Imaging spectropolarimetry with two LiNbO₃ Fabry Pérot interferometers and a spectrograph

¹ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
e-mail: kleintl@astro.phys.ethz.ch
² Istituto Ricerche Solari Locarno (IRSOL), 6605 Locarno Monti, Switzerland
³ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany

Received: 5 October 2010
Accepted: 17 February 2011

Abstract

Context. Narrow-band spectropolarimetry is used to obtain information about the velocity and magnetic field structure of the solar atmosphere. Several types of instruments are suited to these observations, each with different advantages and drawbacks.

Aims. We set up a novel instrument combination using two LiNbO₃ Fabry Pérot interferometers (FPI), a high-resolution grating spectrograph, and the ZIMPOL system for polarimetry at IRSOL. With this system, we can carry out imaging spectropolarimetry of any spectral line from 390 to 660 nm, with a spectral resolution of 30 mÅ at 630 nm.

Methods. We describe the setup, its properties, and calculate the limitations induced by the FPI and the spectrograph. We carry out spectropolarimetric observations of the sunspot AR 11087 in different spectral lines with suitable Landé factors that could be used to derive the magnetic field strength in different height ranges of the solar atmosphere.

Results. The main advantage of our instrument compared to similar systems is that no special prefilters are required for each spectral line. A slight disadvantage is the spatial smearing induced by the dispersion of the finite transmission profiles of the FPI, which however is of the same magnitude as the seeing-limited resolution of 1–2″ at IRSOL.

Conclusions. We demonstrate that this particular instrument combination is well suited to spectropolarimetry at IRSOL.