PROGRESS IN LOW-LIGHT-LEVEL CHARGE-COUPLED DEVICE IMAGING IN ASTRONOMY

Techniques for data acquisition and processing, which permit accurate photometry at a level of 10-4 of the background night-sky level, are described. High quantum efficiency, excellent linearity, and low noise make frametransfer silicon charge-coupled devices (CCD's) nearly ideal imagers at ultralow-light levels in the UV to 900-nm range. Scientific-grade CCD's are now available with less than 10-electron noise from several manufacturers. In astronomy these CCD's permit studies of objects that are 100 times fainter than is possible by using photographic or video-camera techniques. Detection levels of 2 X 10-5 photons m-2 nm-1 sec-1 pixel-1 have been achieved. The efficiency of telescopes of all apertures has increased to the point at which observations that were unthinkable a decade ago are now becoming routine. New techniques for ultradeep imaging with CCD's are described. The telescope is moved randomly between exposures, producing 30-100 disregistered images. Automated image processing on a workstation yields the final image, corrected for CCD imperfections and sky + optics systematics. Faint galaxies and stars of 30th magnitude are detected in 10-h integration on a 4-m telescope, corresponding to 0.02 photon sec-1 pixel-1. These techniques may be applied to longer-wavelength imagers with similar problems of bad pixels, bad lines or columns, background variations, etc. Application to faint-galaxy photometry is reviewed, and mosaic CCD imagers are discussed. © 1990 Optical Society of America.