The VIRGO interferometer needs six main mirrors. The 3 km long Fabry-Perot cavities need four large mirrors (Input and End mirrors, 350 mm diameter, about 100mm thickness and 20 kg mass). Because of their dimensions, we have developed new tools to manipulate them between every step of the process in clean conditions with a total safety. Such large components have never been coated in the past, so it was a great challenge for us to succeed.
The beam splitter is 55 mm thick, it has a diameter of 230 mm and a mass of 5 kg. The recycling mirror is 30mm thick and it has a diameter of 350 mm.

The first set of the VIRGO main mirrors was ready in June 2002 as foreseen by the planning. The picture below shows a real VIRGO end mirror in its characterization mounting during the measurement of the scattering.

VIRGO End Mirror during the scattering measurement

Now, the mirror installation on the VIRGO site in Italy is finished. A summary of the optical performances of these large mirrors is shown on the table 1.

The absorption and scattering requirements (< 5 ppm) are satisfied for each component, especially for the absorption (in the 1 ppm range). The dispersion in the absorption values only comes from the coating design: the higher the reflectivity the lower the absorption.
Some scattering values are higher than 5 ppm, but if we consider only the surface corresponding to the laser beam size, the scattering is in the VIRGO specifications.

Another remarkable result concerns the flatness of these large components. The "8 nm RMS on 150 mm diameter " requirement on the wavefront is always satisfied. Nevertheless, we found that the limiting factor of the wavefront flatness is the substrate by checking that the RMS flatness value is the same with or without coating (Before: 3.38 nm RMS and 25.5 nm PV - After: 3.37 nm RMS and 25.2 nm PV). The coatings reproduce exactly and even smooth the substrate surface .
As the polishers can not guarantee every time better wavefronts, the only solution is to correct the substrate surface before deposition by the "Corrective Coating" technique.

An other Virgo requirement we had to satisfy is the finesse inequality between the two Fabry-Perot cavities: (Df<1%). To fulfill this requirement, the cavity mirrors must have transmission values as close as possible. For instance, the difference between the two Input Mirrors transmission must be lower than 0.2 %.