This paper demonstrates the Geometric Wavefront Sensor (WFS)’s capability to estimate wavefronts of multiple sources on a new optical test-bench in open-loop. The linearity response of the Geometric WFS to individual Zernike modes is measured for each source, and converted into calibration gain factors to generate improved estimations of the wavefront phase aberrations. A novel technique to simulate atmospheric turbulence in the laboratory is explored, and is found to suitably create multiple atmospheric layers artificially. This technique permits for wavefront phase aberrations of multiple sources to be simulated simultaneously with varying degrees of overlapping, i.e. changing the height of the atmospheric turbulence layer, without altering the physical optical path. Finally, atmospheric tomography is demonstrated using the novel technique and calibrated Geometric WFS.
Atmospheric turbulence is the major reason for blurring of the images from ground-based telescopes. In this paper, a model-based wavefront sensorless approach to remove the distortion from the images is proposed. A radial surrogate model is used to simplify the optimisation of an intensity-based objective function. To reduce the number of intensity evaluations required to build the model, the statistics of the atmosphere are extracted and explored. The number of evaluations can be further decreased by employing space-filling sampling and sub-sampling plans. The accuracy of the proposed approach is improved by optimising the associated hyperparameters. This method is shown to efficiently remove the first 8 aberration modes after tip and tilt within the maximum number of evaluations allowed. Results indicate that applying the proposed method to a random atmosphere phase screen can improve the fractional encircled energy by about 20%.
The GPU-based High-order adaptive OpticS Testbench (GHOST) at the European Southern Observatory (ESO) is a new 2-stage extreme adaptive optics (XAO) testbench at ESO. The GHOST is designed to investigate and evaluate new control methods (machine learning, predictive control) for XAO which will be required for instruments such as the Planetary Camera and Spectrograph of ESOs Extremely Large Telescope. The first stage corrections are performed in simulation, with the residual wavefront error at each iteration saved. The residual wavefront errors from the first stage are then injected into the GHOST using a spatial light modulator. The second stage correction is made with a Boston Michromachines Corporation 492 actuator deformable mirror and a pyramid wavefront sensor. The flexibility of the bench also opens it up to other applications, one such application is investigating the flip-flop modulation method for the pyramid wavefront sensor.
The next generation of ground-based optical telescopes, such as the European Southern Observatory’s Extremely Large Telescope (the ELT), will have large support structures (spiders) for the secondary mirror. These spiders have the effect of segmenting the pupil. Without careful control of the wavefront, segment piston (petal modes) errors can develop. We present a flip-flop modulated/unmodulated method for the pyramid wavefront sensor (PWFS) enabling the PWFS to sense petal piston modes. This flip-flop modulation method uses a single PWFS operating in two states: a modulated state and an unmodulated state. An independent controller is used in each state; the modulated state controls the atmospheric turbulence and the unmodulated state only controls petal piston modes. In simulation, we show the flip-flop method working with the wavefront sensor in both K- and R-bands, providing an improvement of 9.9% and 13%, respectively, over a standard modulated PWFS.
This paper investigates the performance and optimization of the Geometric Wavefront Sensor (GWFS) in openloop wavefront sensing, along with the Curvature Wavefront Sensor (CWFS) and Shack-Hartmann Wavefront Sensor (SH-WFS). The GWFS uses a ray tracing process to calculate the displacement of intensity fluctuations from two defocused point source images. Various parameters within the GWFS – such as the signal-to-noise ratio (SNR) sensitivity, the number of Radon angles, the virtual propagation distance, and the number of reconstruction modes – are explored on a laboratory test bench. We found that the GWFS wavefront estimate error experiences an inverse relationship to the SNR, a minimum of 5 Radon angles is required to accurately estimate the single Zernike mode wavefronts (Z4 – Z15), the virtual propagation distance is confined by ray crossing and Fresnel diffraction effects, and the number of reconstruction Zernike modes is limited by noise amplification and over-fitting. This paper demonstrates the capabilities of the GWFS, illustrates the resulting wavefront estimates, and confirms the superior performance of the GWFS compared to the CWFS. The optimized GWFS will be utilized at Mt. John University Observatory (MJUO) in New Zealand for satellite and space debris imaging and tracking
The secondary mirror and associated support structure (spider) will obscure entire rows and columns of sub- apertures of the wavefront sensors (WFS) on the extremely large telescopes. Consequently, piston discontinuities between the segments created by the spider can arise in the estimated wavefront. By optimising the amount of regularisation, the number of modes corrected and the illumination threshold for active subaperture selection, we achieve a closed-loop Strehl of 0.9467 with a spider versus 0.9681 for a case with no spider in K band. The European Southern Observatory’s end-to-end Monte Carlo simulator, OCTOPUS, is used for the numerical evaluation. We also investigate the orientation of the spider relative to the prism and find that when the spider arms are aligned with the edges of the WFS prism, the closed-loop Strehl is at its highest. In our test case of a 2-fold spider and an unmodulated pyramid WFS, we found the difference between best and worst case Strehl to be approximately 18%. We found that an illumination threshold of 20-50% is optimum when a spider arm is paraxial to the wavefront sensor. We also consider the effects of the spider on the reconstruction algorithm, using maximum a posteriori (MAP) reconstructors, one using wavefront slopes and another using the conjugate aperture plane images directly. At high flux both MAP reconstructors perform similarly.
