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Modern Solar Facilities – Advanced Solar Science, 55–58 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c  Universitätsverlag Göttingen 2007 Multi-wavelength observations at the German VTT on Tenerife – Possible combinations of post-focus instruments C. Beck1,* , K. Mikurda1 , L. R. Bellot Rubio2 , T. Kentischer1 , and M. Collados3 1 Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany 2 Instituto de Astrofı́sica de Andalucı́a (CSIC), Granada, Spain 3 Instituto de Astrofı́sica de Canarias, La Laguna, Spain * Email: cbeck@kis.uni-freiburg.de Abstract. To study the small-scale dynamic processes of magneto-convection in the solar photosphere in more detail than currently achieveable, not only the spatial resolution has to be increased, but also the information content of observations. In order to do so, several wavelengths and spectral lines must be observed simultaneously. This is often achieved by coordinated campaigns at several telescopes with different post-focus instrumentation. The German Vacuum Tower Telescope (VTT) on Tenerife offers the possibility to operate several dedicated instruments – spectrometers, polarimeters, imaging systems – at the same time. We describe some of the possible combinations of post-focus instruments, and present examples of multi-wavelength data obtained recently. 1 Introduction Progress in solar physics has been achieved recently on both the observational and the theoretical side. Simulations of the solar atmosphere now extend from photospheric layers up to the chromosphere (e.g. Wedemeyer et al. 2004). The new Swedish 1-m telescope on La Palma has led to the discovery of several peculiarities that went unnoticed before, e.g., the dark cores of penumbral filaments seen by Scharmer et al. (2002) and Sütterlin et al. (2004). The same amount of surprises is to be expected with the advent of the next-generation solar telescopes, like the 1.5-m GREGOR telescope on Tenerife (von der Lühe et al. 2001) or the 4-m ATST in Hawaii (Keil et al. 2003). However, the increase of spatial resolution does not make life much easier for observers. To understand the physical processes on the Sun not only the spatial resolution – which determines the spatial scales that are visible – is critical, but also the information content of the observations – that allows to understand what is happening. In the excitement over the new observational findings, the equally important issue of information content seems to be undervalued. In this contribution, we show how the German Vacuum Tower Telescope on Tenerife can be used to maximize the information content of solar observations at a moderate spatial resolution of around 1 by operating several post-focus instruments simultaneously. 56 C. Beck et al.: Multi-wavelength observations at the German VTT Figure 1. The building of the German VTT (left) with a sketch of the optical layout (middle) and the location of the post-focus instruments in the first floor (right). The light can be divided between the main spectrograph in the basement and the optical lab in the first floor. 2 The German Vacuum Tower Telescope The German Vacuum Tower Telescope consists of a coelostat system on top of the elevenfloor building, and a 70-cm primary mirror of 46 m focal length. The light is fed in downwards by the coelostat, and the focal plane is finally located close to the first floor of the building. Before reaching the focus, the light passes through the Kiepenheuer-Institute adaptive optics system (KAOS, von der Lühe et al. 2003) that improves the spatial resolution by a real-time correction of wavefront aberrations. With good seeing conditions, the diffraction limit of around 0.  6 in infrared wavelengths can be reached. Calibration units for infrared and visible wavelengths can be inserted in front of the adaptive optics system for the derivation of the polarimetric properties of each instrument (cf. middle panel of Fig. 1). In the first floor, the light can be divided between the main spectrograph in the basement and the post-focus instruments located in the first floor. As dichroic beamsplitters are available, this can be done without any loss of light. 3 Post-focus instrumentation and possible combinations The POlarimetric LIttrow Spectrograph (POLIS, Beck et al. 2005) is a stand-alone slitspectrograph vector polarimeter with two channels. The first channel observes the full Stokes vector of the two photospheric Fe I lines at 630 nm, while the second channel observes the intensity profile of the chromospheric Ca II H line. Two slits of 0.  18 and 0.  48 width are available. The spatial sampling...

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