The GREGOR Fabry-Perot interferometer: a new instrument for high-resolution solar observations

Abstract

The GREGOR Fabry-P´erot Interferometer (GFPI) is one of the first-light instruments of the 1.5-meter GREGOR solar telescope currently being commissioned at Observatorio del Teide (OT), Tenerife, Spain. A spectral resolution of R ≈ 250, 000 over the wavelength range from 530-860 nm can be achieved using a tunable dual etalon system. A high spectral resolving power is needed to extract physical parameters (e.g., temperature, plasma velocity and the magnetic field vector) from inversions of photospheric and chromospheric spectral lines. The GFPI is outfitted with a polarimeter, which accurately measures the full Stokes vector. Precision polarimetry is facilitated by a calibration unit in the immediate vicinity of GREGOR's secondary focus. The GFPI operates close to the diffraction limit of GREGOR, thus providing access to fine structures as small as 60 km on the solar surface. The field-of-view (FOV) of 52" × 40" is sufficiently large to cover significant portions of active regions. Large-format, high-cadence CCD detectors are an integral part of the instrument to ensure that scans of spectral lines can be obtained in time spans corresponding to the evolution time scale of solar phenomena such as granulation, evolving magnetic fields or dynamic chromospheric features. Besides describing the technical features of the GFPI and providing a status report on commissioning the instrument, we will use two-dimensional spectropolarimetric data obtained with the Vacuum Tower Telescope (VTT) at OT to illustrate GFPI's science capabilities.