Wide-field Adaptive Optics correction using a single rotating laser guide star

The problem of providing Adaptive Optics (AO) correction over a wide field of view is one that can be alleviated by using multiple conjugate AO (MCAO), or a low-altitude Laser Guide Star (LGS) that is projected to an altitude below any high layer turbulence. A low-altitude LGS can only sense wavefront distortions induced by low-altitude turbulence, which is dominated by a strong boundary layer at the ground. Sensing only the wavefront from this layer provides an AO system with a more spatially invariant performance over the telescope field of view at the expense of overall correction. An alternative method for measuring a ground-layer biased wavefront using a single rotating LGS is presented together with a numerical analysis of the wide-field performance of an AO system utilizing such a LGS. System performance in H and K bands is predicted in terms of system Strehl ratio, which shows that uniform correction can be obtained over fields of view of 200 arcsec in diameter. The simulations also show that the on-axis performance of a LGS utilizing Rayleigh backscattered light will be improved.     

Determination of the profile of atmospheric optical turbulence strength from SLODAR data

Slope Detection and Ranging (SLODAR) is a technique for the measurement of the vertical profile of atmospheric optical turbulence strength. Its main applications are astronomical site characterization and real-time optimization of imaging with adaptive optical correction. The turbulence profile is recovered from the cross-covariance of the slope of the optical phase aberration for a double star source, measured at the telescope with a wavefront sensor (WFS). Here, we determine the theoretical response of a SLODAR system based on a Shack–Hartmann WFS to a thin turbulent layer at a given altitude, and also as a function of the spatial power spectral index of the optical phase aberrations. Recovery of the turbulence profile via fitting of these theoretical response functions is explored. The limiting resolution in altitude of the instrument and the statistical uncertainty of the measured profiles are discussed. We examine the measurement of the total integrated turbulence strength (the seeing) from the WFS data and, by subtraction, the fractional contribution from all turbulence above the maximum altitude for direct sensing of the instrument. We take into account the effects of noise in the measurement of wavefront slopes from centroids and the form of the spatial structure function of the atmospheric optical aberrations.     

High-Spatial-Resolution Imaging Combining High-Order Adaptive Optics,Frame Selection,and Speckle Masking Reconstruction

We present, for the first time, high-spatial-resolution observations combining high-order adaptive optics (AO), frame selection, and post-facto image correction via speckle masking. The data analysis is based on observations of solar active region NOAA 10486 taken with the Dunn Solar Telescope (DST) at the Sacramento Peak Observatory (SPO) of the National Solar Observatory (NSO) on 29 October 2003. The high Strehl ratio encountered in AO corrected short-exposure images provides highly improved signal-to-noise ratios leading to a superior recovery of the object’s Fourier phases. This allows reliable detection of small-scale solar features near the diffraction limit of the telescope. Speckle masking imaging provides access to high-order wavefront aberrations, which predominantly originate at high atmospheric layers and are only partially corrected by the AO system. In addition, the observations provided qualitative measures of the image correction away from the lock point of the AO system. We further present a brief inspection of the underlying imaging theory discussing the limitations and prospects of this multi-faceted image reconstruction approach in terms of the recovery of spatial information, photometric accuracy, and spectroscopic applications.The editors apologize to the authors: due to a misunderstanding during the editorial process, the publication of this paper has been delayed.     

High‐order adaptive optical system for Big Bear Solar Observatory

A high‐order Adaptive Optical (AO) system for the 65 cm vacuum telescope of the Big Bear Solar Observatory (BBSO) is presented. The Coudé‐exit of the telescope has been modified to accommodate the AO system and two imaging magnetograph systems for visible‐light and near infrared (NIR) observations. A small elliptical tip/tilt mirror directs the light into an optical laboratory on the observatory's 2^nd floor just below the observing floor. A deformable mirror (DM) with 77 mm diameter is located on an optical table where it serves two wave‐front sensors (WFS), a correlation tracker (CT) and Shack‐Hartman (SH) sensor for the high‐order AO system, and the scientific channels with the imaging magnetographs. The two‐axis tip/tilt platform has a resonance frequency around 3.3 kHz and tilt range of about 2 mrad, which corresponds to about 25″ in the sky. Based on 32 × 32 pixel images, the CT detects image displacements between a reference frame and real‐time frames at a rate of 2 kHz. High‐order wave‐front aberrations are detected in the SH WFS channel from slope measurements derived from 76 sub‐apertures, which are recorded with 1,280 × 1,024 pixel Complex Metal Oxide Semiconductor (CMOS) camera manufactured by Photobit camera. In the 4 × 4 pixel binning mode, the data acquisition rate of the CMOS device is more than 2 kHz. Both visible‐light and NIR imaging magnetographs use Fabry‐Pérot etalons in telecentric configurations for two‐dimensional spectro‐polarimetry. The optical design of the AO system allows using small aperture prefilters, such as interference or Lyot filters, and 70 mm diameter Fabry‐Pérot etalons covering a field‐of‐view (FOV) of about 180″ × 180″.     

