Tip-tilt Correction Research Articles

Adaptive Optics Tip-Tilt Correction Based on Smith Predictor and Filter-Optimized Linear Active Disturbance Rejection Control Method.

A tip-tilt mirror (TTM) control method is designed to enhance the control bandwidth and ensure the rejection performance of the adaptive optics (AO) tip-tilt correction system. Optimized with the Smith predictor and filter, linear active disturbance rejection (LADRC) is adopted to achieve the tip-tilt correction. An AO tip-tilt correction experimental platform was built to validate the method. Experimental results show that the proposed method improves the control bandwidth of the system by at least 3.6 times compared with proportional-integral (PI) control. In addition, under the same control bandwidth condition, compared with the Smith predictor and proportional-integral (PI-Smith) control method, the system is more capable of rejecting internal and external disturbances, and its dynamic response performance is improved by more than 29%.

Sub-pm linewidth, high pulse energy, high beam quality microsecond-pulse Ti:sapphire laser at 766.699 nm.

In this letter, a sub-pm linewidth, high pulse energy and high beam quality microsecond-pulse 766.699 nm Ti:sapphire laser pumped by a frequency-doubled Nd:YAG laser is demonstrated. At an incident pump energy of 824 mJ, the maximum output energy of 132.5 mJ at 766.699 nm with linewidth of 0.66 pm and a pulse width of 100 µs is achieved at a repetition rate of 5 Hz. To the best of our knowledge, this is the highest pulse energy at 766.699 nm with pulse width of hundred micro-seconds for a Ti:sapphire laser. The beam quality factor M2 is measured to be 1.21. It could be precisely tuned from 766.623 to 766.755 nm with a tuning resolution of 0.8 pm. The wavelength stability is measured to be less than ±0.7 pm over 30 min. The sub-pm linewidth, high pulse energy and high beam quality Ti:sapphire laser at 766.699 nm can be used to create a polychromatic laser guide star together with a home-made 589 nm laser in the mesospheric sodium and potassium layer for the tip-tilt correction resulting in the near-diffraction limited imagery on a large telescope.

Enhanced Observer-Based Repetitive Control for Tip-Tilt Disturbance Rejections Beyond Control Bandwidth in Optical-Stabilized Systems

The stabilization and tracking performance of an image-based tip-tilt system is usually deteriorated by tip-tilt vibrations and beam jitter in optical links due to a limited tip-tilt correction bandwidth. This paper proposes an enhanced observer-based repetitive control (OBRC) method to break the bandwidth limitation of the time delay tip-tilt stabilized loop, which achieves high-frequency disturbance rejections beyond control bandwidth even up to Nyquist frequency. The enhanced repetitive controller is fused with the optimized observer based on Youla parameterization, which improves the stability of the closed-loop and reduces the waterbed effect in non-repetitive frequency. Furthermore, the time delay of the closed-loop system could be compensated by the OBRC. Therefore, disturbance suppressions can be transformed into the design of a repetitive controller, which is effective and easy to be used for tip-tilt vibrations and beam jitter suppression. Finally, simulation and experimental results show that the OBRC effectively suppresses the high-frequency disturbance beyond the control bandwidth of the optical-stabilized system.

О проекте евразийских субмиллиметровых телескопов (ESMT) и возможности применения адаптивной оптики для улучшения качества субмм изображений

Actual problems in submillimeter astronomy require the creation of instruments, which combine high sensitivity, angular resolution, wide field of view, and multi-wavelength (multicolor) spectral range. Currently, new single mm/submm telescopes are in great demand in Eurasia, as well as their inclusion in the Event Horizon Telescope (EHT) VLBI network. The concept of the Eurasian Submillimeter Teselscopes (ESMT) project involves the construction of three structurally identical mm/submm radio telescopes of the 15-21 m class on the Suffa Plateau, Uzbekistan (2400 m or higher), in the Russian Federation (3000 m or higher) and in Tibet, China (higher than 5000 m). The ESMT concept is considered, where the design of the European ALMA antennas is taken as a starting point. The antenna diameter should be increased from 12 m to 21 m, the optical layout and the structure of the truss frame will be changed accordingly, tertiary optics and an “active surface” have been added, and tools for its operational diagnostics have been proposed. The article presents variants of the ESMT optical scheme and tertiary optics, which make it possible to hold a compact cryostat with a KID-matrix in a vertical position to ensure its operability when the telescope is tilted. Comparison of ESMT capabilities with other mm/submm tools and projects is provided. The necessary astroclimatic conditions and characteristics of ESMT sites are considered, estimates of the precipitated water PWV, dPWV and the share of total cloudiness TCC for a number of practically interesting sites obtained using the ERA-Interim and ERA-5 reanalysis databases are presented. The influence of atmospheric turbulence on a radio telescope, radio astronomy methods for suppressing atmospheric fluctuations and possibilities of using adaptive optics, in particular, tip-tilt correction for improving the quality of submm images, are considered.

