A possibility to develop a high-resolution terahertz (THz) spectrometer for a wide range of tasks is considered. The proposed spectrometer consists of a synthesizer based on a Gunn generator and a frequency multiplier based on a semiconductor superlattice as a radiation source, as well as a NbN hot-electron bolometer in a direct-detection mode as a THz radiation receiver. We present some experimental results for the spectrometer: the absorption lines of H 2O (1.92 THz, 17 th harmonic) and of HDO (1.49 THz, 13 th harmonic) have been registered. The results obtained are analyzed and further prospects to improve the spectrometer characteristics are considered.

We consider how passing throw the optical system of a telescope affects polarization of the entered electromagnetic signal. We describe alternating of a ray polarization while reflection at arbitrary oriented surface of a mirror. The polarization distortions, originated from both deviation of the rays from initial direction and difference in reflection coefficients for different polarization under non-ideal reflection, are taken into account. Within Frensel method we derive formulae for estimations, which give relation of the field amplitudes with initial polarization and near the orthogonal-to-initial in the vicinity of the focal point of an optical system. Formulae are derived in several significant cases of optical systems, including single parabolic mirror and system of two mirrors. Using the formulae we estimate the upper limit of the polarization distortions within the Airy disk. Derived estimates coincide with the results of numerical calculations for two-mirror radio telescope of sub-teraherz range.