corrected with “Risley prisms,” which consist in two identical sets of prisms (Breckinridge [*et al.*]{} 1979, Walner, 1990) that can be rotated to produce a tunable chromatic dispersion both in amplitude and direction (Fig. 4a).

Each set is made of two prisms of different dispersion law and roof angles, placed in an upside-down position. These prisms have been designed to have a null mean deviation, and a dispersion allowing atmospheric correction from the zenith down to an elevation of 30$^\circ$ for the blue domain which is the most defavorable (B filter, centered at 450 nm with a 70 nm bandwidth). We used the same combination of Shott glasses (F4, SK10) as the one used for the Kitt Peak speckle camera (Breckinridge [*et al.*]{}, 1979). Wallner (1990) found other combinations which are closer to the atmospheric dispersion curve but we chose the Kitt Peak combination because of its low cost and sufficient efficiency for our purpose. Our Risley prisms reduce the residual dispersion down to a level smaller than 0.01“ for every location of the object in the sky above an elevation of 30$^\circ$, with the 70 nm bandpass B filter which is small compared to the diffraction limit of 0.05” 