning with charge-coupled devices (CCDs) during the dark half of the month. We explored six modes of using CCDs for searches of gamma-ray bursters, debris in geosynchronous space, satellites of asteroids, brown dwarfs, the tenth planet, comets, cometesimals, and various types of asteroids. In the process, we gained experience with cosmic rays and artifacts in CCD observations. Each of these topics is described. I especially note that the existence of cometesimals has not been confirmed by the Spacewatch Telescope, contrary to reports published by others.

This paper describes a new discipline in astrophysics, ’scannerscopy’, of surveying with a CCD rather than with photography at a Schmidt telescope. It uses the CCD in scanning rather than in sequencing of stare exposures as is done at most observatories. This may save telescope time, and flat-fielding is rarely needed. Usually we turn the drive off, but the scanning can be done with the telescope moving. In any case, the motion on the sky is precisely followed by slaving the charge transfer of the CCD to the drift rate of the image, while the CCD is read out continuously during the observing. Our primary application of CCD scanning is on moving objects such as comets and asteroids. We also do routine astronomy with CCDs, in a transit method, and this yields a precision of better than ±0.7 arc sec.

We presently use a Tektronix 2048×2048 CCD, 38 arc min wide, to a limiting magnitude ofV=20.5 (6σ detection). This is successful even for discovering rare and small near-Earth asteroids. 1990 UN with a diameter of 90 m and 1991 BA at 9 m are the smallest natural objects observed outside the Earth's atmosphere to date. In a month with good conditions we find typically 2000 new main-belt asteroids and, on average, nearly two near-Earth asteroids. Only the latter are followed up with astrometry. The goal is to study magnitude-frequency relations, as well as to complete the inventory of dangerous impactors on Earth. We are designing a new CCD-scanning telecope to become an order-of-magnitude more effective in the discovery of elusive objects than the Spacewatch Telescope. The paper also describes possibilities with cameras on spacecraft that pass through the asteroid belt; thousands of small asteroids can be observed with the CCDs of CRAF and CASSINI.