Cryostat, bolometers and cold optics

The cryostat is a $^3\mathrm{He}-^4\mathrm{He}$ dilution refrigerator that cools down to 100 mK. This dilution is the same that will be used for Planck-HFI and does not rely on gravity. The $^3\mathrm{He}-^4\mathrm{He}$ mixing is made through capillaries all around the 100 mK stage. The radiation enters through a large (16 cm) polypropylene window closed by a valve that opens automatically at low pressure. The radiation is the focused through Planck-HFI-like flared corrugated back-to-back horns at the 10 K stage. The lower part of the horns contains the filters in the 1.6 K stage. They filter 4 different frequencies depending on the bolometer: 143, 217, 353 and 545 GHz. The bolometers are located on the focal plane cooled down to 100 mK at the exit of flared horns. The bolometers are spider web bolometers similar to those that will be used for Planck-HFI [18]. Their nominal noise equivalent power (NEP) is $1.4\times 10^{-17}\mathrm{W/\sqrt{Hz}}$.