New challenges for adaptive optics: extremely large telescopes

The performance of an adaptive optics (AO) system on a 100-m diameter ground-based telescope working in the visible range of the spectrum is computed using an analytical approach. The target Strehl ratio of 60 per cent is achieved at 0.5 μm with a limiting magnitude of the AO guide source near R   magnitude~10, at the cost of an extremely low sky coverage. To alleviate this problem, the concept of tomographic wavefront sensing in a wider field of view using either natural guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use three or four reference sources and up to three deformable mirrors, which increase up to 8-fold the corrected field size (up to 60 arcsec at 0.5 μm). Operation with multiple NGS is limited to the infrared (in the J band this approach yields a sky coverage of 50 per cent with a Strehl ratio of 0.2). The option of open-loop wavefront correction in the visible using several bright NGS is discussed. The LGS approach involves the use of a faint ( R ~22) NGS for low-order correction, which results in a sky coverage of 40 per cent at the Galactic poles in the visible.     

基于迭代分形法的大气波前仿真

大口径望远镜受大气湍流的影响,光学分辨率远远小于其自身光路所决定的衍射极限。为了相应的自适应光学系统设计,首先有必要对大气波动进行仿真以提供环境数据。通常的大气波前仿真方法需要通过计算结构函数,得到功率谱函数,进而得到仿真波前,但该方法存在计算速度慢,中间变量存储空间大的问题,给大口径望远镜或者长时间仿真带来很大不便。介绍了一种可行的基于迭代分形法的波前仿真方法,复杂度达到O（N）,可以大大提高波前仿真的速度。     

The GREGOR adaptive optics system

The new 1.5‐m German solar telescope GREGOR at the Observatorio del Teide, Tenerife, is equipped with an integrated adaptive optics system. Although partly still in the commissioning phase, the system is already being used used for most science observations. It is designed to provide diffraction‐limited observations in the visible‐light regime for seeing better than 1.2″. We describe the AO system including the optical design, software, wavefront reconstruction, and performance (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical design of the new solar telescope GREGOR

This article describes the considerations which led to the current optical design of the new 1.5 m solar telescope GREGOR. The result is Gregorian design with two real foci in the optical train. The telescope includes a relay optic with a pupil image used by a high order adaptive optics system (AO). The optical design is described in detail and performance characteristics are given. Finally we show some verification results which prove that – without atmospheric effects – the completed telescope reaches a diffraction limited performance (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Atmospheric and adaptive optics

Atmospheric optics is the study of optical effects induced by the atmosphere on light propagating from distant sources. Of particular concern to astronomers is atmospheric turbulence, which limits the performance of ground-based telescopes. The past two decades have seen remarkable growth in the capabilities and performance of adaptive optics (AO) systems. These opto-mechanical systems actively compensate for the blurring effect of the Earth’s turbulent atmosphere. By sensing, and correcting, wavefront distortion introduced by atmospheric index-of-refraction variations, AO systems can produce images with resolution approaching the diffraction limit of the telescope at near-infrared wavelengths. This review highlights the physical processes and fundamental relations of atmospheric optics that are most relevant to astronomy, and discusses the techniques used to characterize atmospheric turbulence. The fundamentals of AO are then introduced and the many types of advanced AO systems that have been developed are described. The principles of each are outlined, and the performance and limitations are examined. Aspects of photometric and astrometric measurements of AO-corrected images are considered. The paper concludes with a discussion of some of the challenges related to current and future AO systems, particularly those that will equip the next generation of large, ground-based optical and infrared telescopes.     

Solar GLAO and TAO:performance comparisons

Multi-conjugate adaptive optics(MCAO),consisting of several deformable mirrors(DMs),can significantly increase the adaptive optics(AO)correction field of view.Current MCAO can be realized by either star-oriented or layer-oriented approaches.For solar AO,ground-layer adaptive optics(GLAO)can be viewed as an extreme case of layer-oriented MCAO in which the DM is conjugated to the ground,while solar tomography adaptive optics(TAO)that we proposed recently can be viewed as star-oriented MCAO with only one DM.Solar GLAO and TAO use the same hardware as conventional solar AO,and therefore it will be important to see which method can deliver better performance.In this article,we compare the performance of solar GLAO and TAO by using end-to-end numerical simulation software.Numerical simulations of TAO and GLAO with different numbers of guide stars are conducted.Our results show that TAO and GLAO produce the same performance if the DM is conjugated to the ground,but TAO can only generate better performance when the DM is conjugated to the best height.This result has important application in existing one-DM solar AO systems.     

