Abstract.A new readout electronic system for bolometers is presented in this paper. The bolometer resistance is measured in a bridge with a capacitive load, using a periodic square wave bias current. The bias voltages at both ends of the bridge are balanced in order to keep the middle point around zero. Only changes around this zero value are amplified and detected synchronously with the bias signal. These features shift the measurement frequency out of the electrical low frequency noises (JFETs), and reduces the dynamics required from the amplification chain. The bias voltages are fully controlled by computer, and the lockin detection is digital. This readout electronic has many advantages over previous ones. In particular, it proved to be able to read the total power of the radiation reaching the bolometer, and to perform measurements down to low frequencies (≤ 0.1 Hz) without significant additional noise. These features open new observation strategies such as full sky scanning to bolometer instruments on board future submillimetre space projects having high thermal background levels (warm telescope). The different steps of the development of this new readout electronics on the ground-based Diabolo experiment are described, the performances reached are discussed, and a version suitable for the readout of the arrays of bolometers on the COBRAS/SAMBA satellite mission is presented.
Abstract.We have designed and built a largethroughput dual channel photometer, Diabolo. This photometer is dedicated to the observation of millimetre continuum diffuse sources, and in particular, of the SunyaevZel'dovich effect and of anisotropies of the 3K 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 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 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 SZ 7 × 10 −5 and the 3K background anisotropy ∆T T 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.
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