C. Halbgewachs

The Chromospheric Telescope

Aims. We introduce the Chromospheric Telescope (ChroTel) at the Observatorio del Teide in Izana on Tenerife as a new multi- wavelength imaging telescope for full-disk synoptic observations of the solar chromosphere. We describe the design of the instrument and summarize its performance during the first one and a half years of operation. We present a method to derive line-of-sight velocity maps of the full solar disk from filtergrams taken in and near the He i infrared line at 10 830 A. Methods. ChroTel observations are conducted using Lyot-type filters for the chromospheric lines of Ca ii K, Hα ,a nd Hei 10 830 A. The instrument operates autonomically and gathers imaging data in all three channels with a cadence of down to one minute. The use of a tunable filter for the He i line allows us to determine line-shifts by calibrating the line-of-sight velocity maps derived from the filtergram intensities with spectrographic data from the Tenerife Infrared Polarimeter at high spatial and spectral resolution. Results. The robotic operation and automated data reduction have proven to operate reliably in the first one and and half years. The achieved spatial resolution of the data is close to the theoretical limit of 2 arcsec in Hα and Ca ii Ka nd 3a rcsec in Hei. Line-of-sight velocities in He i can be determined with a precision of better than 3-4 km s −1 when co-temporal spectrographic maps are available for calibration. Conclusions. ChroTel offers a unique combination of imaging in the most important chromospheric lines, along with the possibility to determine line-of-sight velocities in one of the lines. This is of interest for scientific investigations of large-scale structures in the solar chromosphere, as well as for context imaging of high-resolution solar observations.

A two-dimensional spectropolarimeter as a first-light instrument for the Daniel K. Inouye Solar Telescope

The Visible Tunable Filter (VTF) is a narrowband tunable filter system for imaging spectropolarimetry. The instrument will be one of the first-light instruments of the Daniel K. Inouye Solar Telescope (DKIST) that is currently under construction on Maui (Hawaii). The DKIST has a clear aperture of 4 meters. The VTF is being developed by the Kiepenheuer Institut für Sonnenphysik in Freiburg, as a German contribution to the DKIST. The VTF is designed as a diffraction-limited narrowband tunable instrument for Stokes spectro-polarimetry in the wavelength range between 520 and 860 nm. The instrument uses large-format Fabry-Perot interferometers (Etalons) as tunable monochromators with clear apertures of about 240 mm. To minimize the influence of gravity on the interferometer plates, the Fabry-Perots are placed horizontally. This implies a complex optical design and a three-dimensional support structure instead of a horizontal optical bench. The VTF has a field of view of one arc minute squared. With 4096x4096 pixel detectors, one pixel corresponds to an angle of 0.014” on the sky (10 x 10 km on the Sun). The spectral resolution is 6 pm at a wavelength of 600 nm. One 2Dspectrum with a polarimetric sensitivity of 5E-3 will be recorded within 13 seconds. The wavelength range of the VTF includes a number of important spectral lines for the measurement flows and magnetic fields in the atmosphere of the Sun. The VTF uses three identical large-format detectors, two for the polarimetric measurements, and one for broadband filtergrams. The main scientific observables of the VTF are Stokes polarimetric images to retrieve the magnetic field configuration of the observed area, Doppler images to measure the line-of-sight flow in the solar photosphere, and monochromatic intensity filtergrams to study higher layers of the solar atmosphere.

The visible tunable filtergraph for the ATST

The Kiepenheuer-Institut will develop for the Advanced Technology Solar Telescope (ATST) a narrowband tunable filter system (Visible Tunable Filter, VTF) for imaging spectroscopy and spectropolarimetry based on large-format Fabry Perot interferometers. A major challenge for the realization of this instrument is the development of large-format Fabry-Perots with a free aperture of about 250 mm. The instrument will operate in the spectral range between 500 and 900 nm with access to a host of magnetically sensitive lines. The instrument is designed to match the diffraction limit of the 4m-aperture ATST and will be able to observe processes on the sun at spatial scales of 35 km. Its multi-line capability, together with a field of view of one arc minute, and the ability to measure polarization states of the incoming light allow to probe different layers of the solar atmosphere within a couple of seconds. The instrument is capable to vary the spectral sampling, the integration time, and the temporal cadence over a wide range without changing or compromising the opto-mechanical setup. This versatility gives unique possibilities to apply different measurement schemes to a variety of science questions. The ATST is a fully funded US project, with the VTF as the only non-US contribution, and is ready to start construction at the Haleakala summit. The VTF is foreseen as one of the ATST’s firstlight instruments and should become operational in 2018.

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)

ChroTel: a robotic telescope to observe the chromosphere of the Sun

The Chromospheric Telescope (ChroTel) is a 10 cm robotic telescope to observe the full solar disk with a 2k × 2k CCD at high temporal cadence. It is located at the Observatorio del Teide, Tenerife, Spain, next to the 70 cm German Vacuum Tower Telescope (VTT). ChroTel contains a turret system that relays a stabilized image of the solar disk into a laboratory within the VTT building. The control design allows a fully robotic operation. Observations are carried out in three chromospheric wavelengths (CaK: 393 nm, Ha: 652 nm, HeI 1083 nm).