A Review of Daytime Atmospheric Optical Turbulence Profile Detection Technology

Atmospheric turbulence has been confirmed as the primary source affecting the quality of ground-based telescope image. To reduce the effect of atmosphere, a good site should be selected, and adaptive optics (AO) should be installed for the telescope. In general, the daytime atmospheric turbulence is more intense than that at night under the effect of solar radiation. Numerous solar telescopes have built AO systems worldwide. Conventional AO is only capable of improving the image quality in a small field of view, whereas it cannot satisfy the needs of a large field of view. The novel wide field adaptive optical system is capable of achieving a large field of view and high-resolution images, whereas the atmospheric turbulence profile should be accurately detected, which is the prerequisite and key parameter of the novel AO system. Moreover, the astronomical high-resolution technology in accordance with the turbulence imaging theory requires more detailed detection of turbulence. Accordingly, a brief review about the latest detection technology of the daytime optical turbulence profile is valuable for astronomical observations. Besides, the parameters of atmospheric turbulence are briefly introduced. Subsequently, SNODAR, SHABAR, MOSP, DIMM+, A-MASP, and other detection technologies of the stratified atmospheric turbulence for daytime are primarily presented, and the advantages and disadvantages of the different technologies are summarized.     

Field-Dependent Adaptive Optics Correction Derived with the Spectral Ratio Technique

In this empirical study, we compare high-resolution observations obtained with the 65-cm vacuum reflector at Big Bear Solar Observatory (BBSO) in 2005 and with the Dunn Solar Telescope (DST) at the National Solar Observatory/Sacramento Peak (NSO/SP) in 2006. We measure the correction of the high-order adaptive optics (AO) systems across the field of view (FOV) using the spectral ratio technique, which is commonly employed in speckle masking imaging, and differential image motion measurements. The AO correction is typically much larger (10^′′ to 25^′′) than the isoplanatic angle and can be described by a radially symmetric function with a central core and extended wings. The full-width at half-maximum (FWHM) of the core represents a measure of the AO correction. The average FWHM values for BBSO and NSO/SP are 23.5^′′ and 18.2^′′, respectively. The extended wings of the function show that the AO systems still contribute to an improved speckle reconstruction at the periphery of the 80^′′×80^′′ FOV. The major differences in the level of AO correction between BBSO and NSO/SP can be explained by different contributions of ground-layer- and free-atmosphere-dominated seeing, as well as different FOVs of the wavefront sensors. In addition, we find an anisotropic spectral ratio in sunspot penumbrae caused by the quasi-one-dimensional nature of penumbral filaments, which introduces a significant error in the estimation of the Fourier amplitudes during the image restoration process.     

Study of the quality of the Zeiss-1000 telescope optical system using the Shack–Hartmann wavefront sensor

The results of studies of the optics of the 1-m Zeiss-1000 telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS) by the Shack–Hartmann (SH) method are presented. Using a Shack–Hartmann wavefront sensor (SH WFS) we have adjusted the telescope optical system by means of shifts and tilts of the secondary mirror. The procedure has significantly reduced the aberrations that appeared during the long-term instrument operation. A new method to investigate the surface quality of the mirrors of the Zeiss-1000 being applied, characteristics close to the diffraction limit are achieved. In general, the entire opto-mechanical telescope system provides an image quality of about 0.5″ at 80%energy level.     

Multi-Conjugate Adaptive Optics with Two Deformable Mirrors – Requirements and Performance

In order to increase the corrected field of view of an adaptive optics (AO) system, several deformable mirrors (DM) have to be placed in the conjugate planes of the dominant turbulent layers (multi-conjugate adaptive optics,MCAO (Beckers, 1988)).The performance of MCAO systems depends on the quality of thewavefront sensing ofthe individual layers and on the number of corrected modes in eachindividual layer as in single layer AO systems. In addition, the increase in corrected field of view depends on the number of guide stars providing information about theturbulence over a sufficiently large area in each turbulent layer. In this article, we investigate these points and provide formulae for calculating the increased field of view with a new approach using the spatial correlation functions of the appliedpolynomials (e.g. Zernike). We also present a new scheme of measuring the individual wavefront distortion of each of the dominantlayers with a Shack-Hartmann-Curvature Sensor using gradientinformation as well as scintillation. An example for the performance of a two layer MCAO system is given for the 3.5-m telescope of the Calar Alto Observatory, Spain, using ameasured C_n ²-profile. The corrected field of view in K-band(2.2 m) can be as large as 3 arcmin with a Strehl ratio above 60%.     

