The visible tunable filter is an imaging spectropolarimeter for solar observations in visible light. The instrument consists of several Fabry–Pérot interferometers (FPIs), a polarization modulator, and a prefilter. It will be one of the first light instruments for the Daniel K. Inouye Solar Telescope (DKIST) on Haleakaláa, Maui, Hawaii. We have developed simulation algorithms to describe the instrument and its impact on scientific observations. Our aim is to study the expected measurement accuracy and to test calibration algorithms. A well-known problem is the surface quality of the glass plates for each FPI. We developed algorithms to describe the influence of a surface microroughness, reflectivity, and figure errors of the individual FPI plates, and the expected total photon flux for scientific data acquisition. This tool is used to derive the limits for manufacturing processes to achieve the measurement accuracy required for science observations with DKIST.
The image quality of ground-based solar telescopes depends on the amount of turbulence in the Earth’s atmosphere, which is strongest in layers close to the ground during daytime. Local optical turbulence affects the performance of adaptive optical systems and reduces the spatial resolution of solar observations. Increased turbulence which is caused by solar irradiation of the infrastructure close to the telescope and obstructions to a free airflow are major concerns, but difficult to detect and to monitor. We have conducted measurements of optical turbulence at the GREGOR solar telescope (Teide Observatory, Tenerife, Canary Islands) to assess quantitatively the influence of the infrastructure on the image quality.
The strength of optical turbulence is determined by the structure function parameter of the refractive index Cn2. We have measured the temporal behavior of the free flow optical turbulence about 30m above ground using a laser scintillometer between the towers of the Vacuum Tower Telescope and of GREOR. Local measurements of Cn2 were taken on the observing platform of GREGOR using three ultrasonic anemometers. Two anemometers are located at the north and south ends of the telescope building, a third one was be placed close to the telescope main mirror cell. Air temperature, pressure, humidity as well as wind speed and direction were measured along with refractive index measurements. An image quality indicator based on an estimate of the Fried parameter r0 was recorded whenever the adaptive optics system GAOS at the GREGOR telescope was operating. Recordings of the net solar radiation were obtained from the GONG experiment which is located a few 100 m from GREGOR. All data were taken between May 2015 and March 2016.
We investigate the relation between optical turbulence, solar irradiance and meteorological parameters. Under almost all conditions, optical turbulence nearby the telescope is stronger than free flow turbulence. We note significant dependencies of the strength and the horizontal gradient of Cn2 on wind direction. The moderate influence on image quality indicates that there is only a thin layer with strong turbulence just above the telescope.
Local atmospheric turbulence at the telescope level is regarded as a major reason for affecting the performance of the adaptive optics systems using wavelengths in the visible and infrared for solar observations. During the day the air masses around the telescope dome are influenced by flow distortions. Additionally heating of the infrastructure close to telescope causes thermal turbulence. Thereby optical turbulence is produced and leads to quality changes in the local seeing throughout the day. Image degradation will be yielded affecting the performance of adaptive optical systems. The spatial resolution of the solar observations will be reduced. For this study measurements of the optical turbulence, represented by the structure function parameter of the refractive index Cn2 were performed on several locations at the GREGOR telescope at the Teide observatory at Tenerife at the Canary Islands / Spain. Since September 2012 measurements of Cn2 were carried out between the towers of the Vacuum Tower Telescope (VTT) and of GREGOR with a laser-scintillometer. The horizontal distance of the measurement path was about 75 m. Additional from May 2015 up to March 2016 the optical turbulence was determined at three additional locations close to the solar telescope GREGOR. The optical turbulence is derived from sonic anemometer measurements. Time series of the sonic temperature are analyzed and compared to the direct measurements of the laser scintillometer. Meteorological conditions are investigated, especially the influence of the wind direction. Turbulence of upper atmospheric layers is not regarded. The measured local turbulence is compared to the system performance of the GREGOR telescopes. It appears that the mountain ridge effects on turbulence are more relevant than any local causes of seeing close to the telescope. Results of these analyses and comparison of nearly one year of measurements are presented and discussed.