GREGOR Telescope - Start of Commissioning.

With the integration of a 1-meter Cesic primary mirror the GREGOR telescope pre-commissioning started. This is the first time, that the entire light path has seen sunlight. The pre-commissioning period includes testing of the main optics, adaptive optics, cooling system, and pointing system. This time was also used to install a near-infrared grating spectro-polarimeter and a 2D-spectropolarimeter for the visible range as first-light science instruments. As soon as the final 1.5 meter primary mirror is installed, commissioning will be completed, and an extended phase of science verification will follow. In the near future, GREGOR will be equipped with a multi-conjugate adaptive optics system that is presently under development at KIS.

The new 1.5m solar telescope GREGOR: first light and start of commissioning

The integration of the three main silicon carbide mirrors into the new 1.5 m solar telescope GREGOR at Izana on Tenerife, Spain is planned during 2006. We expect first light at the end of 2006. A progress report about integration of the optics and mechanics and planning of the commissioning phase of the telescope and post focus instruments will be presented at the meeting. The GREGOR telescope is build by a consortium of the Kiepenheuer Institut fur Sonnenphysik in Freiburg, the Astrophysikalische Institut Potsdam, the Institut fur Astronomie Gottingen and additional national and international Partners.

Development of high reflectivity coatings for large format Fabry-Perot etalons

The Visible Tunable Filter (VTF) is a diffraction-limited narrowband tunable instrument for imaging spectropolarimetry in the wavelength range between 520 and 860 nm. It is based on large-format Fabry Perot. The instrument will be one of the first-light instruments of the 4m aperture Daniel K. Inoue Solar Telescope (DKIST). To provide a field of view of 1 arcmin and a spectral resolution λ/Δλ of about 100.000, the required free aperture of the Fabry Perot is 250mm. The high reflectivity coatings for the Etalon plates need to meet the specifications for the reflectivity over the entire wavelength range and preserve the plate figure specifications of better λ/300, and a micro roughness of < 0.4 nm rms. Coated surfaces with similar specifications have successfully been made for reflecting mirrors on thick substrates but not for larger format Fabry-Perot systems. Ion Beam Sputtering (IBS) based coatings provide stable, homogeneous, and smooth coatings. But IBS coatings also introduce stresses to the substrate that influence the plate figure in our case at the nm level. In a joint effort with an industry partner and a French CNRS research laboratory, we developed and tested processes on small and full size substrates, to provide coated Etalon plates to the required specifications. Zygo Extreme Precision Optics, Richmond, CA, USA, is polishing and figuring the substrates, doing the metrology and FE analysis. LMA (Laboratoire Matériaux Avancés, Lyon, France) is designing and making the IBS coatings and investigating the detailed behavior of the coatings and related processes. Both partners provide experience from manufacturing coated plane optics for gravitational wave detection experiments and EUV optics. The Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany is designing and building the VTF instrument and is leading the coating development. We present the characteristics of the coatings and the substrate processing concept, as well as results from tests on sample size and from full size substrate processing. We demonstrate that the tight specifications for a single Etalon can be reached.

End-to-end simulations of the visible tunable filter for the Daniel K. Inouye Solar Telescope

The Visible Tunable Filter (VTF) is a narrowband tunable filter system for imaging spectroscopy and spectropolarimetry based. The instrument will be one of the first-light instruments of the Daniel K. Inouye Solar Telescope that is currently under construction on Maui (Hawaii). The VTF is being developed by the Kiepenheuer Institut fuer Sonnenphysik in Freiburg as a German contribution to the DKIST. We perform end-to-end simulations of spectropolarimetric observations with the VTF to verify the science requirements of the instrument. The instrument is simulated with two Etalons, and with a single Etalon. The clear aperture of the Etalons is 250 mm, corresponding to a field of view with a diameter of 60 arcsec in the sky (42,000 km on the Sun). To model the large-scale figure errors we employ low-order Zernike polynomials (power and spherical aberration) with amplitudes of 2.5 nm RMS. We use an ideal polarization modulator with equal modulation coefficients of 3-1/2 for the polarization modulation We synthesize Stokes profiles of two iron lines (630.15 nm and 630.25 nm) and for the 854.2 nm line of calcium, for a range of magnetic field values and for several inclination angles. We estimated the photon noise on the basis of the DKIST and VTF transmission values, the atmospheric transmission and the spectral flux from the Sun. For the Fe 630.25 nm line, we obtain a sensitivity of 20 G for the longitudinal component and for 150 G for the transverse component, in agreement with the science requirements for the VTF.

The new 1.5 solar telescope GREGOR: progress report and results of performance tests

The telescope structure including control system and the complete retractable dome of the new 1.5 m solar telescope GREGOR were assembled during 2004 at Izana on Tenerife, Spain. The GREGOR telescope is build by a consortium of the Kiepenheuer Institut fuer Sonnenphysik, the Astrophysikalische Institut Potsdam, the Institut fuer Astrophysik Goettingen and additional national and international Partners. Pointing, tracking and thermal tests were made to verify the proposed performance. The results of these tests and a progress report of the project will be presented.