VIRGO mirrors: wavefront control

The Franco-Italian VIRGO program to detect gravitational waves requires large mirrors whose optical properties are extremely severe: absorption and scattering < 1ppm at 1064 nm, wavefront homogeneity of 9 nm RMS on empty set100mm. To achieve this performance on the wavefront, we have developed an experimental method to correct the wavefront shape to be as plane as possible. A thin silica layer is added through a mask by sputtering, where it is necessary on the surface, on the last layer of the multilayer. A phase retardation is produced. The wavefront is measured before and after correction with a Zygo interferometer. The first test has been done on empty set80mm mirror at 633 nm. The wavefront peak to valley goes from 34 to 14 nm. At this level, we are limited by the interferometer accuracy and repeatability. The optical property modifications due to the corrective coating are small (at 1064 nm) or easy to overcome. The corrected wavefront includes at the moment the reference flat of the interferometer, so the measurements are not absolute. But the feasibility of this correction method is proved and it is powerful.