As shown in the optical designs the R4 echelle 200x800 mm mosaic grating is used in Littrow configuration with a small 0.4° off-plane angle, feed by the 200mm beam diameter F/13.5 Maksutov main collimator used in double pass. A transfer Maksutov collimator coupled to the first one through a field lens casts a white pupil onto the camera entrance pupil, where a grism acts as cross-disperser. The Maksutov layout of the collimators makes it possible to reduce the beam diameter of the transfer collimators to 125mm, thereby making very significant savings on the optical spectrograph cameras possible.
Due to the dispersion of the echelle grating, the overall dimensions of the two spherical mirrors are quite large (blank diameters: M1 = 88cm, M2 = 70cm).
The length of the spectrograph is about 5.5 m, just fitting within the 6m optical table on which it will be installed. M1 is rectangular shape (88x32 cm) with round ends and is cut from a circular mirror with a diameter of 88 cm. Its radius of curvature is 4144 mm. The mirrors are coated with a protected silver coating, giving a reflectivity of 97% in the visible/red part of the spectrum.
The BK7 corrector lens is an off-axis portion of a spherical meniscus lens with a central thickness of 40 mm. Its size (40x30 cm) is cut from a circular lens with a diameter of 50 cm. The two spherical surfaces of the lens segment are inclined with respect to each other, i.e. the lens is a 'spherical wedge'. The edge thickness of the lens therefore had to be controlled carefully during the grinding.
The transfer collimators (blue+red) are very similar to, but smaller than the main collimator. The mirror dimensions are 70x20 cm (red) and 40x20 cm (blue), and the BK7 corrector lenses are 30x25 cm, with a nominal central thickness of 40 mm.
The field lens is a cylinder lens (36x5 cm) with a central thickness of 25 mm.
Last updated:14:55 21/01 2008, by JB