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A microwave device is described which collimates the radiation from a point feed into a parallel beam. The instrument operates by both reflection and diffraction. The offense against the sine condition is corrected perfectly, so that no monochromatic aberration is present for a moderate field even at very high apertures. The expressions of the aberrations are worked out and a graph is given where one can read the maximum values of the aperture and field for a given tolerance. It seems that the device described may be useful for radio astronomy and for rapid scanning.