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Collimators

A paraboloidal mirror, made in the low expansion material Astrosital, with 400 mm clear area and a focal length of 1000 mm, has been cutted into equal parts. One is used off-axis as a F/10 collimator and a second time creating an intermediate focal plane with light dispersed by the echelle grating. The second mirror collimates the light towards the cross-disperser. These items have been realized by Marcon Costruzioni Ottico Meccaniche (WFE = $\lambda/10$) and delivered in December 2004.

Because of the three reflections, the previous two mirrors are critical for the over all efficiency of the spectrograph. A high efficiency reflectivity coating ($>$ 97%) has been defined with the Società Italiana Lavorazione Ottiche within the whole selected spectral range (figure 2.2).

Figure 2.2: An enhnanced silver coating will guarantee a total efficiency of the two mirrors larger than 91% at any wavelengths after the three reflections.
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Innocenza Busa' 2005-11-14