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 Visible Tunable Filter (VTF) is a narrowband tunable instrument for imaging spectropolarimetry in the wavelength range between 520 and 870 nm. It is based on large-format Fabry Perots with a free aperture of 250 mm. The instrument will be one of the first-light instruments of the 4 m aperture Daniel K. Inoue Solar Telescope (DKIST) that is currently under construction on Maui (Hawaii). To provide stable and repeatable spectral scanning by tuning the air gap distance of the Etalons, a metrology system with 20 pm resolution and drift stability of better 100 pm per hour is needed. The integration of the metrology system must preserve the tight optical specifications of the Etalon plates. The HEIDENHAIN LIP 382 linear encoder system has a selected linear scale for low noise high signal interpolation. The signal period is 128nm and the interpolated signal from the sensor can be read out at 128 nm/ 14 bit = 7.8125 pm. To qualify the LIP 382 system for the VTF, we investigated the resolution and stability under nominal VTF operation conditions and verified a mounting concept for the sensor heads. We present results that demonstrate that the LIP 382 system fulfills the requirements for the VTF Etalons. We also present a design for the sensor head mounts.
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.
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.
KEYWORDS: Control systems, Cameras, Optical proximity correction, Photonic integrated circuits, Modulators, Sensors, Fabry–Perot interferometers, Human-machine interfaces, Control systems design, OLE for process control
The Visible Tunable Filter (VTF) is a narrowband tunable filter system for imaging spectroscopy and spectropolarimetry based on large-format Fabry Perot interferometers that is currently built by the Kiepenheuer Institut fuer Sonnenphysik for the Daniel K. Inouye Solar Telescope (DKIST). The control software must handle around 30 motorised drives, 3 etalons, a polarizing modulator, a helium neon laser for system calibration, temperature controllers and a multitude of sensors. The VTF is foreseen as one of the DKISTs first-light instruments and should become operational in 2019.
In the design of the control software we strongly separate between the high-level part interfacing to the
DKIST common services framework (CSF) and the low-level control system software which guarantees real-time performance and synchronization to precision time protocol (PTP) based observatory time. For the latter we chose a programmable logic controller (PLC) from Beckhoff Automation GmbH which supports a wide set of input and output devices as well as distributed clocks for synchronizing signals down to the sub-microsecond level.
In this paper we present the design of the required control system software as well as our work on extending the DKIST CSF to use the OPC Unified Architecture (OPC UA) standard which provides a cross-platform communication standard for process control and automation as an interface between the high-level software and the real-time control system.
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.
Observing the Sun with high angular resolution is difficult because the turbulence in the atmosphere is strongest during day time. In this paper we describe the principles of solar adaptive optics exemplified by the two German solar telescopes VTT and GREGOR at the Observatorio del Teide. With theses systems we obtain near diffraction limited images of the Sun. Ways to overcome the limits of conventional AO by applying multiconjugate adaptive optics (MCAO) are shown.
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.
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.
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.
Solar telescopes equipped with wide field of view Hartmann-Shack wavefront sensors (WFWFS) can be used
to measure the vertical distribution of optical turbulence strength at daytime.1 The method is based on the
computation of the angular covariance of local image displacements within the subapertures of the WFWFS,
similar to the SLODAR method which is used at nighttime telescopes.2 In this paper the basic principles of
the method are summarized, and practical limitations are shown. Moreover, the influence of compensating
ground-layer turbulence with a single conjugated adaptive optics system (SCAO) on the angular covariance
functions is modeled.
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.
The influence of thin film multilayer coatings of Fabry-Perot interferometers (FPI) on polarimetric measurements
is investigated. Because the oblique ray reflectivity of the coatings in general is polarization dependent, the
transmission profile is slightly different for the s- and p-components of light passing through the FPI, resulting
in weak artificial polarization signals. The difference increases with larger angles of incidence and higher design
reflectivity of the coatings. In order to estimate the magnitude of the effect, we perform numerical calculations
with different coating designs and different optical configurations. We conclude that while current slow focal ratio
solar FPI spectrometers are safe, high-precision polarimetric measurements with large aperture solar telescopes
which may require considerably steeper focal ratios may suffer from spurious polarization effects.
We present the results of our first experimental tests of the concept of an alternative wavefront sensor for extended, incoherent
light sources such as the sun. This concept is not associated with subapertures and therefore does not suffer from
involved restrictions. In theory, this wavefront sensor also needs very little light from the telescope. The sensor employs a
liquid crystal display as used in digital video projectors for masking an image plane in an aberrated telescope. We describe
a laboratory setup and an advanced prototype used at the German Vaccum Tower Telescope (VTT), Tenerife.
