Study of a channel selection system

The spectral separation of a mean resolution (visible to infrared) camera which can be carried on board an orbital platform was studied. A dispersive system with a grating which operates in the first three orders, and systems which use a prism or field separation were considered. Channel separation can be achieved by: a dichroic plate which eliminates the thermal channel either by reflecting it or transmitting it, without absorption, from 0.5 to 2.5 microns; a grating spectrometer with a direct image of the Earth given by the telescope as entrance slit, aperture = that of the telescope, two-zone grating of the D2B type; 3 bandpass filters (1 micron) centered on 0.55; 0.65 and 0.765 microns; or 3 dichroic plates (which can act as exit slits for the 3 paths of the spectrometer). The system grating has compact assembly and good performance, but precise positioning of the filters separating the three paths at the system's exit, and of all the optical parts is diffcult. The prismatic system presents great practical problems, but is worth investigating. The separator plate for the thermal band can be replaced by separation in the field, but ground processing becomes too complicated. In every case the greatest problem is the provision of near IR detectors.