Given its unchallenged capabilities in terms of sensitivity and spatial resolution, the combination of imaging spectropolarimetry and numeric Stokes inversion represents the dominant technique currently used to remotely sense the physical properties of the solar atmosphere and, in particular, its important driving magnetic field. Solar magnetism manifests itself in a wide range of spatial, temporal, and energetic scales. The ubiquitous but relatively small and weak fields of the so-called quiet Sun are believed today to be crucial for answering many open questions in solar physics, some of which have substantial practical relevance due to the strong Sun–Earth connection. However, such fields are very challenging to detect because they require spectropolarimetric measurements with high spatial (sub-arcsec), spectral (<100 mÅ), and temporal (<10 s) resolution along with high polarimetric sensitivity (<0.1 % of the intensity). We collect and discuss both well-established and upcoming instrumental solutions developed during the last decades to push solar observations toward the above-mentioned parameter regime. This typically involves design trade-offs due to the high dimensionality of the data and signal-to-noise-ratio considerations, among others. We focus on the main three components that form a spectropolarimeter, namely, wavelength discriminators, the devices employed to encode the incoming polarization state into intensity images (polarization modulators), and the sensor technologies used to register them. We consider the instrumental solutions introduced to perform this kind of measurements at different optical wavelengths and from various observing locations, i.e., ground-based, from the stratosphere or near space.
Advanced Astronomy for Heliophysics Plus (ADAHELI+) is a project concept for a small solar and space weather mission with a budget compatible with an European Space Agency (ESA) S-class mission, including launch, and a fast development cycle. ADAHELI+ was submitted to the European Space Agency by a European-wide consortium of solar physics research institutes in response to the “Call for a small mission opportunity for a launch in 2017,” of March 9, 2012. The ADAHELI+ project builds on the heritage of the former ADAHELI mission, which had successfully completed its phase-A study under the Italian Space Agency 2007 Small Mission Programme, thus proving the soundness and feasibility of its innovative low-budget design. ADAHELI+ is a solar space mission with two main instruments: ISODY+: an imager, based on Fabry–Pérot interferometers, whose design is optimized to the acquisition of highest cadence, long-duration, multiline spectropolarimetric images in the visible/near-infrared region of the solar spectrum. XSPO: an x-ray polarimeter for solar flares in x-rays with energies in the 15 to 35 keV range. ADAHELI+ is capable of performing observations that cannot be addressed by other currently planned solar space missions, due to their limited telemetry, or by ground-based facilities, due to the problematic effect of the terrestrial atmosphere.
This communication presents a family of spectrographs designed for the European Solar Telescope. They can operate in
four different configurations: a long slit standard spectrograph (LsSS), two devices based on subtractive double pass
(TUNIS and MSDP) and one based on an integral field, multi-slit, multi-wavelength configuration. The combination of
them composes the multi-purpose grating spectrograph of EST, focused on supporting the different science cases of the
solar photosphere and chromosphere in the spectral range from 3900 Å to 23000 Å. The different alternatives are made
compatible by using the same base spectrographs and different selectable optical elements corresponding to specific
subsystems of each configuration.
In the context of the conceptual design study for the European Solar Telescope (EST) we have investigated
different metallic mirror coatings in terms of reflectivity, polarization properties and durability. Samples of the
following coating types have been studied: bare aluminum, silver with different dielectric layers for protection
and UV enhancement, and an aluminum-silver combination. From 2009 to 2011 we have carried out a long-term
durability test under realistic observing conditions at the VTT solar telescope of the Observatorio del
Teide (Tenerife, Spain), accompanied by repeated reflectivity measurements in the EST spectral working range
(0.3 - 20 μm), and by polarization measurements in the visible range. The test results allow us to find the
optimum coatings for the different mirrors in the EST beampath and to eventually assess aging effects and
re-coating cycles. The results of the polarization measurements are a valuable input for an EST telescope
polarization model, helping to meet the stringent requirements on polarimetric accuracy.
High-precision full-Stokes polarimetry at near diffraction limited spatial resolution is important to understand
numerous physical processes on the Sun. In view of the next generation of ground based solar telescopes, we have
explored, through numerical simulation, how polarimetric accuracy is affected by atmospheric seeing, especially
in the case of large aperture telescopes with increasing ratio between mirror diameter and Fried parameter. In this
work we focus on higher-order wavefront aberrations. The numerical generation of time-dependent turbulence
phase screens is based on the well-known power spectral method and on the assumption that the temporal
evolution is mainly caused by wind driven propagation of frozen-in turbulence across the telescope. To analyze
the seeing induced cross-talk between the Stokes parameters we consider polarization modulation scheme based
on a continuously rotating waveplate with rotation frequencies between 1 Hz and several 100 Hz.
Further, we have started the development of a new fast solar imaging polarimeter, based on pnCCD detector
technology from PNSensor. The first detector will have a size of 264 x 264 pixels and will work at frame rates of
up to 1kHz, combined with a very low readout noise of 2-3 e- ENC. The camera readout electronics will allow for
buffering and accumulation of images corresponding to the different phases of the fast polarization modulation.
A high write-out rate (about 30 to 50 frames/s) will allow for post-facto image reconstruction. We will present
the concept and the expected performance of the new polarimeter, based on the above-mentioned simulations of
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.
EST is a project for a 4-meter class telescope to be located in the Canary Islands. EST will be optimized for studies of
the magnetic coupling between the photosphere and the chromosphere. This requires high spatial and temporal resolution
diagnostics tools of properties of the plasma, by using multiple wavelength spectropolarimetry. To achieve these goals,
visible and near-IR multi-purpose spectrographs are being designed to be compatible with different modes of use: LsSS
(Long-slit Standard Spectrograph), multi-slit multi-wavelength spectrograph with an integral field unit, TUNIS (Tunable
Universal Narrow-band Imaging Spectrograph), and new generation MSDP (Multi-channel Subtractive Double-pass
Spectrograph). In this contribution, these different instrumental configurations are described.
Imaging spectroscopy of the Sun is a challenging task usually performed with Fabry-Perot etalons. The common setup is a combination of two or three etalons in series and a narrow-band prefilter. The requirement of one, usually expensive prefilter for every desired wavelength limits the number of spectral regions that can be observed.
We present a novel instrument combination consisting of two Fabry-Perot etalons and a grating spectrograph, which allows for observations in any wavelength between 390 nm and 660 nm without the need for narrow-band prefilters. Furthermore, two or more adjacent monochromatic images are projected on the detector, each image corresponding to a different spectral transmission peak of the Fabry-Perot filtergraph. Together with our Zurich Imaging Polarimeter (ZIMPOL) the system is installed at the telescope of the Istituto Ricerche Solari Locarno (IRSOL) where it will be used for two-dimensional spectropolarimetry. We present a description of the instrument and test observations.
The design of an achromatic polarisation modulator is presented. The modulator is based on a combination of three electrically switchable non-achromatic ferroelectric liquid crystal retarders. The design follows the idea by Pancharatnam who first introduced suitable achromatic combinations of crystal retarders. We combined three ferroelectric liquid crystal retarders to create an electrically switchable achromatic halfwave plate which can be used in the
spectral range from 400 nm to 750 nm. Different designs are theoretically modeled and compared under the aspects of their individual response to temperature fluctuations and useful wavelength range. First results of laboratory tests are presented to experimentally evaluate the feasibility of the concept.