The predominant effect of the atmosphere on the incoming wavefront of an astronomical object is the introduction of phase distortion, resulting in an aberrated image from ground-based telescopes. Since wavefront perturbations cannot be directly measured from an image, a wavefront sensor can use intensity variations from a point source to measure specific wavefront aberrations. However, processing of measured aberration data from these sensors can be computationally intensive and this is a challenge for real-time image restoration. To accurately represent such wavefront aberrations with improved processing time, we analyse how the ridgelet transform can be used with the slope-based wavefront sensor i.e., geometric wavefront sensor, in an open-loop configuration. Ridgelet analysis is performed in the Radon domain, where each Radon line integral is computed over N angles, and is represented by a wavelet. Contrasting the behaviour of the ridgelet transform to generate Zernike polynomials with the geometric wavefront sensor, which uses the properties of geometric optics, is the main aim of this paper. We first decompose the image into a Radon domain, and then analyse each line integeral of a Radon transform by a wavelet transform. We show that multi-resolution geometric analysis with ridgelets results in lower wavefront errors, particularly for low photon counts, and computational efficiency of the geometric wavefront sensor is improved by almost a factor of 2.
We present in this paper an analysis of several tip-tilt on-sky data registered on adaptive optics systems installed on different telescopes (Gemini South, William Herschel Telescope, Large Binocular Telescope, Very Large Tele scope, Subaru). Vibration peaks can be detected, and it is shown that their presence and location may vary, and that their origin is not always easy to determine. Mechanical solution that have been realized to mitigate vibrations are presented. Nevertheless, residual vibrations may still affect the instruments' performance, ranging from narrow high frequency vibration peaks to wide low frequency windshake-type perturbations. Power Spectral Densities (PSDs) of on-sky data are presented to evidence these features. When possible, indications are given regarding the gain in performance that could be achieved with adequate controllers accounting for vibration mitigation. Two examples of controller identification and design illustrate their ability to compensate for various types of disturbances (turbulence, windshake, vibration peaks, ...),showing a significant gain in performance.
In modern adaptive optics systems, lightly damped sinusoidal oscillations resulting from telescope structural vibrations have a significant deleterious impact on the quality of the image collected at the detector plane. Such oscillations are often at frequencies beyond the bandwidth of the wave-front controller that therefore is either incapable of rejecting them or might even amplify their detrimental impact on the overall AO performance. A technique for the rejection of periodic disturbances acting at the output of unknown plants, which has been recently presented in literature, has been adapted to the problem of rejecting vibrations in AO loops. The proposed methodology aims at estimating phase and amplitude of the harmonic disturbance together with the response of the unknown plant at the frequency of vibration. On the basis of such estimates, a control signal is generated to cancel out the periodic perturbation. Additionally, the algorithm can be easily extended to cope with unexpected time variations of the vibrations frequency by adding a frequency tracking module based either on a simple PLL architecture or on a classical extended Kalman filter. Oversampling can be also easily introduced to efficiently correct for vibrations approaching the sampling frequency. The approach presented in this contribution is compared against a different algorithm for vibration rejection available in literature, in order to identify drawbacks and advantages. Finally, the performance of the proposed vibration cancellation technique has been tested in realistic scenarios defined exploiting tip/tilt measurements from MACAO and NACO
In this paper, we present simulation work done on AO systems for the E-ELT. We study the influence of the number of
Laser Guide Stars (LGS) on system performance. Then, we investigate the impact of the conjugation height of the M4
adaptive mirror on GL/LT/MC-AO. Finally, we compare the results of a Fourier code and end-to-end models on the
position of the LGS in the field of view.
