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 He-4 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 (He-4/He-3) 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 He-4 and He-3, without pumping on the main He-4 bath.
