The spectrograph is located on a 2m x 6m optical table in the telescope pier, inside a pressure- and temperature-stabilized chamber. It receives light from the polarimeters and permanent focus stations via 44m long fibres, coupled with image slicers. The polarimetric mode provides a resolution of 130.000, while resolutions of 50.000, 130.000 and 250.000 can be obtained with the permanent focus stations through a choice of different fibres. A white-pupil two-arm design with a 200mm main beam diameter, utilizing the largest available monolithic echelle grating and VPH grisms for cross dispersion, is employed. The collimators are nominally off-axis Maksutov systems, allowing for a beam diameter reduction to 125mm in the blue and red transfer collimator arms. The two spectrograph cameras are equipped with a single monolithic 10k x 10k CCD from STA (STA 1600LN). For each spectral order, four spectra are recorded, i.e. two polarization states or object/sky for each telescope. The entire spectral range from 383 to 912 nm can be covered in three exposures.

Overall spectrograph characteristics

Fibre Diam. (μm)Sky Diam. (arcsec) # of slices (n)Slicer Width (μm)Slicer Width (arcsec)R = λ/ΔλR Sampl. (pix)
Spectrograph optical design.
Spectrograph optical design.