The alt-azimuth mounted 1.5-meter GREGOR solar telescope employs a modified Gregory configuration. The primary mirror is an F / 1.67 paraboloid. Originally conceived as a light-weighted Cesic ceramic optics, it is now realized as a light-weighted Zerodur mirror because of manufacturing problems with large-diameter Cesic optics. The elliptical secondary and tertiary mirrors are still made from Cesic ceramics, which has a high thermal conductivity, thus preventing telescope seeing. The effective focal length of the GREGOR solar telescope is 55 m corresponding to a focal ratio of F / 38. A heat stop in the primary focus limits the field-of-view (FOV) to 150 seconds of arc.
The determination of the thermodynamic and magnetic structure of the solar atmosphere at the smallest spatial scales is one of the cornerstones of modern solar physics and the primary science driver for GREGOR. The AIP-operated GREGOR Fabry-Pérot Interferometer (GFPI) is an imaging spectropolarimeter, which has the advantage that post-factum image restoration can be applied improving spatial resolution across the entire FOV, thus, augmenting the real-time correction of the adaptive optics (AO) system. The GFPI provides access to fine structures as small as 60 km on the solar surface, which just reaches the size of the smallest features in magnetohydrodynamical (MHD) simulations.
ADS library of project-related publications: https://ui.adsabs.harvard.edu/public-libraries/Zehtp9UCSGuO0h87ivRMFg