Instrumental polarization effects of the German Vacuum Tower Telescope (VTT) at Tenerife

Polarimetry is an important method to investigate the physics of the solar atmosphere. If the magnetic field strength is not strong enough to produce completely split Zeeman profiles the degree of polarization is a measure for the field strength. Measuring both the circular and linear polarization allows in principle the construction of the magnetic field vector including magnitude and direction. Unfortunately these highly desired measurements are in many cases affected by the instrument's optic itself. Especially telescopes which don't have a rotation symmetry with respect to their optical axis suffer from these problems. This is also the case with our German Vacuum Tower Telescope at Tenerife and this paper shall show the instrumental effects which are to be expected. One of the effects is crosstalk between linear and circular polarization. We show a method where this crosstalk can be considered as a tool and may be used--under certain assumptions--to derive the true size of magnetic elements which may cover only a fraction of the resolution element.