First Observations with ZIMPOL at GREGOR

Example of polarization measurement in the SR I line at 460.7 nm. The measurement has been recorded with a total exposure time of one minute, near the solar limb and with the spectrograph slit oriented perpendicular to the limb. The solar limb is on the bottom. A facula shows up as a bright horizontal structure. The positive Stokes Q direction is defined parallel to the limb. In the Q/I image scattering polarization is visible in the Sr I line which is increasing toward the limb. In the active region Zeeman signatures are visible, symmetric in linear polarization (Q/I and U/I image), and asymmetric in circular polarization (V/I image). The smallest detected polarization signals have an amplitude of 10-4.

In collaboration with the Istituto Ricerche Solari Locarno (IRSOL) another instrument has been installed for the first time at the GREGOR telescope: ZIMPOL. First successful observations have been carried out.

The new instrument is a polarimeter for the visible wavelength range (390-700 nm) combined with the spectrograph at GREGOR. The polarimeter is based on technologies originally developed at ETH Zurich. It has been used for many years at IRSOL and is further developed in collaboration with SUPSI (University of Applied science in Locarno-Manno).

The system consists of two main components: a fast polarization modulator (composed of two ferro-electric liquid crystal modulators and two retarder plates) and a polarizer. It transforms polarization signals into modulated intensity signals. The second component is a camera system with a special masked CCD sensor. It demodulates the modulated intensity signal directly on the sensor. This concept allows a fast polarization modulation of 1 kHz, fast enough to avoid any disturbing seeing effects for the polarization measurements. Thus the polarimeter achieves a high sensitivity and is especially suited for the measurement of very small polarimetric signals.

End of October 2013 the new system has been installed at GREGOR for the first time with the goal to integrate the instrument into the other components (telescope, spectrograph and AO). Several observations have been recorded to demonstrate the performance of the instrument.

Another important point was the calibration of the measurements. Modern solar telescopes like GREGOR often have many inclined mirrors which change the polarisation state. Generally this is a difficult calibration problem. GREGOR, however, has the great advantage that there is a calibration unit installed before of the first tilted mirror. So the telescope and the polarimeter can be fully calibrated. The signals in the recorded data show that the system is already working successfully.