Intermittency and coherent structures in the two-dimensional inverse energy cascade: comparing numerical and laboratory experiments.

We study the internal intermittency in the inverse energy cascade and in the condensation regime of two-dimensional turbulence, using physical and numerical experimental approaches. The analysis confirms that the velocity increments have nearly Gaussian distributions at all scales in the inverse cascade regime; it moreover shows that, in the condensation regime, the probability distribution functions of the velocity increments are non-Gaussian but do not significantly vary with the scale; it follows that one may consider that there is essentially no intermittency (in the usual sense), in the condensation regime. In both regimes, we emphasize that coherent structures (i.e., long-lived vortices) are clearly visible on the vorticity field, and we suggest the non-Gaussianity of the distributions in the condensation regime is due to the presence of a large-scale long-lived structures. The study is supplemented by the analysis of the distribution of energy transfers at various scales.

Hubble Space Telescope primary-mirror characterization by measurement of the reflective null corrector.

The reflective null corrector used to manufacture of the Hubble Space Telescope contains valuable information about the prescription of the primary mirror since an excellent null was achieved between the null-corrector wave front and the primary-mirror wave front. During the Phase I measurements, the leading cause of the spherical aberration, the field lens position error, was discovered and remeasured to an accuracy of +/-0.005 mm. To derive the conic constant of the primary mirror to an accuracy of +/-0.0003, we remeasured the parameters of the reflective null corrector that could contribute to the spherical aberration of the primary mirror.