Global tip-tilt and high-order aberration correction with plenoptic wavefront sensors in closed-loop AO systems.

The plenoptic wavefront sensor (PlWFS) has been proposed as being suitable for operating on extended objects in adaptive optics (AO) systems. We propose a new self-referencing method that uses the cell image of PlWFS as a correlation reference and enables the simultaneous measurement of high-order aberrations and tilt. Simulations have been performed for different operating conditions to verify the feasibility of the proposed method. These show that using the cell image results in better AO performance compared to existing reference generation techniques and allows stable wavefront tilt control with a small residual variance.

Kalman Filtering for Tip-tilt Correction in Adaptive Optics

Abstract Correcting the tip and tilt modes of optical aberrations can mitigate a large proportion of distortions in adaptive optics (AO) systems. We describe a method to greatly improve tip/tilt correction for the Keck telescopes, using a Kalman filter with a physics model conditioned by the expected tip/tilt power spectrum and vibration peaks. The model builds on similar implementations, such as that of the Gemini Planet Imager, by considering the effects of loop delays, the response of the control hardware, and systematics specific to Keck. We demonstrate improved correction in simulation, and discuss how this model could be tuned to a specific telescope and otherwise improved.

DNN-based aberration correction in a wavefront sensorless adaptive optics system.

Existing wavefront sensorless (WFS-less) adaptive optics (AO) generally require a search algorithm that takes lots of iterations and measurements to get optimal results. So the latency is a serious problem in the current WFS-less AO system, especially in applications to free-space optics communication. To solve this issue, we propose a deep neural network (DNN)-based aberration correction method. The DNN model can detect the wavefront distortion directly from the intensity images, thereby avoiding time-consuming iterative processes. Since the tip-and-tilt mode of Zernike coefficients are considered, the tip-tilt correction system is not necessarily required in the proposed method. From our simulation results, the proposed method can effectively reduce the computation time and has an impressive improvement of root mean square (RMS) in different turbulence conditions.

High beam quality narrow linewidth microsecond pulse Ti:sapphire laser operating at 819.710 nm

A sub-pm linewidth high beam quality microsecond (μs) Ti:sapphire laser was reported, which was end-pumped by a frequency-doubled 1064 nm Nd:YAG laser. By inserting three birefringent plates tuner and one etalon into the cavity of the Ti:sapphire laser, a ∼ 0.8 pm linewidth at 819.710 nm was achieved with a maximum output power of 3 W operating at a high repetition rate of 500 Hz with pulse width of ∼25 μs. The beam quality factor M2 was measured to be ∼1.14. The wavelength could be finely tuned from 819.699 to 819.718 nm with tuning resolution of 0.76 pm. To the best of our knowledge, this is the first demonstration on the μs pulse Ti:sapphire laser at 819.710 nm with narrow linewidth and high beam quality simultaneously. Such μs 819.710 nm laser could be used to efficiently generate a polychromatic laser guide star together with a 589 nm laser for the tip-tilt correction of ground-based telescopes.

Space-qualified fast steering mirror for an image stabilization system of space astronomical telescopes.

