1. Radiant energy sensing apparatus comprising: a spectral filter including first and second spectral bandpass portions which pass radiations of long and relatively short wavelength, respectively a centrally located detector element arranged to receive radiation passing through said first spectral bandpass portion of said spectral filter and which produces a set of signals, the amplitudes of which are a function of the position of an image of a point source target incident thereon; and a plurality of separate detectors arranged to receive radiation passing through said first and second spectral bandpass portions of said spectral filter surrounding and equally spaced from the center of the centrally located detector and each arranged to produce an output signal in response to the imaging of a point source target thereon, said output signal having a particular characteristic depending on the spectral content of the point source target.