Fig. 2

Left: DICE calibration beam. A DICE light source consists in a quasi-point source, located at a finite distance from the telescope pupil. Such a beam generates a quasi-uniform illumination of the focal plane. Right: science beam. The light from a distant source, such as a star, may be approximately modelled by a plane wave that fills the entire telescope aperture. Such a beam is focussed on a single point of the focal plane.
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