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Modern Solar Facilities – Advanced Solar Science, 59–62 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c  Universitätsverlag Göttingen 2007 Instrumental developments at the Gregory Coudé Telescope (GCT) at IRSOL M. Bianda1,2,* , R. Ramelli1 , A. Feller2 , J. O. Stenflo2,3 , and G. Küveler4 1 Istituto Ricerche Solari Locarno, Locarno, Switzerland 2 Institute of Astronomy, ETH Zentrum, Zurich, Switzerland 3 Faculty of Mathematics & Science, University of Zurich, Zurich, Switzerland 4 Fachhochschule Wiesbaden, Rüsselsheim, Germany * Email: mbianda@irsol.ch Abstract. The research projects carried out with the Gregory Coudé Telescope (GCT) at Istituto Ricerche Solari Locarno (IRSOL) are mainly focused on high precision polarimetry. The Zurich IMaging POLarimeter (ZIMPOL) developed at ETH Zurich and installed permanently at the GCT at IRSOL allows a polarimetric precision down to 10−5 to be reached. This makes it possible to perform several accurate spectro-polarimetric measurements of scattering polarization and to investigate solar magnetic fields through the signatures of the Hanle and Zeeman effects. The research programs are currently being extended to monochromatic imaging of the Stokes vector with a recently installed Fabry-Perot rapidly tunable filter system with a narrow pass band of about 30 mÅ. The spatial resolution is being improved by the installation of an adaptive optics (AO) system. 1 Introduction The great advances in high precision polarimetry that have been achieved with the introduction of the Zurich IMaging POLarimeter (ZIMPOL) a decade ago opened a new window in solar physics. Polarimetry is in fact a very powerful tool that can be used to study solar magnetic fields as well as the physical processes behind the generation of polarization in atomic and molecular spectral lines. Magnetic field measurements through Zeeman effect signatures, which appear in the presence of strong and oriented magnetic fields, have long been performed at many observatories. With the high polarimetric precision of ZIMPOL it has become possible to extend the magnetic field diagnostics to weak fields and to fields which are tangled on scales below the spatial resolution, which are invisible to the Zeeman effect but get revealed by the Hanle effect (Hanle 1924). Spectro-polarimetry is currently the main field of research at the Istituto Ricerche Solari Locarno (IRSOL). Advantage is taken from the circumstance that a ZIMPOL system is permanently installed at IRSOL. In addition, the GCT is very well suited for polarimetric measurements, since the amount of instrumental polarization is low and stays practically constant during the observing day, since it is a function of declination only. Therefore it can easily be accounted for. With a Fabry-Perot filter system and an adaptive optics system recently installed at IRSOL we plan to start several new interesting projects. 60 M. Bianda et al.: Instrumental developments at IRSOL 2 IRSOL - The institute The observatory at the Istituto Ricerche Solari Locarno (IRSOL), located in southern Switzerland, was constructed in 1958/59 by the Universitäts-Sternwarte Göttingen (USG), Germany. In 1984, after USG moved its observing activity to the facilities at Observatorio del Teide on Tenerife, a local foundation (FIRSOL) acquired the observatory in Locarno. The partially dismantled instrumentation was rebuilt and improved, in collaboration with USG (now IAG), Technical Highschool (FH) of Applied Sciences of Wiesbaden (Germany), and the Institute of Astronomy at ETH Zurich. The scientific collaboration with ETH Zurich allowed the implementation at IRSOL of an important polarimetry observing program, first with a beam exchange polarimeter and then with ZIMPOL. 3 Instrumentation at IRSOL The IRSOL telescope is a 45 cm aperture Gregory coudé type instrument with 25 m effective focal length. A field stop with a pinhole of about 2.5 mm diameter in the prime focus reduces the field of view to a 200 arcsec diameter circular image. The rest of the solar image is reflected away from the main light beam. This reduces heating and scattered light and is of particular advantage when observing low intensity structures like sunspots, spicules, or prominences. The relative orientation of the two folding mirrors M3 and M4 (coudé) changes only with declination and is orthogonal at the time of the equinoxes. As a consequence the instrumental polarization, originating through oblique reflections, is almost constant during the day and virtually vanishes during the equinoxes (Sánchez Almeida et al. 1991). A Gregory coudé type telescope is thus very well suited for polarimetric measurements. An automatic guiding system developed by the FH Wiesbaden (Küveler et al. 1998) is also available. Its operation is based on...

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