A New Technique for Solar Imaging Spectro-polarimetry using Shack-Hartmann and Fabry-Perot

Abstract

A new technique for solar imaging spectro-polarimetry is presented. Using the combination of a Shack-Hartmann (SH) and a Fabry-Pérot (FP) interferometer, high-cadence spectroscopic observations can be obtained at discrete wavelength positions simultaneously, thereby avoiding errors due to non-simultaneity of the wavelength scans. A SH mask is used to generate multiple images of the same field-of-view (FOV). These multiple images when passed through the FP in a collimated-beam arrangement are shifted in wavelength due to the angular dependence of the FP filter transmission profile. Thus, by re-imaging one obtains multiple images of the FOV which are tuned to different wavelength points across the spectral line, in a single exposure. The schematic of the setup and the laboratory simulation of such a configuration is presented. The technique has an advantage of simultaneity over conventional wavelength scanning filtergraphs and has potential for observing highly-dynamic phenomena like solar flares. Also, one can exploit the method to perform snapshot spectropolarimetry by designing a special polarization modulator. The limitation of this technique is that it downgrades the spatial resolution due to the downsampling of the pupil into smaller sub-apertures. However, for large aperture telescopes like 4 meter class telescopes (ATST) this is not a major issue and one can still work at sub-arcsec resolution, though not at the diffraction limit of the full aperture.