Optical spectrograph for the Takhomag-International Space Station space-based spectromagnetograph

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Subject of study. This paper is the second part of a series devoted to the development of the optical spectrograph designed for the Takhomag-International Space Station space-based spectromagnetograph planned to be deployed in the Russian segment of the International Space Station. The first paper of the series published in this journal describes a solar optical telescope in the spectromagnetograph. Aim of study. This study aimed to design an optical spectrograph for the Takhomag-International Space Station spectromagnetograph with properties required for the measurement of the Zeeman effect at selected spectral lines under magnetic fields characteristic of the solar photosphere. Method. The design of the Takhomag-International Space Station spectrograph is a classic variation of the off-axis lens scheme of the Littrow spectrograph. FeI 6301.5 Å and FeI 6302.5 Å lines were chosen as working spectral lines. The lens-based spectrograph design enabled it to satisfy the requirements of its limited size without optical elements with aspherical surface shapes. Main results. Despite the hard constraints on mass and dimensions owing to the conditions of operation, the spectrograph enables the formation of almost aberrationless images (Marechal parameter exceeding λ/20) of the solar photosphere spectrum with an angular resolution of 0.35″ by the Rayleigh criterion that corresponds to the angular resolution of the solar optical telescope of the Takhomag-International Space Station spectromagnetograph with a spectral range of 2.52 Å and spectral resolution of approximately 30 mÅ. The spectral resolution is smaller than the widths of the spectral lines used in the range of 42.4 mÅ in intense spots to 49.1 mÅ in a calm photosphere that results from the chaotic motion of atoms along the line of sight and the unresolvable structure of microturbulent velocities. Practical significance. The development of the Takhomag-International Space Station spectromagnetograph will facilitate the solution of relevant problems of solar and plasma physics and provide the groundwork for preparing more complex missions involving solar research from small distances.