The 1m balloon-borne solar telescope Sunrise will be equipped with a wave-front sensing system for automatic in-flight focusing and alignment of the telescope and for high-precision image tracking. A six-element wavefront sensor measures low order aberrations of the telescope, including defocus and coma. The correction is achieved by moving the focusing mirror and the telescope secondary, respectively, in a closed-loop circuit. The same system measures image motion. The instrument requirements for the tracking are a dynamical range of about 30 Hz and a precision of about 0.005 arcs in the sky. The image motion signal feeds a closed-loop control system that drives both the tip-tilt mirror assembly and the mirrors that are needed for focusing and alignment. The tip-tilt unit is a dual-stage system, built at the Kiepenheuer-Insitut, consisting of a slow component with a large range of about 60 arcs and a fast component with a short range and high bandwidth. A breadboard-version of the Correlating Wavefront Sensor has been successfully tested at the German Vacuum Tower Telescope on Tenerife in summer of 2005. A closed-loop bandwidth of 80 Hz was measured for the tracking system. The wave-front sensor detected image aberrations pre-set by the telescope's adaptive optics system with the required accuracy. Sunrise will be flown in long duration stratospheric balloon flights, with a first scientific flight in 2009.
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.
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.
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.
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.
We observe that a multi-conjugate adaptive optics (MCAO) system produces rapid fluctuations of irradiance of a few percent in the compensated focal plane when observing an extended target, such as the solar surface. Such fluctuations are absent in the uncompensated focal plane. The fluctuations are apparently correlated with the local curvature of deformable mirror in the plane conjugated to the high altitude turbulence layer. They can be explained by the field dependent change of effective focal length introduced by a high altitude turbulence layer, which is related to image distortion. Using a simple geometric model of the MCAO system, we are able to reproduce the observed irradiance effects. We propose to introduce a slightly undersized aperture stop at the location of the nominal exit pupil in order to remove the fluctuations in the compensated field.
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.
SUNRISE is a 1m solar telescope for the visible and near UV wavelength range. It will be flown in long duration stratospheric
balloon flights in Antarctica, with a first scientific flight in 2007. In this paper, we describe the development of a wave-front sensing system that will be used for the automatic in-flight alignment of the SUNRISE telescope and for high-precision
tracking. The system is based on the principles of an adaptive optics system. A 19-element wavefront sensor is used to determine low order aberrations of the telescope, including defocus and spherical aberrations. The correction is achieved by controlling the position of the telescope secondary and a focusing mirror in closed-loop. In addition to these quasi-static aberrations, the system will also measure image motion with a dynamical range of at least 30 Hz and with a precision of about 0.005 arcs. To this end, the image displacement measured in all sub-apertures is averaged and used as
tip-tilt correction signal. This signal will feed a second closed-loop system that drives the tip-tilt mirror assembly. The
tip-tilt mirror unit is designed as a dual-stage system that consists of a slow component with a large range of 60 arcs and a fast component with high bandwidth.
GREGOR is the new 1.5 m solar telescope assembled on Tenerife, Spain, by the German consortium of the Kiepenheuer Institut fur Sonnenphysik, the Astronomischen Institut Potsdam, the Universitats-Sternwarte Gottingen and other national and international Partners. The refurbishment of the building is almost finished. The manufacturing of the telescope structure and the optics is still in progress. After the integration of the new complete retractable dome in July 2004 the telescope structure, optic and post focus instruments will be assembled during the rest of the year. First light is planned during May 2005.
KEYWORDS: Mirrors, Telescopes, Solar telescopes, Point spread functions, Adaptive optics, Space telescopes, Diffraction, Off axis mirrors, Optical components, Modulation transfer functions
A concept is presented for an extremely large high-resolution solar telescope with an aperture of 11 m and diffraction limited for visual wavelengths. The structure of GISOT will be transparent to wind and placed on a transparent stiff tower. For efficient wind flushing, all optics, including the primary mirror, will be located above the elevation axis. The aperture will be of the order of 11 m, not rotatively symmetrical, but of an elongated shape with dimensions 11 x 4 m. It consists of a central on-axis 4 m mirror with on both sides 3 pieces of 2 m mirrors. The optical layout will be kept simple to guarantee quality and minimize stray light. A Coudé room for instruments is planned below the telescope.
