Frank Heidecke

The Filter Imager SuFI and the Image Stabilization and Light Distribution System ISLiD of the Sunrise Balloon-Borne Observatory: Instrument Description

We describe the design of the Sunrise Filter Imager (SuFI) and the Image Stabilization and Light Distribution (ISLiD) unit onboard the Sunrise balloon borne solar observatory. This contribution provides the necessary information which is relevant to understand the instruments’ working principles, the relevant technical data, and the necessary information about calibration issues directly related to the science data.

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.

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.

Testbed for the multi-conjugate adaptive optics system of the solar telescope GREGOR

We present the latest concept of the multi-conjugate adaptive optics system for the 1.5-meter solar telescope Gregor. This system will employ three deformable mirrors in order to compensate for seeing introduced by the ground layer, and by shear winds in 5 and 15 km above the telescope ground. Thus, the compensated field of view will grow compared to ground layer compensation only. We describe the design and the used components and present a testbed which is used to improve control algorithms and to test all the components before installing them at the Gregor telescope.

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)

The 2012 Status of the MCAO Testbed for the GREGOR Solar Telescope

We look back on two years of experience with the laboratory MCAO testbed for the GREGOR solar telescope. GREGOR’s MCAO features four adaptive mirrors, i. e. one tip-tilt mirror, and three DMs to compensate for turbulence around 0 km, 5 km, and 15.5 km above ground. Two different Hartmann-Shack wavefront sensor units are used for wavefront tomography. A sensor with a narrow field of view and smaller subapertures is dedicated to high-order aberrations on the optical axis. This sensor directly follows the pupil plane DM and does not see the high-altitude DMs. The second sensor features larger subapertures and 19 guide regions spread over a wide field of view for off-axis wavefront sensing. We show that high-altitude DMs cause rapidly changing pupil distortions and thus misregistration, which renders the interaction of a pupil-plane DM and a subsequent wavefront sensor non-linear. We rewrote the control software for cleaner and more flexible code, and we switched to modal wavefront reconstruction from direct reconstruction. The original digital interfacing of the DMs high-voltage electronics didn’t prove to be reliable. Thus, we developed a new interface board that is based on CameraLink/ChannelLink technology to transmit the DM commands from the control computer. In this paper we present the innovations and some of the first experimental performance measurements with two DMs. One DM failed before scientific grade data was recorded with three DMs. This DM will be replaced soon. We conclude that GREGOR’s MCAO system is now ready for first on-sky tests at the telescope.

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.

System model of an image stabilization system

The Polarimetric and Helioseismic Imager (PHI) instrument is part of the remote instruments for the ESA Solar Orbiter (SO), which is scheduled to launch in 2017. PHI captures polarimetric images from the Sun to better understand our nearest star, the Sun. A set of images is acquired with different polarizations, and afterwards is processed to extract the Stokes parameters. As Stokes parameters require the subtraction of the image values, in order to get the desired quality it is necessary to have good contrast in the image and very small displacements between them. As a result an Image Stabilization System (ISS) is required. This paper is focused in the behavior and the main characteristics of this system. This ISS is composed of a camera, a tip-tilt mirror and a control system. The camera is based on a STAR1000 sensor that includes a 10 bits resolution high-speed Analog-to-Digital Converter (ADC). The control system includes a Correlation Tracking (CT) algorithm that determines the necessary corrections. The tip-tilt mirror is moved based on this corrections to minimize the effects of the spacecraft (S/C) drift and jitter with respect to the Sun. Due to its stringent requirements, a system model has been developed in order to verify that the required parameters can be satisfied. The results show that the ISS is feasible, although the margins are very small.

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.