A new instrument for solar bi-dimensional spectroscopy, the Interferometric BIdimensional Spectrometer (IBIS), has been successfully installed at the Dunn Solar Telescope of the National Solar Observatory (USA-NM) in June 2003. This instrument is essentially composed of a series of two Fabry-Perot interferometers and a set of narrow-band interference filters, used in a classic mount and in axial-mode. It has been designed to take monochromatic images of the solar surface with high spectral (R ≥ 200 000), spatial ≃ 0.2″), and temporal resolution (several frames s−1). IBIS has a circular field of view, 80″ in diameter and, with suitable interference filters, it can be used in the wavelength range 580 – 860 nm. The wavelength stability of the instrumental profile is very high, the maximum drift in 10 hours amounting to ≃10 m s−1. In this paper the criteria used in the design and the expected instrumental characteristics are described.

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Ibis: a new post-focus instrument for solar imaging spectroscopy

Atomic-level polarization and Zeeman effect diagnostics in the neutral helium triplet at 10830 A in principle allow full vector magnetometry of fine-scaled chromospheric fibrils. We present high-resolution spectropolarimetric observations of superpenumbral fibrils in the He I triplet with sufficient polarimetric sensitivity to infer their full magnetic field geometry. He I observations from the Facility Infrared Spectropolarimeter are paired with high-resolution observations of the Hα 6563 A and Ca II 8542 A spectral lines from the Interferometric Bidimensional Spectrometer from the Dunn Solar Telescope in New Mexico. Linear and circular polarization signatures in the He I triplet are measured and described, as well as analyzed with the advanced inversion capability of the "Hanle and Zeeman Light" modeling code. Our analysis provides direct evidence for the often assumed field alignment of fibril structures. The projected angle of the fibrils and the inferred magnetic field geometry align within an error of ±10°. We describe changes in the inclination angle of these features that reflect their connectivity with the photospheric magnetic field. Evidence for an accelerated flow (~40 m s–2) along an individual fibril anchored at its endpoints in the strong sunspot and weaker plage in part supports the magnetic siphon flow mechanism's role in the inverse Evershed effect. However, the connectivity of the outer endpoint of many of the fibrils cannot be established.

https://iopscience.iop.org/article/10.1088/0004-637X/768/2/111/pdf

He I VECTOR MAGNETOMETRY OF FIELD-ALIGNED SUPERPENUMBRAL FIBRILS

Adaptive optics (AO) has become an indispensable tool at ground-based solar telescopes. AO enables the ground-based observer to overcome the adverse effects of atmospheric seeing and obtain diffraction limited observations. Over the last decade adaptive optics systems have been deployed at major ground-based solar telescopes and revitalized ground-based solar astronomy. The relatively small aperture of solar telescopes and the bright source make solar AO possible for visible wavelengths where the majority of solar observations are still performed. Solar AO systems enable diffraction limited observations of the Sun for a significant fraction of the available observing time at ground-based solar telescopes, which often have a larger aperture than equivalent space based observatories, such as HINODE. New ground breaking scientific results have been achieved with solar adaptive optics and this trend continues. New large aperture telescopes are currently being deployed or are under construction. With the aid of solar AO these telescopes will obtain observations of the highly structured and dynamic solar atmosphere with unprecedented resolution. This paper reviews solar adaptive optics techniques and summarizes the recent progress in the field of solar adaptive optics. An outlook to future solar AO developments, including a discussion of Multi-Conjugate AO (MCAO) and Ground-Layer AO (GLAO) will be given.

/pdf/solar-adaptive-optics-2ifcrjexdi.pdf

Solar Adaptive Optics

This paper presents high-resolution (06) spectropolarimetric observations of very quiet solar regions. We make determinations of the magnetic flux density, intrinsic field strength, and magnetic filling factor in these regions. Our results confirm other previous works in that most of the internetwork flux is in the form of intrinsically strong fields. However, in contrast to other recent observations, we do not find a significant increase in the amount of unsigned magnetic flux relative to that measured at an angular resolution of 1'', nor do we find that most internetwork lanes are populated by strong flux. We discuss possible causes for this disparity. The lack of additional flux with increased resolution may be used to set constrains on the typical spatial scales at which mixed polarities occur (smaller than 06).

