Calibration and first light of the Diabolo photometer at the millimetre and infrared Testa Grigia Observatory

We have designed and built a large-throughput dual channel photometer, Diabolo. This photometer is dedicated to the observation of millimetre continuum diffuse sources, and in particular, of the Sunyaev-Zel'dovich effect and of anisotropies of the 3 K background. We describe the optical layout and filtering system of the instrument, which uses two bolometric detectors for simultaneous observations in two frequency channels at 1.2 and 2.1 mm. The bolometers are cooled to a working temperature of $0.1 \rm\,K$ provided by a compact dilution cryostat. The photometric and angular responses of the instrument are measured in the laboratory. First astronomical light was detected in March 1995 at the focus of the new Millimetre and Infrared Testa Grigia Observatory (MITO) Telescope. The established sensitivity of the system is of $7 \rm\,mK_{RJ}\ s^{1/2}$. For a typical map of at least 10 beams, with one hour of integration per beam, one can achieve the rms values of $y_{\rm SZ} \simeq 7\ 10^{-5}$ and the 3 K background anisotropy ${\Delta T\over T} \simeq 7\ 10^{-5}$, in winter conditions. We also report on a novel bolometer AC readout circuit which allows for the first time total power measurements on the sky. This technique alleviates (but does not forbid) the use of chopping with a secondary mirror. This technique and the dilution fridge concept will be used in future scan-modulated space instrument like the ESA PLANCK mission project.