The OSER project 3 2 Limitations from spatial and temporal coherences

The OSER project (Optical Scintillation by Extraterrestrial Refractors) is proposed to search for scintillation of extragalactic sources through the galactic – disk or halo – transparent H2 clouds, the last unknown baryonic structures. This project should allow one to detect column density stochastic variations in cool Galactic molecular clouds of order of ∼ 3 × 10 −5 g/cm 2 per ∼ 10 000 km transverse distance. 1 The transparent baryonic matter and its possible signature Considering the results of baryonic compact massive objects searches (Lasserre et al. 2000; Afonso et al. 2003; Alcock et al. 2000), cool molecular hydrogen (H 2) clouds should now be seriously considered as a possible major component of the Galactic baryonic hidden matter. It has been suggested that a hierarchical structure of cold H 2 could fill the Galactic thick disk (Pfenniger & Combes 1994) or halo (De Paolis et al. 1995), providing a solution for the Galactic hidden matter problem. This gas should form transparent " clumpuscules " of 10 AU size, with a column density of 10 24−25 cm −2 , and a surface filling factor smaller than 1%. Refraction through such an inhomogeneous transparent H 2 cloud (hereafter called screen) distorts the wave-front of incident electromagnetic waves (Fig. 1, see Moniez 2003 for details). The extra optical path induced by a screen at distance z 0 can be described by a function δ(x 1 , y 1) in the plane transverse to the observer-source line. The amplitude in the observer's plane after propagation is described by the Huygens-Fresnel diffraction theory: