White Pupil Fibre-Fed Echelle Spectrograph of the Russian-Turkish 1.5 m Telescope: Optical Design and Analysis

This study includes optical designs of two Fibre-Fed Échelle Spectrographs (FES) and optical analyses of the spectra (orders) obtained from the designs. Optical analyses of both designs were performed by interpreting the results obtained from the Spot Diagram, Footprint Diagram and Encircled Energy Diagram in the ZEMAX (OpticStudio) optical design program.Prisms were used as a cross-disperser for the first spectrograph type. In the second design, a grism was used as a cross-disperser.The operating wavelength range of both spectrographs was determined as 390-900 nm. m = 113 order and its 501.148 nm central wavelength were taken into consideration for the prism cross-disperser FES (prism FES) optical design and optimization process. On the other side, m=89 and its 636.256 nm central wavelength were taken into consideration for the grism cross-disperser FES (grism FES) optical design and optimization process.A camera with a focal length of 155 mm and a focal ratio of f/2 was used for both designs. A classical couple-charged device (CCD) with a pixel size of 13.5x13.5 microns was used to determine the optical parameters of the camera and analyze its image quality on the focal plane.During the analysis following the tolerance process, the RMS and geometric spectrum image thickness values of m=113 were observed to be 2.3–2.8 pixels and 5.8–7.6 pixels for prism FES, respectively. In addition, the distance between the central order m=113 and its adjacent orders was measured as 17 pixels. When a similar analysis was performed for the grism FES, it was observed that the RMS and geometric spectral image thickness values of m=89 were in the range of 2.3-2.45 pixels and 5.7-8.1 pixels, respectively. The distance between the central order m=89 and its adjacent orders was measured as 25 pixels.