We present here the ongoing upgrades at Themis Solar Telescope. Adaptive optics systems are installed with excellent results for imaging purposes at most existing solar telescopes, but the combination of polarimetric measurements with adaptive optics is quite challenging and not usually fully addressed from the design up. Adaptive optics within a specially re-designed optical path are being combined with THEMIS unique high polarimetric precision and high spectral resolution.
Themis routinely delivers high SNR polarized spectra with a polarimetric accuracy of 10-4 to 10-5. The polarimetric system uses a modulated dual beam principle with an analyzer located at the prime focus. Due to this configuration, Themis has been called a "polarization -free" telescope (although a better qualification would be "calibration-free"). Speckle imaging has been successfully implemented showing that the telescope has excellent imaging capabilities whenever the seeing is above r0=8cm. The present upgrades aim at correcting a number of limiting issues: i) Polarized spectra require high-SNR hence long exposure times, and the mapping the solar surface is also a time consuming process: as a result, the spectropolarimetry spatial resolution is never better than 0.5" which is no longer a competitive number, ii) The current dual beam polarimetry requires a severe field limitation to 15" in one spatial direction, which is both an observing limitation and makes impossible the implementation of any wavefront sensor, and iii) There is a flaw at system level in the placement of the current scanning device.
To solve what precedes, a 2 years study was necessary. A re-designed optical path starting from the M2 is being implemented, changing the f/ratio from f/63 to f/52.9. The optical relay is simpler with an improved transmission, and an AO bench will be located just before the spectrograph entrance. This AO is designed to start operating at r0=4.7 cm (half of the time at our site) with a ~100 actuator high-speed and high-stroke DM. The new polarimetric concept is now a superimposed modulated dual beam with no field limitation: this is the most challenging part of the work as it implies to control the behaviour of the Mueller matrix of a time-varying complex optical system. Based on a complete model of the optical path, with detailed simulations of the effect of ad-hoc surface coatings, deviation from the ideal situation can be controlled to an almost arbitrary low level. We expect from this renewed telescope a very large improvement of the spatial resolution of the polarized spectra, an excellent performances in spectrolarimetric mapping of the solar surface. It will available during a second exploitation period within 2018 -2025 and will then stop operating at the dawn of the future European Solar Telescope (EST).