Characterization of Magnetic Flux in the Quiet Sun. II. The Internetwork Fields at High Angular Resolution

We study the evolution of the flows and horizontal proper motions in and around a decaying follower sunspot based on time sequences of two-dimensional spectroscopic observations in the visible and white-light imaging data obtained over 6 days from 2005 June 7 to 12. During this time period the sunspot decayed gradually to a pore. The spectroscopic observations were obtained with the Fabry-Perot-based Visible-Light Imaging Magnetograph (VIM) in conjunction with the high-order adaptive optics (AO) system operated at the 65 cm vacuum reflector of the Big Bear Solar Observatory (BBSO). We apply local correlation tracking (LCT) to the speckle-reconstructed time sequences of white-light images around 600 nm to infer horizontal proper motions, while the Doppler shifts of the scanned Fe I line at 630.15 nm are used to calculate line-of-sight (LOS) velocities with subarcsecond resolution. We find that the dividing line between radial inward and outward proper motions in the inner and outer penumbra, respectively, survives the decay phase. In particular the moat flow is still detectable after the penumbra disappeared. Based on our observations, three major processes removed flux from the sunspot: (1) fragmentation of the umbra, (2) flux cancelation of moving magnetic features (MMFs; of the same polarity as the sunspot) that encounter the leading opposite polarity network and plages areas, and (3) flux transport by MMFs (of the same polarity as the sunspot) to the surrounding network and plage regions that have the same polarity as the sunspot.

Flow field evolution of a decaying sunspot

The National Solar Observatory and the New Jersey Institute of Technology have developed two 97 actuator solar adaptive optics (AO) systems based on a correlating Shack-Hartmann wavefront sensor approach. The first engineering run was successfully completed at the Dunn Solar Telescope (DST) at Sacramento Peak, New Mexico in December 2002. The first of two systems is now operational at Sacramento Peak. The second system will be deployed at the Big Bear Solar Observatory by the end of 2003. The correlating Shack-Hartmann wavefront sensor is able to measure wavefront aberrations for low-contrast, extended and time-varying objects, such as solar granulation. The 97-actuator solar AO system operates at a loop update rate of 2.5 kHz and achieves a closed loop bandwidth (0dB crossover error rejection) of about 130 Hz. The AO system is capable of correcting atmospheric seeing at visible wavelengths during median seeing conditions at both the NSO/Sacramento Peak site and the Big Bear Solar Observatory. We present an overview of the system design. The servo loop was successfully closed and first AO corrected images were recorded. We present first results from the new, high order AO system.

