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The measured sensitivities are incorporated in a Fortan program SIGNAL, which can be used to predict the number of object and sky photons which will be detected as a function of above-atmosphere apparent magnitude (or intensity in Jy), wavelength band (U, B, V, R or I), detector, exposure time and sky brightness and, for spectroscopy, diffraction grating and slit width. For the few instruments for which no empirical data are available, the program calculates throughput on the basis of measured or estimated efficiencies for the telescopes, cameras, gratings and detectors. The program can be used before observing to estimate the exposure time needed for a particular experiment, and at the telescope to check that the expected number of photons (countsgain) arrives at the CCD. The program may be obtained by email from or by anonymous ftp via the ING information page on the Cambridge server

Tue Aug 15 16:42:46 BST 1995