To predict the performance of the E-ELT three sets of toolkits are developed at ESO: i) The main structure and associated optical unit dynamical and feedback control toolkit, ii) Active optics and phasing toolkit, and iii) adaptive optics simulation toolkit. There was a deliberate policy not to integrate all of the systems into a massive model and tool. The dynamical and control time scale differences are used to separate the simulation environments and tools. Therefore, each toolkit contains an appropriate detail of the problem and holds sufficient overlap with the others to ensure the consistency of the results. In this paper, these toolkits together with some examples are presented.
Variations of the sodium layer altitude and atom density profile induce errors on laser-guide-star (LGS) adaptive
optics systems. These errors must be mitigated by (i), optimizing the LGS wavefront sensor (WFS) and the
centroiding algorithm, and (ii), by adding a high-pass filter on the LGS path and a low-bandwidth
natural-guide-star WFS. In the context of the ESO E-ELT project, five centroiding algorithms, namely the centre-of-gravity
(CoG), the weighted CoG, the matched filter, the quad-cell and the correlation, have been evaluated in closedloop
on the University of Victoria LGS wavefront sensing test bed. Each centroiding algorithm performance is
compared for a central versus side-launch laser, different fields of view, pixel sampling, and LGS flux.
Simulations of adaptive optics (AO) for the European extremely large telescope (EELT) are presented. For Shack-Hartmann wavefront sensors for the laser guide star (LGS) based systems, the simulations show that without the Rayleigh fratricide effect, central projection of the laser is preferable to side projection, the correlation or
matched filter centroiding algorithms offer superior performance to a traditional center-of-gravity approach, the optimum sampling of the detector is approximately 1.5 pixels per FWHM of the non-elongated spot, and that at least 10×10 pixels are required. The required number of photo-detection events from the LGS per frame per
subaperture is of the order of 1000. Correction of segmentation errors with a Shack-Hartmann wavefront sensor
(WFS) has also been investigated; atmospheric turbulence dominates these segmentation errors. The pyramid
WFS is also simulated for the EELT, showing that modulation of the pyramid will be necessary.
The near-Infrared Multi-Object Spectrograph (IRMOS) for TMT is one of the most powerful astronomical instruments ever envisioned. The combination of the collecting area of TMT, the unique image-sharpening capabilities of the Multi-Object Adaptive Optics (MOAO) system, and the multiplexing advantage of the multi-object integral-field spectra provided by the IRMOS back-end make it capable of addressing some of the leading scientific challenges of the coming decades. Here we present an overview of one potential IRMOS concept and then focus on the MOAO system. In particular we will describe our concept for the laser and natural guide star wavefront sensors, deformable mirrors and the calibration system of MOAO. For each of these design elements, we describe the key trade studies which help define each subsystem. From results of our studies, we assemble a MOAO ensquared energy budget. We find that 50% of the energy is ensquared within the 50 milli-arcsecond spatial pixel of the IRMOS integral field units for a wavelength of 1.65μm. Given the requirements placed on the MOAO system to achieve this performance, large ensquared energies can be achieved with even finer plate scales for wavelengths longer than 1.5μm.
A Monte Carlo sky coverage model for laser guide star adaptive optics systems is presented. This model provides
fast Monte Carlo simulations of the tip/tilt (TT) wavefront error calculated with minimum variance estimators
over natural guide star constellations generated from star models. With this simulation code we are able to
generate a TT error budget for the Thirty Metre Telescope (TMT) facility Narrow Field Infra-Red Adaptive
Optics System (NFIRAOS), and perform several design trade studies. With the current NFIRAOS design, the
median TT error at the galactic pole with median seeing is calculated to be 65 nm or 1.8 mas.
