Millimetric observations with a high-altitude 2.6 m ground-based telescope

High atmospheric performances are necessary to ensure efficient sub/millimetre cosmological observations from ground. Low atmospheric components fluctuations along the line of sight are a must for best detector applications. Such site constraints are attained only at in specific places around the world: high-altitude observatories or, equivalently, polar regions. We are currently involved in cosmological observations with the MITO project from an Alpine ground station which satisfies such requirements: the Testa Grigia mountain at 3500 m a.s.l., AO Italy. The Chacaltaya laboratory at 5200 m a.s.l. could also be an appropriate mm-site. One of the goals of MITO is the multifrequency observation of nearby rich clusters of galaxies for measuring the Sunyaev-Zel'dovich effect. Combined S-Z and X-ray measurements yield the Hubble constant and other cosmological information. A dedicated instrument has been designed to minimize spurious contaminations on the signals. The telescope is a 2.6 m Cassegrain with a wobbling subreflector and a 4-band single pixel photometer installed at the focal plane. The bolometric detectors are cooled down to 300 mK by a double stage He3-He4 fridge.