The telescope will not be housed in a dome-like construction, which interferes with the open principle. Instead the telescope will be protected by a foldable tent construction with a diameter of the order of 30 m, which doesn’t form any obstruction during observations, but can withstand the severe weather circumstances on mountain sites.
Because of the nature of the solar scene, extremely high resolution in only one dimension is sufficient to solve many exciting problems in solar physics and in this respect the concept of GISOT is very promising.
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.
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.
Ceramics technologies were successfully applied to a series of lightweight mirrors with different sizes and requirements. Several joining and optical surface optimizations were applied. Besides the classical rib-structured mirrors also the application of sandwich mirrors with Cesic foam and/or honeycomb structures are going to be tested. For all processes relatively simple straightforward processes can be applied which keeps the products relatively cost-effective.
We present an overview of the optical setup and control algorithms for the multi-conjugate adaptive optics (MCAO) system of the 70cm German Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife. The system is designed to remove the strong differential tip/tilt of the present AO system across a field of 30 arcseconds at visible wavelengths. It will consist of two Shack-Hartmann wavefront sensors (WFS) and two deformable mirrors (DM) plus a separate Tip/Tilt mirror. Both wavefront sensors will be situated in the pupil plane of the telescope. One determines the high order wavefront aberrations for the center of the field of view (FOV), the other measures only low order wavefront aberrations, but covers a large FOV in each subaperture. A 35 actuator bimorph mirror and a 37 actuator membrane mirror will correct the ground layer and the tropopause, respectively. For wavefront reconstruction, the mirror eigenmodes will be used. The system will have first light in the first quarter of 2003. Scientific operation is expected to start in April 2003 or July 2003.
We present the optical setup and wavefront reconstruction algorithms for the multi-conjugate adaptive optics (MCAO) system at the 70 cm German Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife. The system is designed to increase the corrected field of view (FOV) from about 10 arcseconds to 30 arcseconds in the visible. It will consist of two Shack-Hartmann wavefront sensors (WFS) and two deformable mirrors (DMs). Both wavefront sensors will be situated in the pupil plane of the telescope. One determines the high order wavefront aberrations for the center of the FOV, the other measures only low order wavefront aberrations, but covers a large FOV in each subaperture. A 35 actuator bimorph mirror and a micro mirror will correct the ground layer and the tropopause, respectively. The system will have first light in early 2002. Scientific operation is expected to start in the second half of 2002.
We describe an adaptive optics system for the 1.5m diameter solar telescope GREGOR which is currently developed for the Teide Observatory on Tenerife. In a first development step, the AO will provide compensation of 77 modes of wavefront deformation, corresponding to the first 10 radial degrees of a Karhunen-Loeve decomposition of wavefront error. We estimate that such a performance will render GREGOR nearly diffraction limited at visible wavelengths in conditions which correspond to the best 25% of the seeing In Tenerife. The AO uses a Shack-Hartmann wavefront sensor which operates on fine structure anywhere on the solar disk. The required control bandwidth will be about 200 Hz. We show how such a system can be realized using existing technology. Substantial increases in performance in terms of corrected wavefront error and field requires significant technological advances, in particular in the field of large field high speed detectors.
We describe principles, design and present status of MIDI, the mid-infrared interferometric instrument for the VLTI, which is planned to come into operation at the ESO Very Large Telescope Interferometer during the second half of 2001.
AMBER is the near-IR instrument for the VLTI, which will offer the possibility of combining two or three beams from either the 8 meter VLT main telescopes or the 1.8 meter auxiliary telescopes. With spectral dispersion up to 10,000 high visibility accuracy and the ability to obtain closure phases, AMBER will offer the means to perform high quality interferometric measurements in the 1 - 2.5 micron range initially, with later extensions to other portions of the spectrum. These design characteristics, coupled to the VLT interferometer potential, open up the access to investigation of several classes of objects, from stellar to extragalactic astronomy. We will review the projected performance in terms of sensitivity and angular resolution, and illustrate the potential applications in some key research areas. In particular, we will present the work of the AMBER Science Group, which is evaluating simulated data of source models and interferometric outputs for the purpose of defining the criteria for observations.
