New Design Approach for a Very-High Resolution Spectrograph for the VLT Combined Focus

To achieve very-high spectral resolution (R > 100 000) with large telescopes (D > 8 m) new solutions have been investigated, like anamorphic pupil slicing, elliptical pupils, slanted VPH gratings, starting from the initial design of CODEX, as proposed by B. Delabre. Generally, for a given slit width, the spectral resolution of a spectrograph is proportional to the length of the grating and inversely proportional to the telescope diameter. For ESPRESSO, as in CODEX, a trick has been used: i.e. to slice the pupil, allowing to reduce the grating length by a factor two. The price to pay is a detector area that is larger by the same amount. An anamorphic cross-disperser is used to create a fast beam in the cross-dispersion direction and so reduce the required detector area.