Laser guide star (LGS) adaptive optics systems for extremely large telescopes must handle an important effect that is negligible for current generation telescopes. Wavefront errors, due to improperly focusing laser wavefront sensors (WFS) on the mesospheric sodium layer, are proportional to the square of the telescope diameter. The sodium layer, whose mean altitude is approximately 90 km, can move vertically at rates of up to a few metres per second; a few seconds lag in refocusing can substantially degrade delivered image quality (15 m of defocus can cause 120 nm residual wavefront error on a 30-m telescope.) As well, the range of temporal frequencies of sodium altitude focus, overlaps the temporal frequencies of focus caused by atmospheric turbulence. Only natural star wavefront sensors can disentangle this degeneracy. However, applying corrections with representative focus mechanisms having modest control bandwidths causes appreciable tracking errors. In principle, electronic offsets measured by natural guide star detectors could be rapidly applied to laser WFS measurements, but to provide useable sky coverage, integrating sufficient photons causes an unavoidable time delay, again resulting in potentially serious focus tracking errors. However, our analysis depends on extrapolating to temporal frequencies greater than 1 Hz from power spectra of sodium profile time series taken at 1-2 minute intervals. In principle, with a pulsed laser, (e.g. 3-μs pulses) and dynamic refocusing on a polar-coordinate CCD, this focus tracking error may be eliminated. This result is an additional benefit of dynamic refocusing beyond the commonly recognized amelioration of LGS WFS spot elongation.
We present a design of a thermal-infrared optimized adaptive optics system for the TMT 30-meter telescope. The
approach makes use of an adaptive secondary but during an initial implementation contains a more conventional
ambient-temperature optical relay and deformable mirror. The conventional optical relay is used without sacrificing the
thermal background by using multiple off-axis laser guide stars to avoid a warm dichroic in the common path. Three
laser guide stars, equally spaced 75" off axis, and a "conventional" 30×30 deformable mirror provide a Strehl > 0.9 at
wavelengths longer than 10 microns and the LGS beams can be passed to the LGS wavefront sensors with pickoff
mirrors while a one-arcminute field is passed unvignetted to the science instrument and NGS WFSs. The overall design
is relatively simple with a wavefront correction similar to existing high-order systems (e.g. 30×30) but still provides
competitive performance over the higher-order TMT NIR AO design at wavelengths as short as 3 microns due to its
reduced thermal emissivity. We present our figures of merit and design considerations within the context of the science
drivers for high-spectral resolution NIR/MIR spectroscopy at 5-28 microns on a 30-meter ground-based telescope.
In this paper, we provide an overview of the adaptive optics (AO) program for the Thirty Meter Telescope (TMT) project, including an update on requirements; the philosophical approach to developing an overall AO system architecture; the recently completed conceptual designs for facility and instrument AO systems; anticipated first light capabilities and upgrade options; and the hardware, software, and controls interfaces with the remainder of the observatory. Supporting work in AO component development, lab and field tests, and simulation and analysis is also discussed. Further detail on all of these subjects may be found in additional papers in this conference.
Although many of the instruments planned for the TMT (Thirty Meter Telescope) have their own closely-coupled adaptive
optics systems, TMT will also have a facility Adaptive Optics (AO) system, NFIRAOS, feeding three instruments
on the Nasmyth platform. This Narrow-Field Infrared Adaptive Optics System, employs conventional deformable mirrors
with large diameters of about 300 mm. The requirements for NFIRAOS include 1.0-2.5 microns wavelength range,
30 arcsecond diameter science field of view (FOV), excellent sky coverage, and diffraction-limited atmospheric turbulence
compensation (specified at 133 nm RMS including residual telescope and science instrument errors.) The reference
design for NFIRAOS includes six sodium laser guide stars over a 70 arcsecond FOV, and multiple infrared tip/tilt sensors
and a natural guide star focus sensor within instruments. Larger telescopes require greater deformable mirror (DM)
stroke. Although initially NFIRAOS will correct a 10 arcsecond science field, it uses two deformable mirrors in series,
partly to provide sufficient stroke for atmospheric correction over the 30 m telescope aperture, but mainly to improve
sky coverage by sharpening near-IR natural guide stars over a 2 arcminute diameter "technical" field. The planned upgrade
to full performance includes replacing the ground-conjugated DM with a higher actuator density, and using a deformable
telescope secondary mirror as a "woofer." NFIRAOS feeds three live instruments: a near-Infrared integral field
Imaging spectrograph, a near-infrared echelle spectrograph, and after upgrading NFIRAOS to full multi-conjugation, a
wide field (30 arcsecond) infrared camera.