First results from the NSO/NJIT solar adaptive optics system

The planned 4-meter diameter of the European Solar Telescope (EST) is aimed at providing high spatial resolution and large photon collecting area, in order to understand in particular the mechanisms of magnetic coupling in the chromosphere and the photosphere. To reach its goals in the visible and the near-infrared, EST is designed with both a conventional and a multi-conjugate adaptive optics (AO) of similar complexity than the ones featured for the Extremely Large Telescopes. In addition, the AO on EST has to face a particularity of solar AO: the wavefront sensing on extended sources with measurement fields of about 10” in size. Reviewing recent literature together with an independent analysis, we investigate the impact of extended-field sensing in AO for large solar telescopes. Sensing modeling and its effect on reconstruction performance are analyzed, thanks to simulations performed with the Fractal Iterative Method for tomography (FRiM-3D), showing the difficulty to correct high altitude turbulence. We introduce a new approximate direct model of extended-source sensing which greatly improves the quality of the end-to-end simulations for EST AO. Next, we try to improve the conventional solar AO correction by using this new model in the reconstruction. Our simulations do not show significant benefits from using such tomographic model in this conventional AO configuration and under typical atmospheric conditions. 1 Context: the EST adaptive optics The European Solar Telescope (EST) [1] does not fit in the range of 20 meter-class telescopes, and so is not properly considered as an Extremely Large Telescope (ELT). In fact, it is designed to have a 4 meter-diameter primary mirror, like the Advanced Technology Solar Telescope (ATST) [2] currently under construction. Nevertheless, the solar physics requirements of high Strehl at visible wavelengths and over a wide field-of-view (FOV) with these telescopes lead to the design of adaptive optics (AO) of similar complexity [3] to the ones planned for the ELTs. The construction of EST is foreseen to start in 2019, in order for the telescope to see its first light in 2025. It is not the scope of this paper to detail neither the high-resolution requirements for EST nor its AO systems, but the main ambitious goals are summarized here [4]. It is expected to have correction abilities over a wide range of elevations down to 15 degrees, as a specific need for solar observations. It is required to provide a corrected FOV of 1 arcmin in diameter with a homogeneous Strehl ratio at 500 nm greater than 30% for r0 = 7 cm, 50% for r0 = 14 cm and a CBechetP@ing.puc.cl Third AO4ELT Conference Adaptive Optics for Extremely Large Telescopes Florence, Italy. May 2013 ISBN: 978-88-908876-0-4 DOI: 10.12839/AO4ELT3.13377

Wide-field wavefront sensing in solar adaptive optics : modeling and effects on reconstruction

Multiconjugate adaptive optics has been proposed in order to extend the size of the corrected field of view with respect to the classical AO field of view. In order to achieve this, a three dimensional measurement of the turbulent volume is needed to collect the information to command the several deformable mirrors. This can be done by using tomography, in which several WFS are used, each of them coupled to a sky region. Here we report the experimental demonstration of such evaluation for solar observations. In addition, we confront these results on turbulence distribution with a study of AO corrected images by using multi point large field of view wavefront sensing with the new Dunn Solar Telescope adaptive optics system. This yields to information on the AO system performances and provide useful estimate of the PSF variation across the field. The results from this article provides an important step forward for building a full solar multi-conjugate adaptive optics system for the Dunn Solar Telescope and in a longer term for the future 4 meter ATST telescope.

Three-dimensional high-order wavefront sensing, anisoplanatism, and evaluation of the feasibility of solar MCAO

Concept for Solar Multi-Conjugate Adaptive Optics at Big Bear Observatory

An adaptive optics system with a single deformable mirror is being implemented on the THEMIS 90cm solar telescope. This system is designed to operate in the visible and is required to be as robust as possible in order to deliver the best possible correction in any atmospheric conditions, even if wavefronts are sensed on some low-contrast solar granulation. In extreme conditions, the images given by the subapertures of the Shack-Hartmann wavefront sensor get randomly blurred in space, in the set of subapertures, and the distribution of blurred images is rapidly changing in time, some of them possibly fading away. The algorithms we have developed for such harsh conditions rely on inverse problem approach. As an example, with the gradients of the wavefronts, the wavefront sensor also estimates their errors, including their covariance. This information allows the control loop to promptly optimize itself to the fast varying conditions, both in space (wavefront reconstruction) and in time. A major constraint is to fit the calculations in a low-cost multi-core CPU. An overview of the algorithms in charge of implementing this strategy is presented, focusing on wavefront sensing.

https://hal.archives-ouvertes.fr/hal-02414753/document

Maud Langlois

Papers

First results from the NSO/NJIT solar adaptive optics system

Wide-field wavefront sensing in solar adaptive optics : modeling and effects on reconstruction

Three-dimensional high-order wavefront sensing, anisoplanatism, and evaluation of the feasibility of solar MCAO

Concept for Solar Multi-Conjugate Adaptive Optics at Big Bear Observatory

The wavefront sensing making-of for THEMIS solar telescope