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Heterodyne spectroscopy combines high instrumental transmission with virtually unlimited spectral resolution. For 250 K bodies like Mars and Venus resolution is limited to 107 (= 30 m/s) because of photon statistics whereas for solar measurements the cm/s velocity range, desirable for study of solar oscillations, is feasible. Since a single heterodyne receiver is conserving its etendu, smaller collecting area results in larger field of view. The signal remains unchanged as long as the source is large enough to fill the beam. For Mars, Venus and the Sun heterodyne observation with 1m aperture is competitive.