Design of the optics for an imaging classic mount multi-étalon spectro- polarimeter for the next generation of ground-based solar telescope

In designing the optics of an imaging classic mount multi-etalon spectro-polarimeter as a post-focus instrument for the next generation of ground-based solar telescopes (Advanced Technology Solar Telescope, European Solar Telescope), many constraints must be considered. The large entrance pupil diameter of the telescope (4 m), the demanded large field of view (>= 90 arcsec), high spectral resolving power (>= 200000), and limited field-dependent blue-shift of the instrumental profile (<= 3 FWHM) require Fabry-Perot interferometers of large diameter (>= 200 mm), lighted by highly collimated beams. This implies large optical elements and long optical paths. Moreover, to use interference pre-filters, placed between the interferometers to reduce the inter-reflections in axial-mount, with a relatively small diameter (<= 70 mm), a "pupil adapter" should be included, with a further increase of the optical path length. Although a multi-etalon spectro-polarimeter works in quasi-monochromatic light, the Fraunhofer lines of interest cover a wide range of wavelengths (850 nm - 1650 nm), demanding a good chromatic aberration control. Finally, a low instrumental polarization (<= 0.5 %) is required to allow a high polarimetric precision. In this paper a diffraction limited optical solution is described, fulfilling all the above requirements in a relative small volume.