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The possibilities of ground based infrared astronomy are discussed. It is shown that telescopes used up to now are not optimized and that the Diffraction Limited Detectivity conditions (DLD) cannot be reached, in the near infrared, by the larger telescopes. The maximum useful diameters for near i.r. and far i.r. telescopes have been evaluated and a new parameter (i.e. visibility) is introduced in order to describe the performances of i.r. ground based telescopes.

The far infrared atmospheric windows between 30 cm −1 and 5 cm −1 have been investigated during many months of observations at the Alpine Station of Testa Grigia (3500 m a.s.l.). The seasonal and diurnal variation of the atmospheric transmittance for each window have been measured. The contribution of the minor atmospheric components is also discussed.

We have described an instrument intended to measure the atmospheric transmission through the windows between 300 and 3000 μm. The radiometer operates automatically, scanning the sky in steps of 0.7° from the horizon to the zenith, with the time constant as selected by the operator. The spectral distribution of the radiation is measured using a Michelson interferometer with an optical path difference of ± 10 cm. Using a 77°K reference blackbody, the atmospheric transmission can be measured with an accuracy better than 1 per cent. Some preliminary results are described, obtained at the Alpine station of Testa Grigia (3500 m above sea level).

Different kinds of far i.r. bolometers are compared between 100 and 5 cm−1 in various optical arrangements. A Golay cell has been used as reference detector. The germanium bolometer seems to be about ten-times more sensitive than the other two detectors, while the carbon bolometer shows the highest responsivity.

The polarization of the Sun's radiation has been measured in the 30-10 cm −1 spectral region through the atmospheric windows at the high altitude observatory of Testa Grigia, Italy, at 3500 m a.s.l. The results are in agreement with those previously obtained during a balloon flight in the 100-5 cm −1 region. The spectral dependence of the linearly polarized component has been investigated and a possible interpretation of the obtained data is suggested.

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.

During a balloon borne experiment intended to study the state of polarization of the far infrared radiation in the 100–2000 micron range, a signal was detected from the Sun corresponding to a linear polarization of at least 6,4 % assuming a constant polarization over the whole infrared bandwidth observed (wavelengths longer than 500 microns cannot contribute significantly to the observed signal even if they are completely polarized). The vibrational plane is parallel to the Sun equator within 5°.

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.

We have measured the temperature of the Sun in the 300–1000 μm region through the atmospheric windows at the high altitude observatory of Testa Grigia, Italy. 3500 m a.s.l. The far i.r. detector used in this experiment is a 150cm flux collector with a germanium bolometer. A set of interferential filters coupled to a specially designed radiometer allow us to isolate and measure the atmospheric windows transmittance with a precision of 1 per cent. The results obtained are in agreement with the HSRA model of the Sun.

Interference filters adequate to isolate the atmospheric windows in the 30-5 cm −1 spectral region are described. The filters have a half-bandwidth of about 2 cm −1 and, with suitable blocking, a rejection factor larger than 10 8 is obtained up to the visible region.

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.