Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) for SUNRISE-3: optical design and performance

The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is a near-IR spectro-polarimeter instrument newly designed for Sunrise III, which is a balloon-borne solar observatory equipped with a 1 m optical telescope. To acquire high-quality 3D magnetic and velocity fields, SCIP selects the two wavelength bands centered at 850 nm and 770 nm, which contain many spectrum lines that are highly sensitive to magnetic fields permeating the photosphere and chromosphere. To achieve high spatial and spectral resolution (0.21 arcsec and 2 × 105), SCIP optics adopt a quasi-Littrow configuration based on an echelle grating and two high-order aspheric mirrors. Using different diffraction orders of the echelle grating, dichroic beam splitter, and polarizing beam-splitters, SCIP can obtain s- and p-polarization signals in the two wavelength bands simultaneously within a relatively small space. We established the wavefront error budget based on tolerance analysis, surface figure errors, alignment errors, and environmental changes. In addition, we performed stray light analysis, and designed light traps and baffles needed to suppress unwanted reflections and diffraction by the grating. In this paper, we present the details of this optical system and its performance.