A space-qualified fast steering mirror (SQ-FSM) was designed, built, and tested at the National Astronomical Observatories of the Chinese Academy of Sciences for an image stabilization system of space astronomical telescopes, which is used for the tip-tilt correction of small jitter of the satellite platform; this achieved image stability in a closed-loop manner. Its design primarily faces four challenges involving (1)sustaining the specified sine and random vibration without launch lock, as well as shock response spectrum experiments; (2)surface form error of a clear aperture of ϕ120 mm less than 1/50λ root mean square (RMS, λ=632.8 nm) with a relatively rigid mirror support; (3)resonance frequency of at least 800Hz and as high as possible; (4)minimum reaction force and torque in order to decrease its unfavorable influence on the satellite platform. To achieve these goals, the global optimizations and compromises have to be made throughout the design process. The study reviews the detailed design of the SQ-FSM with respect to the four challenges, mainly by keeping the mirror and its support lightweight, mirror bonding and solidification, actuator and its stiffness, flexure support of the mirror and its holder, material optimization for weight, stiffness, and coefficient of thermal expansion, as well as finite element analysis on statics and dynamics. The performances are also measured and expatiated, including the surface form, resonant frequency, tip-tilt stroke, vibration and shock response spectrum experiments, etc., which validate the performances of the SQ-FSM.

Laser-only Adaptive Optics Achieves Significant Image Quality Gains Compared to Seeing-limited Observations over the Entire Sky

Abstract Adaptive optics laser guide-star systems perform atmospheric correction of stellar wavefronts in two parts: stellar tip-tilt and high-spatial-order laser correction. The requirement of a sufficiently bright guide star in the field-of-view to correct tip-tilt limits sky coverage. In this paper, we show an improvement to effective seeing without the need for nearby bright stars, enabling full sky coverage by performing only laser-assisted wavefront correction. We used Robo-AO, the first robotic AO system, to comprehensively demonstrate this laser-only correction. We analyze observations from four years of efficient robotic operation covering 15000 targets and 42000 observations, each realizing different seeing conditions. Using an autoguider (or a post-processing software equivalent) and the laser to improve effective seeing independent of the brightness of a target, Robo-AO observations show a 39% ± 19% improvement to effective FWHM, without any tip-tilt correction. We also demonstrate that 50% encircled energy performance without tip-tilt correction remains comparable to diffraction-limited, standard Robo-AO performance. Faint-target science programs primarily limited by 50% encircled energy (e.g., those employing integral field spectrographs placed behind the AO system) may see significant benefits to sky coverage from employing laser-only AO.

Random Vibration Analysis of the Tip-tilt System in the GMT Fast Steering Secondary Mirror

A random vibration analysis was accomplished on the tip-tilt system of the fast steering secondary mirror (FSM) for the Giant Magellan Telescope (GMT). As the FSM was to be mounted on the top end of the secondary truss and disturbed by the winds, dynamic effects of the FSM disturbances on the tip-tilt correction performance was studied. The coupled dynamic responses of the FSM segments were evaluated with a suggested tip-tilt correction modeling. Dynamic equations for the tip-tilt system were derived from the force and moment equilibrium on the segment mirror and the geometric compatibility conditions with four design parameters. Statically stationary responses for the tip-tilt actuations to correct the wind-induced disturbances were studied with two design parameters based on the spectral density function of the star image errors in the frequency domain. Frequency response functions and root mean square values of the dynamic responses and the residual star image errors were numerically calculated for the off-axis and on-axis segments of the FSM. A prototype of on-axis segment of the FSM was developed for tip-tilt actuation tests to confirm the ratio of tip-tilt force to tip-tilt angle calculated from the suggested dynamic equations of the tip-tilt system. Tip-tilt actuation tests were executed at 4, 8 and 12 Hz by measuring displacements of piezoelectric actuators and reaction forces acting on the axial supports. The derived ratios of rms tip-tilt force to rms tip-tilt angle from tests showed a good correlation with the numerical results. The suggested process of random vibration analysis on the tip-tilt system to correct the wind-induced disturbances of the FSM segments would be useful to advance the FSM design and upgrade the capability to achieve the least residual star image errors by understanding the details of dynamics.

