Free-forms optics into astronomical use: the case of an all-mirror anamorphic collimator

Up to now, optical design of astronomical instrumentation has been based onto "classical" surfaces, i.e. plano surfaces, spherical surfaces, and some classes of aspheric surfaces. More complex surfaces (like cylinders), are used from time to time to correct for aberrations. A new class of more general surfaces, i.e. non-rotationally symmetrical and completely "free-form" surfaces have been recently introduced. Their use in astronomy, however, has not widely used so far. We propose a new layout for an anamorphic collimator based onto two "free-form" cylinder surfaces, giving diffraction limited images. This collimator can be used to create an elliptical pupil, and allows reducing size of optical systems, too. Two interesting cases of application in astronomy are shown: a very-high resolution spectrograph for large telescopes, and an interferometer cavity to test large plano optics.