EST Adaptive optics performance estimations

We give a short overview of the Adaptive Optics (AO) and Multi‐conjugate Adaptive Optics (MCAO) system of the planned 4 m European Solar Telescope (EST). The optimization process of the AO / MCAO parameters is shown, including the parameters and layout of the Shack‐Hartmann wavefront sensor setup and the DMs. We show the expected performance of the AO and MCAO system (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Curvature Wavefront Sensor For Solar Adaptive Optics

Active or adaptive optics often require the ability to characterize wavefront aberrations using natural extended sources. The task becomes especially challenging when dealing with widely extended sources such as the solar granulation. We propose a new approach based on the processing of oppositely defocused images. This method, which is a generalization of a technique known as curvature sensing, derives the wavefront curvature from the difference between two oppositely defocused images and determines the second momenta of the point spread function. The proposed method measures the wavefront aberration from the images themselves, requires little computational resources, is fast enough to be used in a real-time adaptive optics system and is particularly adapted to random patterns such as solar granulation or spot penumbras whose morphology evolves during the observation. We envision the application of the method to real-time seeing compensation in solar astronomical telescopes, and to the correction of optical system aberrations in remote sensing instrumentation. This effort is directed towards building a curvature sensor for the real-time applications.     

Estimating stellar atmospheric parameters based on Lasso features

With the rapid development of large scale sky surveys like the Sloan Digital Sky Survey （SDSS）, GAIA and LAMOST （Guoshoujing telescope）, stellar spectra can be obtained on an ever-increasing scale. Therefore, it is necessary to estimate stel- lar atmospheric parameters such as Teff, log g and [Fe/H] automatically to achieve the scientific goals and make full use of the potential value of these observations. Feature selection plays a key role in the automatic measurement of atmospheric parameters. We propose to use the least absolute shrinkage selection operator （Lasso） algorithm to select features from stellar spectra. Feature selection can reduce redundancy in spectra, alleviate the influence of noise, improve calculation speed and enhance the robustness of the estimation system. Based on the extracted features, stellar atmospheric param- eters are estimated by the support vector regression model. Three typical schemes are evaluated on spectral data from both the ELODIE library and SDSS. Experimental results show the potential performance to a certain degree. In addition, results show that our method is stable when applied to different spectra.     

The Wave-Front Correction System for the Sunrise Balloon-Borne Solar Observatory

This paper describes the wave-front correction system developed for the Sunrise balloon telescope, and it provides information about its in-flight performance. For the correction of low-order aberrations, a Correlating Wave-Front Sensor (CWS) was used. It consisted of a six-element Shack??C?Hartmann wave-front sensor (WFS), a fast tip-tilt mirror for the compensation of image motion, and an active telescope secondary mirror for focus correction. The CWS delivered a stabilized image with a precision of 0.04?arcsec (rms), whenever the coarse pointing was better than ???45?arcsec peak-to-peak. The automatic focus adjustment maintained a focus stability of 0.01?waves in the focal plane of the CWS. During the 5.5?day flight, good image quality and stability were achieved during 33?hours, containing 45?sequences, which lasted between 10 and 45?min.     

A quantitative treatment of solar ‘seeing’, II

A previously described microthermal measurement technique is applied to diagnose the causes and locations of sources of harmful ‘seeing’ effects in solar telescopes. Examples of the investigation of a compact 30-cm refractor at Culgoora Observatory and the massive 60-in. McMath reflector at Kitt Peak Observatory are described. The separate contributions from various parts of the telescopes and their immediate environments to the overall modulation transfer function and to the rms wavefront deformation are determined. The effectiveness of ventilation equipment fitted to these telescopes is assessed. In particular, it is shown that rms wavefront deformations ranging from λ/8 to 2λ may originate within the immediate atmospheric environments of these telescopes. Since the locations of these disturbances are determined there is a possibility of effecting improvements for example by mounting a telescope adequately high above the floor of its supporting tower. In the McMath reflector the major contribution to bad ‘seeing’ generally arises in the upper part of the instrument in the vicinity of the heliostat.

     

Infrared Observations from the New Solar Telescope at Big Bear

The 1.6 m clear aperture solar telescope in Big Bear is operational and with its adaptive optics (AO) system it provides diffraction limited solar imaging and polarimetry in the near-infrared (NIR). While the AO system is being upgraded to provide diffraction limited imaging at bluer wavelengths, the instrumentation and observations are concentrated in the NIR. The New Solar Telescope (NST) operates in campaigns, making it the ideal ground-based telescope to provide complementary/supplementary data to SDO and Hinode. The NST makes photometric observations in Hα (656.3 nm) and TiO (705.6 nm) among other lines. As well, the NST collects vector magnetograms in the 1565 nm lines and is beginning such observations in 1083.0 nm. Here we discuss the relevant NST instruments, including AO, and present some results that are germane to NASA solar missions.