Akademik Veri Yönetim Sistemi
Turkish Journal of Physics, cilt.49, sa.2, ss.44-62, 2025 (ESCI)
This study presents optical designs of two fibre-fed èchelle spectrographs (FES), which differ from each other in terms of cross-dispersers, 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. In the first spectrograph design, prisms were used as the cross-disperser. Specifically, two symmetric N-F2 glass prisms with 42-degree apex angles were used as the cross-disperser. In the second design, a grism was used as the cross-disperser. This grism was formed by inserting a volume-phase holographic grating between two symmetrical, thin N-BK7 prisms with 8-degree apex angles. The operating wavelength range for both spectrographs was set between 390 and 900 nm. For the prism cross-disperser FES (prism FES) optical design and optimization, the 113th order (m = 113) with a central wavelength of 501.148 nm was considered. In contrast, for the grism cross-disperser FES (grism FES) optical design and optimization, the 89th order (m = 89) with a central wavelength of 636.411 nm was considered. If the parameters of the grism FES were applied to the prism design, a larger camera diameter would be required. Therefore, a higher order and lower wavelength were chosen for the prism FES design and optimization. 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 with a pixel size of 13.5 × 13.5 µ was used to determine the optical parameters of the camera and analyse 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. It can be said that the distance between the orders increases towards the ultraviolet wavelength, and decreases towards the near-infrared wavelength for the prism FES. The opposite situation is observed for the grism FES. Finally, when all spectra obtained from both spectrographs were analysed, it was found that the prism FES had a higher spectrum line tilt.