Thomas Berkefeld

KAOS: Adaptive optics system for the Vacuum Tower Telescope at Teide Observatory

We are completing the integration of a solar adaptive optics system KAOS at the 70 cm diameter Vacuum Tower telescope (VTT) on Tenerife. The system is capable to compensate some 30 modes of wavefront aberration with closed-loop bandwidth of about 100 Hz anywhere on the solar disk. We describe the design goals, the main characteristics of KAOS and present a first demonstration of its performance.

Adaptive optics development at the German solar telescopes

We present an overview of the past achievements and future developments of adaptive optics (AO) for the German solar telescopes. The basic design considerations as well as the differences in wavefront sensing between solar and stellar AO are presented. The AO systems at the 0.7m Vacuum Tower Telescope and the new 1.5m GREGOR Telescope, Tenerife, the image stabilization and active optics for the 1.0mSUNRISE balloon-borne telescope, and the ongoing multiconjugate adaptive optics (MCAO) development for GREGOR are described. The MCAO for GREGOR also serves as a test bed for the planned European Solar Telescope.

Multi-conjugate solar adaptive optics with the VTT and GREGOR

After the successful demonstration of the solar multi-conjugate adaptive optics (MCAO) system at the German 70cm Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife, in the last years, we are continuing the development of the system as a testbed for the future MCAO of the 150cm GREGOR solar telescope. We describe an improved reconstruction scheme that increases the number of corrected off-axis degrees of freedom and will be tested at the VTT in September 2006. We present a modified optical setup of the GREGOR MCAO that has the advantage of being adjustable to a wide height range of the turbulence.

Results of the multi-conjugate adaptive optics system at the German solar telescope, Tenerife

We present the optical setup, reconstruction scheme and observational results of the Multi-conjugate Adaptive Optics (MCAO) system at the German 70cm Vacuum Tower Telescope, Observatorio del Teide, Tenerife. The system serves as a testbed for the future MCAO of the new 1.5m GREGOR solar telescope and is an extension of the conventional Adaptive Optics (CAO) system. We demonstrate that the use of one additional MCAO wavefront sensor and one additional deformable mirror increases the corrected field of view from 10 to 35 arcseconds.

Multi-Conjugate Adaptive Optics with Two Deformable Mirrors – Requirements and Performance

In order to increase the corrected field of view of an adaptive optics (AO) system, several deformable mirrors (DM) have to be placed in the conjugate planes of the dominant turbulent layers (multi-conjugate adaptive optics,MCAO (Beckers, 1988)).The performance of MCAO systems depends on the quality of thewavefront sensing ofthe individual layers and on the number of corrected modes in eachindividual layer as in single layer AO systems. In addition, the increase in corrected field of view depends on the number of guide stars providing information about theturbulence over a sufficiently large area in each turbulent layer. In this article, we investigate these points and provide formulae for calculating the increased field of view with a new approach using the spatial correlation functions of the appliedpolynomials (e.g. Zernike). We also present a new scheme of measuring the individual wavefront distortion of each of the dominantlayers with a Shack-Hartmann-Curvature Sensor using gradientinformation as well as scintillation. An example for the performance of a two layer MCAO system is given for the 3.5-m telescope of the Calar Alto Observatory, Spain, using ameasured Cn2-profile. The corrected field of view in K-band(2.2 μm) can be as large as 3 arcmin with a Strehl ratio above 60%.

The Tesat transportable adaptive optical ground station

Tesat together with Synopta have built a Transportable Adaptive Optical Ground Station (TAOGS) under contract of German Aerospace Center DLR for communication with the 1st and 2nd generation of Tesat’s spaceborne Laser Communication Terminals (LCTs), which employ coherent homodyne optical communication with 1064 nm and binary phase shift keying (BPSK) modulation. The TAOGS is able to communicate with space segments on low earth orbit (LEO, high pointing and tracking dynamics, 5.625 Gbps), and with space segments on geostationary orbit (GEO, low pointing dynamics, up to 40,000 km distance, optical data rate of 2.8125 Gbps and user data rate of 1.8 Gbps). After an alignment and testing phase at the location of Izana, Tenerife, using the TDP1 LCT on geostationary Alphasat as counter terminal, the TAOGS is now fully functioning. Several up-links, down-links and bi-directional links have been performed. Experimental results of some of these links are presented. An outlook to further activities is given.

