A two-stage He-3-He-4 fridge for bolometric photometry

We describe the design, construction and performance of a double stage He-3 -He-4 refrigerator, built to cool down a multiband bolometric photometer at the MITO telescope. The fridge was optimized to work without external pump s, with the main cryostat providing a 4.2 K thermostat at sea level and a 4 .0 K one at high mountain pressure conditions. The measured ultimate temper ature of the fridge is 290 mK, with a hold time of 81 h. The external heat input on the cold flange is similar to 35 mu W, with the main bath at 4.0 K . The recycle time is 8 h with a heat input on the thermostat during recycl ing of similar to 6800 J. The cryostat can operate without any relevant cha nges to performance tilted down to 50 degrees from the vertical position, a s needed at the telescope focal plane. (C) 1999 Elsevier Science Ltd. All r ights reserved.