The sensitivity of an ideal heterodyne spectrometer approaches the quantum detection limit provided the local oscillator power is sufficiently large and the shot noise dominates all other sources of noise. The post-integration minimum-detectable-number of photons/sec for an ideal heterodyne system is B/T when;B is the IF bandwidth and T is the integration time. For astronomical observations, however, a number of factors (A.) tend to degrade the sensitivity, a fact which becomes significant particularly when the laser power is insufficient. A discussion and an evaluation of the degradation in sensitivity is given for a heterodyne spectrometer employing a Hecate photodiode mixer and tunable diode lasers.
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