We propose a wavefront error compensation system based on a spatial light modulator (SLM) and on the phase diversity technique for both wavefront sensing and wavefront correction. The phase diversity technique has been successfully used in balloon borne instruments (IMaX and TuMag) and in space-borne instruments (SO/PHI) to determine the wavefront error of the system of solar spectropolarimeters. The method usually requires either two cameras observing the same scene with a known phase difference (typically a defocus), or a mechanism that introduces a phase diversity plate within the optical path. Processing the focused and defocused images with the phase diversity algorithm yield the point spread functions of the instrument, allowing scientific images to be restored a posteriori. Here, we demonstrate that liquid crystal-based SLMs can successfully be used to introduce a phase shift instead of a physical mechanism. SLMs can provide different levels of defocusing as well as other wavefront aberrations. Their flexibility enables different procedures for a dynamic wavefront retrieval system. Moreover, the approach allows direct acquisition of corrected images since SLMs can also be used for wavefront compensation. They also reduce system complexity, as no mechanism and only a single camera are required, and they help eliminate non-common optical path effects. The compactness and low power requirements of SLMs can be of great advantage for small platforms with onboard optical instrumentation. We present preliminary results using the IMaX optical layout as the baseline for our demonstrator for a solar spectropolarimeter space instrument.
SUNRISE III mission is a one-meter aperture telescope onboard a balloon within NASA Long Duration Balloon Program. Three post-focus instruments are used for studying the Sun’s dynamics and magnetism, among which the Tunable Magnetograph (TuMag) is a tunable imaging spectropolarimeter. TuMag is a diffraction-limited imager, a high sensitivity polarimeter (< 10-3 ), and a high-resolution spectrometer ( ~ 65 mÅ). It will be able to study solar magnetic fields at high spatial resolution (~100 km 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 525.02 nm and 525.06 nm, and the Mg I b2 line at 517.27 nm. It will be sensitive to the solar vector magnetic fields and line-of-sight velocities, 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. In this paper the TuMag end-to-end tests carried out during the verification phase are described. These tests are performed to characterize and calibrate the instrument. Specifically, they determine the polarimetric and spectroscopic performances of the instrument as well as the image quality. The availability of a singular facility, an ISO6 clean room with a coelostat on the building roof, allowed the use of solar light during the verification campaign. This was key to a complete instrument verification due to the unique spectroscopic and polarimetric characteristics of solar light.
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