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Noise measurements in the frequency regions 5-200 HZ and 5.2 x 10/sup -4/ -8.3 x 10/sup -3/ Hz have been performed in the wavelength region between 300 and 3000 mu m from the high altitude observatory of Tests Grigia, Italy (3.500 m). In the high frequency region a specially designed Ge bolometer operating in background-limited-infrared-photoconductor conditions matched to a 1.5-m telescope has been used, while at low frequency a radiometer designed for atmospheric transmittance measurements was employed. In both regions no excess noise with respect to the photon noise relative to 300-K blackbody has been detected.

The possibility of detecting diffuse sky radiation in the (250÷3000) μm range is discussed on the basis of recent measurements performed at the mountain observatory of Testa Grigia (3500 m a.s.l.) on the atmospheric emission and on the atmospheric noise. It is shown how, in many cases, the limits assigned by the residual atmospheric emission are well below the sensitivity of the available infra-red detectors; as a consequence some new experiments of background astronomy are suggested.

Atmospheric transmittance in the band of 300–2000 μm was measured in the far i.r. Solar Observatory of Testa Grigia, 3480m above standard. These measurements are compared with the meteorological data observed at Plateau Rosa in the last 7yr.

In this paper we are dealing with the “throughput” ΩS and with the optical bandwidth Δλ of an IR-detector when it is employed at the focus of a telescope to observe point-like sources. We have found that the maximum signal-to-noise ratio is obtained for ΩS≃ λ2 and Δλ ≃ λ. Under these optimized conditions the quantum noise of the atmosphere is lower than 10−14 W. Hz− 1 2 in the 300–2000 micron range. Because of this the best available far infrared detectors may be used in the atmospheric window at ground based observatories without background limitations.

The photocurrent pinch-off, observed in high resistivity thin CdS crystals subject to a high electric field (field effect), is strongly dependent on the atmosphere surrounding the crystal. The minimum pinch-off voltage has been measured in different atmospheres. This gives a method for the evaluation of several physical quantities relevant to the surface and bulk properties of CdS.

Infrared carbon bolometer detectors operating at liquid helium temperature (LHT) have been constructed. The properties of carbon bolometers which depend upon the thickness of the elements have been investigated, particularly with respect to their responsivity. Responsivities up to 106 V/W may be obtained by decreasing the thickness of the element, without any appreciable loss in the far infrared absorption. By using evaporated gold contacts, a NEP lower than 10−12 W/Hz 1 2 has been obtained in two cases. In practice carbon bolometers appear to be competitive with other liquid helium cooled bolometers.

The investigation of hole mobilityμp in CdS has shown a number of apparently contradictory experimental results. In the present work we examine in some detail the kinetics of the photocurrent under field effect. One finds that the photocurrent pinch-off and the complete recovery give respectivelyμp=0 andμp ≠ 0. The contradiction can be overcome assuming a «surface hole mobility»μps=0 and 24 bulk hole mobility»μpb≠0. Based on this assumption, a simple mathematical model has been developed which appears to describe correctly the photocurrent kinetics under field effect. Furthermore, the results of various authors are well interpreted separating the observations into two groups according to whether the measurement ofμp is done moving the holes at the surface of CdS (μps=0) or through the bulk only (μpb≠0).

We have evaluated the accuracy in maintaining the differential modulation parallel to the horizon, which is required to reduce the atmospheric background below the detector noise in an infrared telescope operating under optimized conditions.

We have measured the sky temperature trough the 7-10cm−1 atmospheric window from the Alpine station of Testa Grigia(3500m. a.s.l.). An upper limit of 2.7°K has been found.

Weber's theoretical calculations concerning photoconductivity of semiconductors in a transverse electric field are extended to two particular models for CdS single crystals. An agreement with experiment is found when photoexcitation is weak; the presence of surface states is pointed out.