In the last few years, we have undertaken a number of studies and experiments to assess the impact of various environmental factors on the performance of the ESO very large telescope interferometer (VLTI). The investigated topics include atmospheric turbulence, wind loads on the telescope structure, vibrations created by equipment, natural thermal variation, thermal load from electronics, natural and man-made seismic noise, as well as acoustic noise. A first report of this activity was given in a previous paper. This paper presents the final results obtained in 1995. The main outcome is the very good confidence that the VLT 8 m telescope and the infrastructure design is adequate for interferometric use at optical wavelengths down to the visible.
The interferometric mode of the ESO very large telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8 m diameter and by several auxiliary telescopes of the 2 m class. While the position of the 8 m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 stations distributed on the top of the observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8 m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferometric mode is the high sensitivity due to the 8 m pupil of the main telescopes, with the potential for adaptive optics compensation in the near- infrared spectral regime. The VLT interferometer is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is initially funded, and more capability may be added later while experience is gained and further funding becomes available. The scientific program is now defined by a team which consists of a VLTI scientist at ESO and fifteen astronomers from the VLT community. ESO has recently decided to resume the construction of the VLTI which was delayed in December 1993, in order to achieve first interferometric fringes with two of the 8 m telescopes around the year 2000, and routine operation with 2 m auxiliary telescopes from 2003 onwards. This paper presents an overview of the recent evolution of the project and its future development.
The optical path difference model (OPD model) determines where to position the delay lines in order to compensate for on-axis delay as seen from an astronomical object of known coordinates. This model is equivalent to a pointing model but applied to the interferometric delay. The objective is to reduce the time to search for fringes and to improve the delay lines blind tracking accuracy. This aspect is of prime importance not only when considering the overall operational efficiency of the interferometer but also its ability to quickly observe a set of program objects even after relocation of the auxiliary telescopes. The optical path difference model is based on a precise knowledge of the interferometer configuration by including a set of calibration measurements. This paper describes the main characteristics of the model and includes the results of a simulation developed to fit telescope axis misalignments which contribute to optical path difference errors.
KEYWORDS: Stars, Telescopes, Space telescopes, Space operations, Interferometers, Mirrors, Observatories, Laser metrology, Interferometry, Active optics
Two versions of a kilometric interferometer with equivalent science capabilities have been studied, one located on the Moon and the other operating as a free-flyer. It has been found that the Moon is not the ideal site for interferometry because of tidal and micro-meteorite induced disturbances, the need for long delay lines and the large temperature swings from day to night. Automatic deployment of the Moon- based interferometer would be difficult and site preparation and assistance by man appear to be essential. The free-flyer would be implemented as a very accurately controlled cluster of independent satellites placed in a halo orbit around the 2nd Lagrange point of the Sun-Earth system. Both versions could attain the required scientific performances and each one needs the same type of metrology control. The free-flyer is intrinsically advantageous because of its reconfiguration flexibility, quasi-unlimited baseline length and observation efficiency (the Moon-based interferometer cannot be operated during the lunar day because of stray light). The free-flyer is better suited for implementation in the near or mid-term future, but the Moon-based version could be considered in the long term when a human presence would permit maintenance and upgrading leading to a longer lifetime with continuous performance enhancement.
The SOHO ultraviolet coronagraph spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliographic height of 12 R. In this paper, the salient features of the design of the UVCS instrument are described. An overview of the UVCS test and calibration activities is presented. The results from the calibration activity have demonstrated that the UVCS can achieve all its primary scientific observational goals.
The very large telescope interferometer (VLTI) will relay stellar beams from each individual telescope to the combining facility through an air path, as opposed to vacuum. Internal air turbulence will induce optical path fluctuations which have been taken into account in the global VLTI error budget and in the expected performance of the delay line control system. This paper presents experimental data used to validate the assessment of these turbulence effects. A comparison with theoretical models developed for the free atmosphere is included in order to investigate their applicability in the VLTI delay line tunnel. Optical path fluctuations were measured by a laser interferometer working in the 0-80m range, in an underground tunnel representative of the VLTI beam transport facilities. The derived index structure coefficient, Cn2, inside the tunnel has been compared with high sensitivity temperature measurements.
