Oskar von der Luhe

Field dependent amplitude calibration of adaptive optics supported solar speckle imaging

Adaptive optics supported solar speckle imaging requires the calibration of the sources Fourier amplitudes with the transfer function of atmosphere and optics. We present analytical models for the relevant transfer functions of an adaptive optics systems. The models include the effect of an arbitrary correction as well as anisoplanatism. The proposed models have been compared with observational data using measurements of alpha-Orionis and of the solar surface delivering both a direct and indirect method (using the spectral ratio technique) for validation. We find that measurements and model agree to a satisfactory degree.

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.

KISIP: a software package for speckle interferometry of adaptive optics corrected solar data

We present a speckle interferometry code for solar data taken with the help of an adaptive optics (AO) system. As any AO correction is only partial there is a need to use post-facto reconstruction algorithms to achieve the diffraction limit of the telescope over a large field of view most of the observational time. However, data rates of current and future solar telescopes are ever increasing with camera chip sizes. In order to overcome the tedious and expensive data handling, we investigate the possibility to use the presented speckle reconstruction program in a real-time application at telescope sites themselves. The program features Fourier phase reconstruction algorithms using either an extended Knox-Thompson or a triple correlation scheme. The Fourier amplitude reconstruction has been adjusted for use with models that take the correction of an AO system into account. The code has been written in the C programming language and optimized for parallel processing in a multi-processor environment. We analyze the scalability of the code to find possible bottlenecks. Finally, the phase reconstruction accuracy is validated by comparison of reconstructed data with satellite data. We conclude that the presented code is capable to run in future real-time reconstruction applications at solar telescopes if care is taken that the multi-processor environments have low latencies between the processing nodes.

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.

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.

Clear widens the field for observations of the Sun with multi-conjugate adaptive optics

The multi-conjugate adaptive optics (MCAO) pathfinder Clear on the New Solar Telescope in Big Bear Lake has provided the first-ever MCAO-corrected observations of the Sun that show a clearly and visibly widened corrected field of view compared to quasi-simultaneous observations with classical adaptive optics (CAO) correction. Clear simultaneously uses three deformable mirrors, each conjugated to a different altitude, to compensate for atmospheric turbulence. While the MCAO correction was most effective over an angle that is approximately three times wider than the angle that was corrected by CAO, the full 53′′ field of view did benefit from MCAO correction. We further demonstrate that ground-layer-only correction is attractive for solar observations as a complementary flavor of adaptive optics for observational programs that require homogenous seeing improvement over a wide field rather than diffraction-limited resolution. We show illustrative images of solar granulation and of a sunspot obtained on different days in July 2016, and present a brief quantitative analysis of the generalized Fried parameters of the images.

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.

Zernike basis optimization for solar adaptive optics by using information theory.

Karhunen-Loève functions represent the best choice for modal wavefront reconstruction. They are usually built up as a linear combination of Zernike polynomials by using principal component analysis methods; thus they are ordered by covariance. Using Shannon information theory, we provide a best reordering procedure based on the concept of mutual information. This enhances reconstruction efficiency, allowing us to reduce the basis dimension while maintaining the same fitting error in wavefront reconstruction.

Optical and thermal design of the main optic of the solar telescope GREGOR

The optical and thermal design of the 1.5 m solar telescope GREGOR is presented. The three first main mirrors of GREGOR will be made from Cesic, a silicon carbide material. One major constraint of large solar telescopes is the thermal load of the structure and the mirrors. The mirrors are heated by the solar radiation and introduce potentially harmful mirror seeing. GREGOR will use an active mirror cooling system and an open telescope structure to reduce these negative effects. A thermal analysis shows that the equilibrium temperature of the Cesic Mirror without active cooling is 6° above ambient temperature. Additional cooling will reduce the temperature difference of the optical surface and ambient air to below 0.1° K. With tempered airflow (about 2.5 m3/s per square meter mirror surface) the temperature gradient on the surface of the face sheet is less than 0.1°K. The telescope will have an open structure and a complete retractable dome to support mirror and structure cooling by wind.