C. Arnault, F. Barone, F. Bellachia, R. Bilhaut, D. Boget, T. Carron, D. Castellazzi, D. Dufournaud, F. Cavalier, R. Chiche, F. Chollet, M. Dehamme, L. Derome, C. Drezen, R. Flaminio, F. Garufi, X. Grave, P. Heusse, O. Lodygenski, F. Marion, L. Massonnet, L. Milano, R. Morand, B. Mours, P. Roudier, V. Sannibale, D. Verkindt, M. Yvert L.A.L. /IN2P3-CNRS et Universite de Paris-Sud, F-91405 Orsay, France I.N.F.N. Sez. di Napoli e Dipartimento di Scienze Fisiche dell’Univ. Federico II, Pad.19, I-80125 Napoli, Italy

The commissioning of the VIRGO central interferometer occasioned the implementation and tests of various algorithms for the characterization of the non-Gaussianity, non-stationarity and non-linearity of the dark fringe data. This library of prototypes will serve as groundwork for the near commissioning of VIRGO (full scale). We make a summary of the activities on that subject including the description of the selected algorithms and some results obtained with the data of the engineering runs.

The automatic control of the suspended mirrors is a major task in operating an interferometric gravitational wave antenna. To reach the extreme sensitivity required for this kind of detector, an accurate alignment and a stable locking of the interferometer on its working point are crucial. The solution of this problem is particularly complex in the case of a multistage pendulum, such as the suspension system for seismic isolation adopted in VIRGO. A precise knowledge of the suspension mechanical transfer functions (TFs) for different forces applied in the control servo-loops represents essential information to reach the goal. In this article, we describe the apparatus we developed to measure the VIRGO suspension TF and we report the results thus obtained on full-scale suspensions at the VIRGO site. Preliminary results for the implemented control system of the last suspension stage are also presented.

The VIRGO Central Interferometer (CITF) is a short suspended interferometer operated with the central area elements of the VIRGO detector. The main motivation behind the CITF is to allow the integration and debugging of a large part of the subsystems of VIRGO while the construction of the long arms of the antenna is being completed. This will permit a faster commissioning of the full-size antenna. In fact, almost all the main components of the CITF, with the exception of the large mirrors and a few other details, are the same as those to be used for the full-size detector. In this paper the present status of the VIRGO CITF is reported.

The present status of the VIRGO antenna, with special care of the noise limit, is presented. The VIRGO project (a French-Italian collaboration) has been approved in 1993 and it is now in the construction phase. The end of the commissioning phase is foreseen within 2001.

A cryogenic refractometer is described. It enables preparation of liquefied samples and measurement of their refractive index at low temperatures (down to 20 K). The liquid sample can be a pure liquefied gas or a mixture. The composition of the liquid mixture can be estimated from the volume and pressure of each gas measured during the liquefying process. The determination of absolute values of the refractive index necessitates a calibration, therefore the accuracy is limited to 10−3 index unit. Sensitivity and reproductibility are better (5× 10−4) and could be improved to 10−4. The refractive index of pure liquid methane was determined at several temperatures (91–106 K) and 670 nm. The results obtained agree with the published data. The refractive index of mixtures of liquid methane and ethane, with and without dissolved nitrogen, was also determined at 94 K and 1.5 bar total pressure, for which conditions no data were previously available.

We describe in this paper a “pseudo-double beam” photoacoustic spectrometer using a wide aperture monochromator (F/2) and a very sensitive cell. The double signals required to get the normalized value, i.e., the sample signal and a carbon black reference signal, are obtained by a slow (1.7 Hz) alternating movement of the two cells support, the sample and the reference being alternately placed at the focus point of the beam which is fixed in space and time. The modulation of the light flux is independently ensured by a mechanical chopper. Such an original device associates good performances in resolution (0.7 nm), signal-to-noise ratio (1200 for HO2O3 at 450 nm with a 5 nm bandwidth) and normalization in a wide spectral range.