High-precision identification of a tip–tilt control system for the compensation of time delay

The resolution of ground-based large aperture telescopes is decreased severely due to the effect of atmospheric turbulence. Adaptive optics systems (AOSs) have been widely used to overcome this, and low-order aberrations [tip–tilt (TT)] are corrected by a TT mirror. In the tip/tilt TT correction loop, the time delay affects the correction performance significantly and a predicted signal compensation method (PSCM) has been used to reduce its effect. However, the performance of the PSCM is reduced obviously due to the low identification accuracy of the TT AOS model. In this paper, a nonlinear least squares subspace identification (NLSSI) method is presented to obtain a high-precision model of the TT AOS. The system is identified with the subspace method first, and then the identified parameters are modified in the frequency domain. By using this method, a TT correction system is identified. Compared with the subspace identification method, the identification accuracies of the time domain and frequency response are increased 2 and 5 times, respectively, with the NLSSI method. Furthermore, with the NLSSI method, the −3 dB error rejection bandwidth is increased from 69 to 76 Hz. Finally, an adaptive correction experiment is performed on a 1.23 m telescope, and the astronomical observation results show that the correction accuracy is increased to 1.5 times with the NLSSI method. Moreover, the peak intensity of the image is improved by 11% with the NLSSI method. This work is very helpful to improve the TT correction accuracy of AOS, particularly for extreme adaptive optics and faint target observation.

Impact of turbulence on high-precision ground-satellite frequency transfer with two-way coherent optical links

Bidirectional ground-satellite laser links suffer from turbulence-induced scintillation and phase distortion. We study the impact of turbulence on coherent detection and the related phase noise that restricts time and frequency transfer precision. We evaluate the capacity to obtain a two-way cancellation of atmospheric effects despite the asymmetry between up- and downlink that limits the link reciprocity. For ground-satellite links, the asymmetry is induced by point-ahead angle and possibly the use, for the ground terminal, of different transceiver diameters, in reception and emission. The quantitative analysis is obtained thanks to refined end-to-end simulations under realistic turbulence and wind conditions as well as satellite kinematics. These temporally resolved simulations allow characterizing the coherent detection in terms of time series of heterodyne efficiency and phase noise for different system parameters. We show that tip-tilt correction on ground is mandatory at reception for the downlink and as a pre-compensation of the uplink. Besides, thanks to the large tilt angular correlation, the correction is shown to be efficient on uplink despite the point-ahead angle. Very good two-way compensation of turbulent effects is obtained even with the asymmetries. The two-way differential phase noise is reduced to $1{\mathrm{rad}}^{2}$, with the best fractional frequency stability below $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ after 1-s averaging time.

Impact of WFIRST-AFTA line-of-sight jitter distributions on phase-induced amplitude apodization complex mask coronagraph science yield

A framework for evaluating the science yield of a coronagraph in the presence of a variety of line-of-sight jitter environments is described and the use of a tip-tilt threshold for improving science yield is proposed. The current expectations of the WFIRST-AFTA mission are used for specific distributions of line-of-sight jitter, including the current expectations for tip-tilt correction using a low-order wavefront sensor/control. The effect of the residual tip-tilt on the phase-induced amplitude apodization complex mask coronagraph (PIAACMC) architecture is considered, because the performance of the PIAACMC architecture is expected to be dominated by tip-tilt sensitivity, implying that this treatment has a large impact on the final science yield. The most important outcomes of this study are that the rms residual tip-tilt expected after correction is 0.6 mas rms/axis and that by eliminating some science frames during analysis through a tip-tilt threshold, the number of planets observable increases by ∼25% for the 550-nm imaging channel. The number of known radial velocity planets expected to be observed ranges from 29 to 78 at 550 nm and from 9 to 12 at 890 nm.

Time delay compensation method for tip-tilt control in adaptive optics system.

The time delay engendered by wavefront sampling and data processing inevitability exists in almost all the wavefront sensor (WFS) based adaptive optics (AO) systems. Also, when WFS is used for tip-tilt aberration detection, the time delay significantly reduces the tip-tilt correction performance of the AO system. In this paper, we focus on researching time delay in a tip-tilt (TT) control system and introduce a predicted signal compensation method (PSCM) to compensate the time delay by modifying the WFS detected signals. Based on a precise model of a TT dynamic control system, the detection delay of TT corrections included in a WFS detected signal can be compensated. Experiments are conducted in the lab: the pure integrator (I), proportional and integral (PI) wavefront TT controllers, and these controllers with PSCM are compared to test the efficiency of the PSCM for TT corrections. For the PI controller, the rejection bandwidth increases from 52 to 62Hz by using PCSM; meanwhile, the open-loop phase margin increases from 45 to 60deg. In addition, astronomical observation results are also given based on the PI wavefront TT controller. The PSCM improves the Strehl ratio by a factor of 1.3. The new method is proven to improve the AO system closed-loop performance not only for increasing the closed-loop rejection bandwidth but also in favor of the error attenuation at low frequency. Furthermore, the method does not introduce more noise to the system.

