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 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.
Proc. SPIE. 9152, Software and Cyberinfrastructure for Astronomy III
KEYWORDS: Human-machine interfaces, Fabry–Perot interferometers, Cameras, Sensors, Control systems, Modulators, Photonic integrated circuits, Optical proximity correction, 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
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 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.
Laser frequency combs (LFC) provide a direct link between the radio frequency (RF) and the optical frequency
regime. The comb-like spectrum of an LFC is formed by exact equidistant laser modes, whose absolute optical
frequencies are controlled by RF-references such as atomic clocks or GPS receivers. While nowadays LFCs
are routinely used in metrological and spectroscopic fields, their application in astronomy was delayed until
recently when systems became available with a mode spacing and wavelength coverage suitable for calibration
of astronomical spectrographs. We developed a LFC based calibration system for the high-resolution echelle
spectrograph at the German Vacuum Tower Telescope (VTT), located at the Teide observatory, Tenerife, Canary
Islands. To characterize the calibration performance of the instrument, we use an all-fiber setup where sunlight
and calibration light are fed to the spectrograph by the same single-mode fiber, eliminating systematic effects
related to variable grating illumination.
EST (European Solar Telescope) is a 4-m class solar telescope, which is currently in the conceptual design phase. EST
will be located at the Canary Islands and aims at observations with the best possible spectral, spatial and temporal
resolution and best polarimetric performance, of the solar photosphere and chromosphere, using a suite of instruments
that can efficiently produce two-dimensional spectropolarimetric information of the thermal, dynamic and magnetic
properties of the plasma over many scale heights, and ranging from λ=350 until 2300 nm.
In order to be able to fulfill the stringent requirements for polarimetric sensitivity and accuracy, from the very beginning
the polarimetry has been included in the design work. The overall philosophy has been to use a combination of
techniques, which includes a telescope with low (and stable) instrumental polarization, optimal full Stokes polarimeters,
differential measurement schemes, fast modulation and demodulation, and accurate calibration.
The current baseline optical layout consists of a 14-mirror layout, which is polarimetrically compensated and nonvarying
in time. In the polarization free F2 focus ample space is reserved for calibration and modulators and a
polarimetric switch. At instrument level the s-, and p-planes of individual components are aligned, resulting in a system
in which eigenvectors can travel undisturbed through the system.
The solar telescope ChroTel is designed as a robotic telescope so that no user interaction is necessary for observation.
The telescope will start tracking in the morning as soon as weather conditions are appropriate and will process a user
defined observation routine until sunset. Weather conditions and system status are continuously monitored to close the
telescope shutter in case of bad weather or to drive to the stow position in case of an error. The ChroTel control software
was programmed in LabVIEW.
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).
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.
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.
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.