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The characteristics and performance of a new class of 3He refrigerators are discussed. We introduce a 3He refrigerator, which allows a temperature of 296 m K to be reached with a starting point of 4.2 K, without pumping on the main 4He bath. The operating principle is based on the single-expansion helium liquefier: gas cooling by isothermal compression and adiabatic expansion.

We introduce a mini 3 He refrigerator, operating at ∼300 mK starting from 4.2 K without pumping on the main 4 He bath. The innovative idea is that the present one is suitable for a very fast operation; for its use, it is sufficient a storage 4 He Dewar. In this way we drastically reduce the time required to cool it down, because there is no need for a classic cryostat. This prototype is particularly aimed for all those operations in which it is necessary to test a large number of samples that do not require long duration measurements at low temperature.

In this paper it is shown how submillimetre and millimetre flux density measurements of sky fluctuations can be used to infer the characteristics of atmospheric turbulence. Two different approaches have been followed: first, by considering the observed process as due to the atmospheric turbulence, we used the r.m.s. sky fluctuations to derive some physical parametres characterizing the phenomenon; second, by investigating the power spectrum and the autocorrelation function, we derive an empirical model (the AR model) to try to deduce the physical process. In this case, the fitted power spectrum agrees strikingly with that predicted by the theory of a stationary turbulent flow for the atmosphere in the sampled frequency range. This kind of modeling can help to disentangle the atmospheric fluctuations from the extraterrestrial signal in ground-based astrophysical measurements. Measurements of the water vapour content and atmospheric transmission at the Italian Base in Antarctica, where the data have been taken, at the wavelengths of 350 μm, 1 and 2mm are also reported.

The sensitivity increase in bolometers obtained by operating temperatures of 1 K (helium-4) and 0.3 K (helium-3), with respect to standard bolometers cooled at pumped 4He temperature, represents in infrared and submillimetric astronomy a remarkable step forward in applications both on large ground telescopes and on stratospheric balloons. The characteristics are introduced of a double stage refrigerator (4He/3He) which allows a temperature of nearly 300 mK to be reached, starting from a temperature of 4.2 K. No moving parts and external links are involved, except for the electric leads, so as to exploit only the condensation and cryosorption cycles of 4He and 3He, without pumping on the main 4He bath.

The new generation of miniature 3He refrigerators, exploiting only the condensation and cryosorption cycles of 3He without moving parts and external links, represents the most common adopted solution in low temperature physics for cooling detectors such as bolometers at 0.3 K. The characteristics and performance of an improved 3He refrigerator working at a temperature of nearly 0.3 K are discussed. We introduce a new class of refrigerator (patented) which allows one to reach the operating temperature in a much shorter time than with standard 3He refrigerators, without using any thermal switch. The system has the potential to be very useful in all applications where several modifications are required.

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 fluctuations in the stratospheric emission in the 800–2000 μm wavelength range have been studied by means of a special correlator operating at 0.3 K. The observed noise is about three times larger than expected from a grey body at a temperature of 250 K and an emissivity of 5 × 10−4. This fact has been interpreted as proof that about 2% of the stratospheric lines were saturated.

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).

Analysis and short review of the interaction between electromagnetic field and living tissues: experimental data suggest that a complex and still undefined model can explain their effects on the electrochemical environment inside and outside the cell.

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.

We describe the design, operation and performances of a FIR photometer suitable for ground-based and balloon-borne astronomical observations in the millimetric and submillimetric regions.

A description is given of a photometer which is suitable for astronomical observations through millimetric atmospheric windows. It consists of a Ge-Ga bolometer operating at 1.0 K, with a throughput AW = 0.3 cm2 sr and a NET = 1 mK Hz-1/2. Two metallic mesh filters set the windows at about 10 and 5 cm-1.

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.

A method using an oxyacetylene torch flame is described; it makes it possible to obtain easily and rapidly, without skill or special equipment, a tungsten micro-electrode tip in a very few seconds.

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.

Polyethylene embedded with glass microspheres has shown the same optical properties as Fluorogold™ in the far-infrared. The polarizing properties of Fluorogold (laminated in thin foils) are avoided by pressing the polyethylene/glass powder between hot plates. The new fiters are suitable for long-wave pass filters with intrinsic polarization less than 10−4