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The Tunable Magnetograph (TuMag) is one of the three post-focus instruments onboard the SUNRISE III mission. It consists of a one-meter aperture telescope onboard a balloon within NASA Long Duration Balloon Program to study the solar dynamics. TuMag is a diffraction-limited imager, a high sensitivity polarimeter and a high resolution spectrometer. It will be able to study solar magnetic fields at high spatial resolution (~100km on the solar surface). It will make images of the solar surface magnetic field after measuring the state of polarization of light within three selected spectral lines: the Fe I lines at 525.02nm and 525.06nm, and the Mg I b2 line at 517.27nm. It will allow to be sensitive to physical quantities, and specifically to the magnetic fields, in the photospheric and chromospheric layers. TuMag will be the first solar magnetograph onboard an aerospace platform with the capability of tuning the solar line to be observed. TuMag consists of an Optical-Unit and an Electronic Unit to control it. The optical design is an optical relay of the telescope post-focal intermediate image where the light analysis is carried out in several stages. The polarization analysis is carried out with a polarization modulator based on Liquid Crystal Variable Retarders developed for the Solar Orbiter mission in operation currently. The spectral lines are scanned during the observation using a LiNbO3 etalon in double-pass configuration with a 65mÅ bandwidth. Additionally, to remove undesired orders of the Fabry-Perot interferometer, three narrow bandpass filters with a ~1.5 Å FWHM (Full Width at Half Maximum) are consecutively inserted in the optical path using a high precision and thermal controlled filter wheel. In this paper the optical, mechanical and thermal design of the TuMag optical unit is described as well as a brief summary of the results obtained during the manufacturing, assembling, integration and verification phases
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