[en] The design of a computer controlled grating spectrometer for medium resolution (lambda/Δlambda approximately 200) astronomical observations in the 10 μm window is described. The grating and spectrometer optics are cooled to liquid helium temperature and an array of five photoconductive detectors is employed; the advantages of this configuration are discussed and a short analysis of the performance of such a system is made. 8 to 13 μm spectrophotometric observations of five planetary nebulae are presented and the continuum emission seen from these, and three other planetary nebulae previously observed, is identified with optically thin thermal radiation by dust grains. A model is developed in which the emission observed from these objects can be attributed to varying contributions from four different grain materials and, by synthesizing the spectra from these components, the amounts and temperatures of the dust components are inferred. It is found that the planetary nebulae can be divided into two classes indicative of either carbon rich or oxygen rich grain material and the relationship of this result to the evolution of the objects is discussed. A computer model to simulate the ionisation structure of planetary nebulae is described and is employed in interpreting the infrared line intensities observed in eight planetary nebulae and in deriving abundances for argon, neon and sulphur. (author)