Characterization of a Commercial, Front-Illuminated Interline Transfer CCD Camera for use as a Guide Camera on a Balloon-Borne Telescope

We report results obtained during the characterization of a commercial front-illuminated progressive scan interline transfer CCD camera. We demonstrate that the unmodified camera operates successfully in temperature and pressure conditions (-40°C, 4 mBar) representative of a high altitude balloon mission. We further demonstrate that the centroid of a well-sampled star can be determined to better than 2% of a pixel, even though the CCD is equipped with a microlens array. This device has been selected for use in a closed-loop star-guiding and tip-tilt correction system in the BIT-STABLE balloon mission.

Optimal closed-loop bandwidth of tip-tilt correction loop in adaptive optics system

Based on the most widely used proportional-integrator controller of the tip-tilt correction loop in an adaptive optical system, the relationship between the optimal closed-loop bandwidth and the atmosphere turbulence as well as the sensor noise is analyzed. Numerical simulation is also performed by using the time serial signals that follow the power spectrum density of the global tilt of Kolmogorov turbulence. Results show that with different atmosphere turbulences and sensor noises, the correction capability of the tip-tilt correction loop can be brought into full play by choosing appropriate closed-loop bandwidth and the residual variance can be reduced.

Spartan Infrared Camera, a High-Resolution Imager for the SOAR Telescope: Design, Tests, and On-Telescope Performance

ABSTRACTThe Spartan Infrared Camera provides tip-tilt corrected imaging for the SOAR Telescope in the 900–2500 nm spectral range with four 2048 × 2048 HAWAII-2 detectors. The camera has two plate scales: high-resolution () for future diffraction-limited sampling in the H and K bands and wide-field () to cover a 5′ × 5′ field, over which tip-tilt correction is substantial. The design is described in detail. Except for CaF2 field-flattening lenses, the optics are aluminum mirrors to thermally match the aluminum cryogenic-optical box in which the optics mount. The design minimizes the tilt of the optics as the instrument rotates on the Nasmyth port of the telescope. Two components of the gravitational torque on an optic are eliminated by symmetry, and the third component is minimized by balancing the optic. The optics (including the off-axis aspherical mirrors) were aligned with precise metrology. For the detector assembly, Henein pivots are used to provide frictionless, thermally compliant, lubricant-free, and thermally conducting rotation of the detectors. The heat load is 14 W for an ambient temperature of 10°C. Cooling down takes 40 hr. An activated-charcoal getter controls permeation through the large Viton O-ring for at least nine months. We present maps of the image distortion, which amount to tens of pixels at the greatest. The wavelength of the narrowband filters shift with position in the sky. The measured Strehl ratio of the camera itself is 0.81–0.84 at λ1650 nm. The width of the best K-band image was 260 mas in unexceptional seeing measured after tuning the telescope and before moving the telescope. Since images are normally taken after pointing the telescope to a different field, this supports the idea that the image quality could be improved by better control of the focus and the shape of the primary mirror. The instrument has proved to be capable of producing images that can be stitched together to measure faint, extended features and to produce photometry that agree internally to better than 0.01 mag and are well calibrated to 2MASS stars in the range of 12 < K < 16.

Experimental investigation of segmented adaptive optics for spatial laser array in an atmospheric turbulence condition

We describe a concept of using segmented adaptive optics (AO) to correct the higher-order wavefront aberrations (HOWAs) in an atmospheric turbulence condition. The effects of piston and tip-tilt corrections are analyzed by numerical simulation. An experiment test-bed is built for verification. Seven spatial beamlets are combined into a hexagonal array by use of a pyramid, and an active segmented mirror is used for phase modulation. The AO control is implemented by stochastic parallel gradient descent (SPGD) algorithm. Experimentally, after segmented AO is enabled, the HOWA in warmed air environment has been effectively corrected and the peak intensity of far-field has been enhanced by 5 times.

基于能动分块反射镜的七路激光阵列倾斜校正与相干合成实验研究

基于能动分块反射镜的七路激光阵列倾斜校正与相干合成实验研究