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Prisms were chosen as the cross-dispersing elements of the spectrograph for a number of reasons, one of which is that the inter-order separation is much less dependent on wavelength than in the case of a diffraction grating. The prisms are made of extremely homogeneous fused silica (Corning 7940 fused slica class 2 grade A), which transmits down to 3000Å . This material is specified as having a refractive index homogeneity of 110.
The prism size demanded by the optical design is beyond the limits of current manufacturing technology. Therefore in order to provide sufficient cross-dispersion, three smaller prisms are used in series, each of which is composed of three smaller parts optically contacted together by means of a UV transmitting fluid (Cargille matching fluid 06350).
The layout and nominal dimensions of the prism assembly are summarised in Fig. 3.