Although many of the instruments planned for the TMT (Thirty Meter Telescope) have their own closely-coupled adaptive optics systems, TMT will also have a facility Adaptive Optics (AO) system feeding three instruments on the Nasmyth platform. For this Narrow-Field Infrared Adaptive Optics System, NFIRAOS (pronounced nefarious), the TMT project considered two architectures. One, described in this paper, employs conventional deformable mirrors with large diameters of about 300 mm and this is the reference design adopted by the TMT project. An alternative design based on MEMS was also studied, and is being presented separately in this conference. The requirements for NFIRAOS include 0.8-5 microns wavelength range, 30 arcsecond diameter output field of view (FOV), excellent sky coverage, and diffraction-
limited atmospheric turbulence compensation (specified at 133 nm RMS including residual telescope and science instrument errors.) The reference design for NFIRAOS includes multiple sodium laser guide stars over a 70 arcsecond FOV, and an infrared tip/tilt/focus/astigmatism natural guide star sensor within instruments. Larger telescopes require greater deformable mirror (DM) stroke. Although initially NFIRAOS will correct a 10 arcsecond science field, it uses two deformable mirrors in series, partly to provide sufficient stroke for atmospheric correction over the 30 m telescope aperture, but mainly to partially correct a 2 arcminute diameter "technical" field to sharpen near-IR natural guide stars and improve sky coverage. The planned upgrade to full performance includes replacing the groundconjugated DM with a higher actuator density, and using a deformable telescope secondary mirror as a "woofer." NFIRAOS incorporates an instrument rotator and selection of three live instruments: a near-Infrared integral field Imaging
spectrograph, a near-infrared echelle spectrograph, and after upgrading NFIRAOS to full multi-conjugation, a wide field (30 arcsecond) infrared camera.
The Shack-Hartmann wavefront sensor operates by subdividing the complex field in the aperture plane of the telescope with a lenslet array and forming low resolution images of the object. An alternative wavefront sensing scheme can be derived from placing a lenslet array at the focal plane of the aperture and forming low resolution images of the aperture. This arrangement can be viewed as the generalisation of the pyramid sensor and enables direct comparisons of the pyramid sensor with the Shack-Hartmann sensor. In particular, in this paper the performance of the reconstructor of the two sensors is investigated. Simulation results demonstrate that the lenslet array at the focal plane has equivalent performance to the Shack-Hartmann sensor in open loop when no modulation is applied to the lenslet array. However, when the array is modulated in a manner akin to the pyramid sensor, subdivision at the focal plane provides a significantly better wavefront estimate than the Shack-Hartmann sensor.
The pyramid wavefront sensor detects wavefront aberrations by subdividing the complex field at the origin of the focal plane into quadrants. This paper investigates wavefront sensors, such as the
pyramid sensor, that subdivide the complex field at the origin of the focal plane into N equal segments. This can be physically realized with either an N-sided prism or by a lenslet array in
the focal plane. An alternative reconstruction technique for the pyramid is also proposed to increase the performance of the sensor.
The predominant effect of the atmosphere on the incoming wavefront of an astronomical object is the introduction of a phase distortion, resulting in a speckle image at the ground-based telescope. Deconvolution from wavefront sensing is an imaging technique used to compensate for the degradation due to atmospheric turbulence, where the point spread function is estimated from the wavefront sensing data. An accurate calibration of the wavefront sensor is critical to estimating the point spread function and hence reconstructing the object. This paper investigates the calibration of a Shack-Hartmann wavefront sensor used for deconvolution from wavefront sensing.
The predominant effect of the atmosphere on the incoming wavefront of an astronomical object is the introduction of a phase distortion, resulting in a speckle image at the ground-based telescope. Deconvolution from wavefront sensing is an imaging technique used to compensate for the degradation due to atmospheric turbulence, where the point spread function is estimated from the wavefront sensing data. However in this approach any information in the speckle images regarding the point spread function is not utilised. This paper investigates the joint application of wavefront sensing data and speckle images in reconstructing the point spread function and the object in a Bayesian framework. The results on experimental data demonstrate the feasibility of this approach even under very low light levels.
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