SNDICE: a direct illumination calibration experiment at CFHT

We present the first results of the SuperNova Direct Illumination Calibration Experiment (SNDICE), installed in January 2008 at the Canada France Hawaii Telescope. SNDICE is designed for the absolute calibration of the instrumental response of a telescope in general, and for the control of systematic errors in the SuperNova Legacy Survey (SNLS) on Megacam in particular. Since photometric calibration will a critical ingredient for the cosmological results of future experiments involving instruments with large focal planes (like SNAP, LSST and DUNE), SNDICE functions also as a real-size demonstrator for such a system of instrumental calibration. SNDICE includes a calibrated source of 24 LEDs, chosen for their stability, spectral coverage, and their power, sufficient for a flux of at least 100 electron/s/pixel on the camera. It includes also Cooled Large Area Photodiode modules (CLAPs), which give a redundant measurement of the flux near the camera focal plane. Before installing SNDICE on CFHT, we completed a full calibration of both subsystems, including a spectral relative calibration and a 3D mapping of the beam emitted by each LED. At CFHT, SNDICE can be operated both to obtain a complete one-shot absolute calibration of telescope transmission in all wavelengths for all filters with several incident angles, and to monitor variations on different time scales.