Abstract

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 frame-transfer 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 × 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

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