KEYWORDS: Telescopes, Interferometry, Space telescopes, Interferometers, Turbulence, Control systems, Astatine, Mirrors, Large telescopes, Observatories
The interferometric mode of the ESO very large telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8m diameter and by several auxiliary telescopes of the 2m class. While the position of the 8m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 stations distributed on the top of the observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferrometric mode is the high sensitivity due to the 8m pupil of the main telescopes which will be compensated by adaptive optics in the near-infrared spectral regime. The VLT interferometer (VLTI) part of the VLT program is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is initially funded, and more capability may be added later while experience is gained and further funding becomes available. Major subsystems of the present baseline VLTI include: three auxiliary telescopes, three delay lines which permit combining the light from up to four telescopes, and a laboratory which contains an imaging beam combiner telescope, and enough space to accomodate a number of experimental setups. This paper presents a general overview of the recent evolution of the project and its future development.
The stringent and specific requirements associated with visible aperture synthesis projects call for a sound engineering effort in the design and development phase to assess the instrumental performance. An important area of effort concerns the influence of the natural or man-made environmental factors on the global performance of the interferometer. This paper discusses the major environmental factors affecting the Very Large Telescope Interferometer (VLTI) and presents the results of a number of studies aimed at evaluating the effects of such environmental factors.
The interferometric mode of the ESO Very Large Telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8-m diameter and by several auxiliary telescopes of the 2-m class. While the position of the 8-m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 distributed on the top of the Observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8-m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferometric mode is the high sensitivity due to the 8-m pupil of the main telescopes which will be compensated by adaptive optics in the near-IR spectral regime. The VLT Interferometer (VLTI) part of the VLT Programme is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is funded, and more capability may be added later while experience is gained and further funding becomes available. Major subsystems of the present baseline VLTI include: three auxiliary telescopes, three delay lines which permit combining the light from up to four telescopes, and a laboratory which contains an imaging beam combiner telescope and enough space to accommodate a number of experimental setups. This paper presents a general overview of the recent evolution of the project and its future development.
We present a simple procedure which estimates aberrations in the exit pupil from point source images taken at the compensated focus. The procedure is based on an iterative technique described by J. Fienup. The measured point spread function of the system produced with a reference source at the input to the adaptive optics system, and the exact, properly scaled shape of the (centrally obscured) exit pupil is all that is needed for the procedure to perform. The results can be obtained quickly, e.g., as part of a daily maintenance procedure. We present some sample cases which were produced with the Come-On Plus adaptive optics system at the ESO 3.6 m telescope on La Silla.
The design of two holosteric configurations have been optimized for a maximum center/edge deflexure of 400 micrometers . Their thickness distribution is given for active zones of 16 mm in diameter. The curvature action is obtained from an air-pressure chamber that generates onto the rear side of the mirrors (1) a uniform pressure up to 9 Atm or (2) a central force up to 11 daN. The control of the curvature is made by an accurate pressure gauge. Some preliminary results are shown as obtained on metal prototype VCMs from a first fabrication cycle as well as an X-ray device for testing the machining validity of the boundaries at the edge of mirrors.
KEYWORDS: Sensors, Detection and tracking algorithms, Mirrors, Servomechanisms, Fourier transforms, Telescopes, Solar telescopes, Active optics, Observatories, Control systems
We present a tracking system that stabilizes atmospheric and instrumental image motion in vacuum tower telescopes. The system is designed to lock on low contrast features, such as solar granulation or other small scale structure. A matrix diode array rapidly scans the scene of interest, usually with a field of 5 arcsec. Images are cross-correlated in real time with a previously recorded reference image of the same area. The drive signal for the image motion corrector, a small, articulated mirror, is generated by measuring the position of the cross correlation maximum. Reference pictures are updated every 30 s in order to adapt to the changing small scale solar features. Performance tests show that the residual image motion in the tracked image is 0.05 arcsec rms compared to a typical 0.5 arcsec rms for the untracked image. The system locks on any small scale structure anywhere on the sun. The bandwidth of the servo system is 40 Hz, or sufficient to stabilize image motion on a meter-class solar telescope.
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