Large-aperture diffraction-limited spectro-polarimetry with FISS-SP

Context. Small-scale events measuring only tens of kilometres can have significant implications for the overall energy state of various layers of the solar atmosphere. Current spectro-polarimetric observations lack either spatial or spectral resolution for a comprehensive study of these small-scale events. Aims. The slit-scanning spectro-polarimetric instrument described here is designed for spectral image reconstruction and, in combination with the excellent optical performance of the 1.6 metre Goode Solar Telescope, yields spectral hypercubes of the highest spatial and spectral resolution. Additionally, the instrument offers a huge spectral window of more than 30 Å, allowing many solar absorption lines to be observed simultaneously. Methods. We extended the existing Fast Imaging Solar Spectrograph (FISS) instrument with polarimetric capabilities, new customized cameras, and a context imager. We applied numerical methods to measure and correct for field-dependent instrumental and atmospheric degradations, to obtain diffraction-limited spectro-polarimetric scans. Results. In this work we present the instrument design, the data reduction workflow, and the first-light results. Compared to a typical HINODE/SP dataset, we find a higher signal-to-noise ratio in our data within the resolution limits of the respective telescopes when utilizing the signal of all simultaneously observed spectral lines. Conclusions. We have obtained the first diffraction-limited full Stokes spectro-polarimetric datasets recorded with a slit-scanning spectrograph on a telescope with an aperture exceeding 1.5 metres.