High precision and thermally controlled Filter Wheel

The Tunable Magnetograph (TuMag) is one of the three post-focal instruments onboard SUNRISE III, a balloon borne solar observatory aimed to the study of solar dynamics TuMag is able to make Sun observations at three absorption solar lines: the Fe I lines at 525.02nm and 525.06nm, and the Mg I b2 line at 517.27nm. The spectral lines are scanned during the observation using a LiNbO3 etalon in double-pass configuration with a 65m angstrom bandwidth Additionally, to remove undesired orders of the Fabry-Perot interferometer, a narrow band filter with a similar to 1.5 angstrom FWHM is inserted in the optical path, and needs to be centered to the corresponding solar line. These filters feature high thermal sensitivity and incidence angle dependence. For this reason, they require a thermal stabilization of +/- 0.5 degrees C and an angular position accuracy better than +/- 0.03 degrees. In order to switch between filters and hence the observation solar line, a Filter Wheel has been designed which comply with stringent thermal and mechanical requirements and it is presented in this work.