High-resolution solar spectroscopy with TESOS – Upgrade from a double to a triple system

We present the characteristics and demonstrate the performance of the Triple Etalon SOlar Spectrometer (TESOS) operated at the German Vacuum Tower Telescope (VTT) on Tenerife. The Fabry–Pérot interferometer TESOS is ideally suited for precise measurements of photospheric and chromospheric motion. Installed in 1997 and equipped with two etalons, TESOS has recently been completed with a third etalon and upgraded with two high-speed, backside-illuminated CCD cameras. The image scale of 0.089 arc sec pixel^–1 is adapted to the resolution of the telescope. The improved system enables frame rates up to 5 frames per second. The spectral resolution of 300000 allows for spectral diagnostics of weak photospheric lines, including individual CH-lines within the G-band at 430.6 nm.     

The Wave-Front Correction System for the Sunrise Balloon-Borne Solar Observatory

This paper describes the wave-front correction system developed for the Sunrise balloon telescope, and it provides information about its in-flight performance. For the correction of low-order aberrations, a Correlating Wave-Front Sensor (CWS) was used. It consisted of a six-element Shack??C?Hartmann wave-front sensor (WFS), a fast tip-tilt mirror for the compensation of image motion, and an active telescope secondary mirror for focus correction. The CWS delivered a stabilized image with a precision of 0.04?arcsec (rms), whenever the coarse pointing was better than ???45?arcsec peak-to-peak. The automatic focus adjustment maintained a focus stability of 0.01?waves in the focal plane of the CWS. During the 5.5?day flight, good image quality and stability were achieved during 33?hours, containing 45?sequences, which lasted between 10 and 45?min.     

A fast tracking mirror for adaptive optics

A piezo driven tilt mirror was developed and built as one component of an image motion compensation system for a solar telescope. The bandwidth of the mirror with a diameter of 60 mm is about 1 kHz with negligible phase shift between input signal and mirror response up to 900 Hz. The tilt range is 1.6 mrad. Special care was taken to maintain the surface quality of the mirror to better than /15 after fixing it to the substrate.     

A Laser Frequency Comb System for Absolute Calibration of the VTT Echelle Spectrograph

A wavelength calibration system based on a laser frequency comb (LFC) was developed in a co-operation between the Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany and the Max-Planck-Institut für Quantenoptik, Garching, Germany for permanent installation at the German Vacuum Tower Telescope (VTT) on Tenerife, Canary Islands. The system was installed successfully in October 2011. By simultaneously recording the spectra from the Sun and the LFC, for each exposure a calibration curve can be derived from the known frequencies of the comb modes that is suitable for absolute calibration at the meters per second level. We briefly summarize some topics in solar physics that benefit from absolute spectroscopy and point out the advantages of LFC compared to traditional calibration techniques. We also sketch the basic setup of the VTT calibration system and its integration with the existing echelle spectrograph.     

The GREGOR Broad‐Band Imager

The design and characteristics of the Broad‐Band Imager (BBI) of GREGOR are described. BBI covers the visible spectral range with two cameras simultaneously for a large field and with critical sampling at 390 nm, and it includes a mode for observing the pupil in a Foucault configuration. Samples of first‐light observations are shown (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A retrospective of the GREGOR solar telescope in scientific literature

In this review, we look back upon the literature, which had the GREGOR solar telescope project as its subject including science cases, telescope subsystems, and post‐focus instruments. The articles date back to the year 2000, when the initial concepts for a new solar telescope on Tenerife were first presented at scientific meetings. This comprehensive bibliography contains literature until the year 2012, i.e., the final stages of commissioning and science verification. Taking stock of the various publications in peer‐reviewed journals and conference proceedings also provides the “historical” context for the reference articles in this special issue of Astronomische Nachrichten/Astronomical Notes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

POLIS: A spectropolarimeter for the VTT and for GREGOR

The polarimetric Littrow Spectrograph POLIS is designed for vector polarimetry at high angular and spectral resolution. It measures the magnetic field simultaneously in the photosphere and the chromosphere of the sun. Both branches of the polarimetry unit are dual beam systems with a single rotating modulator for both wavelengths and polarizing beam splitters in front of each CCD camera. POLIS has been installed at the VTT on Tenerife and has seen First Light on 17 May 2002. A modified version of POLIS will be developed for the balloon mission Sunrise . That version will have UV capabilities down to 200 nm.