The Spectrograph Fibre Positioner
Inside the spectrograph, the blue double-fibre is attached to a positioning stage, which has several degrees of freedom. The fibre is now permanently mounted on its stage, and will never need to be removed by the user. Note that both fibres (`star' and `sky') come together into a single mounting ferrule, which keeps them separated by 180 m. The exit holes are displaced horizontally, since the diffracted light from the échelle grating is dispersed vertically (with all the orders overlapping). This arrangement ensures that the cross dispersed spectra from the two fibres are interleaved.