For Virgo

Scattering and transmission. Figure 28 shows the layout of the seattering measurement bench. Scattering is measured over almost Ttt steradians, with two 1 dead angles, in the direetion of the source and of the deteetor. A typieal value of the amount of seattered light is 4 10 over 150 mm. On the baek side, one measures the transmission. A t5 ical value of the transmission obtained at LMA for VIRGO is 42.9 0.210 over a diameter of 50 mm. 

Wavefront. A critieal parameter of eoatings, for VIRGO or other instruments, is the wavefront rms error. It is measured with the bench in Fig. 29,... 

Figure 27. The relative reflexion bench for VIRGO coatings.

Since cryogenics is not involved in interferometer detector (but the proposal of cooling mirrors has been done for VIRGO), we no longer deal with this type of detector. 

In this lecture we recall the basic principles of the two main coating families dielectric and metallic coatings. In a second part, we describe the coatings deposition techniques and we address their performances and limitation. Most of the examples given in this lecture have been developed for the gravitational waves interferometers VIRGO and LJGO (see Ch. 18). 

Figure 21 shows the large coater for the VIRGO optics at LMA (Lyon). The technology is DIBS. Its size is 2x2.2x2.2 m. The inner pressure is 2 x 10 mBar. The coater is located in a 150 m class 1 room whith metrology and control. In a class 1 room, tolerance is <10 particles of 0.1 jum and <1 particle of 0.5 /xm xfoot xs l. 

Figure 28. Layout of the bench for scattered light measurements (right), and a VIRGO bulk being measured (left).

Finally, microroughness and defects are micromapped, down to a threshold of 0.3 /rm. Table 1 summarizes the LMA-VIRGO measurements of optics for the VIRGO end mirror, and compares them with VIRGO requirements. For all parameters, average values are significantly better than the specifications. 

Wavefront accuracies of < A/1000 are a target within the 4-5 coming years, in particular for relatively gravitationally faint sources observed with VIRGO. Extremely large telescopes (see Ch. 7) are also a big challenge for researchers... 

In natural melts, the presence of high field strength ions such as Fe , TF, and P, which, like silicon, preferentially assume a tetrahedral coordination with oxygen, complicates the structure, and the constitution of the anion matrix may not be deduced on the basis of the equations in section 6.1.2. Structural parameters valid for compositionally complex melts were proposed by Mysen et al. (1980) and Virgo et al. (1980) on the basis of the results of Raman spectroscopy. These parameters are NBO/Si and NBO/T (NBO = Non-Bridging Oxygen T groups all tetrahedrally coordinated cations—i.e., Fe ", TF, P ", ... 

Virgo D., Mysen B. O. and Kushiro I. (1980). Anionic constitution of 1-atmosphere silicate melts Implications for the strnctnre of igneons melts. Science, 208 1371-1373. 

I was confident that whatever choice she made would be the wrong one. The previous day, just for my own amusement, I had cast Grazia s horoscope, using the date and lime her parents had given the Maestro. The stars were very bad for her at the moment and had been for several days, with Mercury in the house of Virgo. Next weekend her fortunes should improve dramatically. Curiously, my own horoscope showed the reverse— good now, bad later. 

Mysen, B. O., D. Virgo, and F. A. Seifert (1982). The structure of silicate melts implications for chemical and physical properties of natural magma. Rev. Geophys. Space Phys. 20, 353-83. 

The Fe " content in xenolith spinels is large and unlike garnet or pyroxenes can be measured reliably from electron microprobe methods (Wood and Virgo, 1989) and is useful for oxygen barometry of spinel peridotites (Wood et al, 1990 Ballhaus et al., 1991). Representative analyses from different peridotite facies are shown in Table 3. 

Luth R. W., Virgo D., Boyd F. R., and Wood B. J. (1990) Ferric iron in mantle derived garnets imphcations for thermo-barometry and for the oxidation state of the mantle. Contrib. Mineral. Petrol. 104, 56—72. 

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