GREGOR: the new 1.5m solar telescope on Tenerife

The new 1.5 m high resolution telescope will be build up on the reused solar tower of the German 45 cm Gregory Coude Telescope at the Teide Observatory, Izana, Tenerife. The new telescope is a Gregory type with open telescope structure, alt-azimuth mount, complete retractable dome, and a pool of well established and new developed post focus instruments. An adaptive optics system provides the capability for diffraction limited observations at visible wavelengths and the polarimetry device in the secondary focus reduces the perturbation due to instrumental polarization in an efficient way. We describe the main optical characteristics and the focal plane instrumentation with respect to the latest status of the project.

Magnetic fields of opposite polarity in sunspot penumbrae

Context. A significant part of the penumbral magnetic field returns below the surface in the very deep photosphere. For lines in the visible, a large portion of this return field can only be detected indirectly by studying its imprints on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe a narrow layer in the very deep photosphere, providing the possibility of directly measuring the orientation of magnetic fields close to the solar surface. Aims. We study the topology of the penumbral magnetic field in the lower photosphere, focusing on regions where it returns below the surface. Methods. We analyzed 71 spectropolarimetric datasets from Hinode and from the GREGOR infrared spectrograph. We inferred the quality and polarimetric accuracy of the infrared data after applying several reduction steps. Techniques of spectral inversion and forward synthesis were used to test the detection algorithm. We compared the morphology and the fractional penumbral area covered by reversed-polarity and three-lobed Stokes V profiles for sunspots at disk center. We determined the amount of reversed-polarity and three-lobed Stokes V profiles in visible and infrared data of sunspots at various heliocentric angles. From the results, we computed center-to-limb variation curves, which were interpreted in the context of existing penumbral models. Results. Observations in visible and near-infrared spectral lines yield a significant difference in the penumbral area covered by magnetic fields of opposite polarity. In the infrared, the number of reversed-polarity Stokes V profiles is smaller by a factor of two than in the visible. For three-lobed Stokes V profiles the numbers differ by up to an order of magnitude.

GREGOR MCAO looking at the Sun

A multi-conjugate adaptive optics systems has been deployed at the 1.5-meter solar telescope GREGOR for on-sun experiments of MCAO in November 2013. GREGOR MCAO incorporates three deformable mirrors (DMs) conjugate to 0, 8, and 25 km line of sight distance. Two correlating Shack-Hartmann wavefront sensor units are deployed: a high-order on-axis wavefront sensor (OA-WFS) with 10-cm subapertures and 10 arcsec field of view, and a low-order multi-direction wavefront sensor (MD-WFS) with 50-cm subapertures that sample the wavefront in 19 guide regions distributed over one arcminute. The MCAO loop was closed repeatedly in November ’13, as well as in January and May ’14. However, in particular strong static aberrations that were not removed well by the system, derogated the image in the MCAO compensated focal plane. GREGOR MCAO is now permanently installed and available for experiments that shall advance the development of solar MCAO.

Second-generation adaptive optics for the 1.5 m solar telescope GREGOR, Tenerife

We present the optical setup and properties of the second-generation adaptive optics (AO) for the 1.5 m solar telescope GREGOR. The system will consist of a high order AO system correcting about 200 degrees of freedom on-axis at a bandwith of 200 Hz and a multi-conjugate (MCAO) extension that uses one additional deformable mirror to correct the low-order aberrations across a field of one arcminute at a bandwidth of 50 Hz. Diffraction limited observations will be possible for seeing better than 1.2 arcsec. First light is expected in 2007.