Low-order Adaptive Optics System Research Articles

Correction of static and non-common path aberrations in an adaptive optics system using inherent calibration data.

For low-order adaptive optics systems, a method that is able to correct for system aberrations in the final focal plane is presented. The paper presents a novel figure of merit, corresponding to the integrated non-normalized tip-tilt-free optical transfer function. The inherent singular value decomposition modal content of the interaction matrix is used to optimize this figure of merit. The method has proven to be stable and robust, providing a simple mean to facilitate diffraction limited imaging in an experimental setup for ophthalmic applications.

SOWAT: Speckle Observations with Alleviated Turbulence

Adaptive optics (AO) systems and image reconstruction algorithms are indispensable tools when it comes to high-precision astrometry. In this paper, we analyze the potential of combining both techniques, i.e., by applying image reconstruction on partially AO-corrected short exposures. Therefore we simulate speckle clouds with and without AO corrections and create synthetic observations. We apply holographic image reconstruction to the obtained observations and find that (i) the residual wavefronts decorrelate slowlier and to a lower limit when AO systems are used, (ii) the same reference stars yield a better reconstruction, and (iii) using fainter reference stars we achieve a similar image quality. These results suggest that holographic imaging of speckle observations is feasible with ∼2–3× longer integration times and ∼3 mag fainter reference stars, to obtain diffraction-limited imaging from low-order AO systems that are less restricted in sky coverage than typical high-order AO systems.

Kalman filtering to suppress spurious signals in adaptive optics control

In many scenarios, an adaptive optics (AO) control system operates in the presence of temporally non-white noise. We use a Kalman filter with a state space formulation that allows suppression of this colored noise, hence improving residual error over the case where the noise is assumed to be white. We demonstrate the effectiveness of this new filter in the case of the estimated Gemini Planet Imager tip-tilt environment, where there are both common-path and non-common-path vibrations. We discuss how this same framework can also be used to suppress spatial aliasing during predictive wavefront control assuming frozen flow in a low-order AO system without a spatially filtered wavefront sensor, and present experimental measurements from Altair that clearly reveal these aliased components.

Development of a low-order Adaptive Optics system at Udaipur Solar Observatory

A low-order Adaptive Optics (AO) system is being developed at the Udaipur Solar Observatory and we present in this paper the status of the project, which includes the image stabilization system and calibration of wavefront sensor and deformable mirror. The image stabilization system comprises of a piezo driven tip-tilt mirror, a high speed camera (955 fps), a frame grabber system for sensing the overall tilt and a Linux based Intel Pentium 4 control computer with Red Hat Linux OS. The system operates under PID control. In the closed loop, an rms image motion of 0.1–0.2 arcsec was observed with the improvement factor varying from 10–20 depending on the external conditions. Error rejection bandwidth of the system at 0 dB is 80–100 Hz. In addition to that, we report the on-going efforts in the calibration of lenslet array and deformable mirror for sensing and correcting the local tilt of the wavefront.

Improving the Sensitivity of Astronomical Curvature Wavefront Sensor Using Dual-Stroke Curvature

ABSTRACTCurvature wavefront sensors measure wavefront phase aberration by acquiring two intensity images on either side of the pupil plane. Low-order adaptive optics (AO) systems using curvature wavefront sensing (CWFS) have proved to be highly efficient for astronomical applications: they are more sensitive, use fewer detector elements, and achieve, for the same number of actuators, higher Strehl ratios than AO systems using more traditional Shack-Hartmann wavefront sensors. In higher-order systems, however, curvature wavefront sensors lose sensitivity to low spatial frequencies wavefront aberrations. This effect, often described as “noise propagation,” limits the usefulness of curvature wavefront sensing for high-order AO systems and/or large telescopes. In this paper, we first explain how this noise propagation effect occurs and then show that this limitation can be overcome by acquiring four defocused images of the pupil instead of two. This solution can be implemented without significant technology development and can run with a simple linear wavefront reconstruction algorithm at >kHz speed. We have successfully demonstrated in the laboratory that the four conjugation planes can be sequentially obtained at >kHz speed using a speaker–vibrating membrane assembly commonly used in current curvature AO systems. Closed loop simulations show that implementing this scheme is equivalent to making the guide star 1 to 1.5 magnitude brighter for the configuration tested (188 actuator elements on 8-m telescope). Higher sensitivity gains are expected on curvature systems with higher number of actuators.

Mapping sodium distribution in the exosphere of Mercury with tip-tilt image stabilization

Mapping the sodium emission about Mercury is a difficult observational problem, since Mercury is seen either against the bright daytime sky, or against a dark sky at very high air masses. The distribution of sodium emission over the surface of Mercury is non-uniform, and changes over time. These effects give clues to the processes that produce the sodium and control its distribution, so that improved mapping of sodium emissions over the Mercury surface will help clarify their relative importance. We have adapted an image stabilizer utilizing a piezoelectric driven tip-tilt correction mirror for daytime spectral imaging of Mercury. The image stabilizer, which was originally developed for solar observations at the McMath–Pierce solar telescope, results in a noticeable improvement in spatial resolution of our Mercury sodium images. In this paper we give initial results from use of the tip-tilt image stabilizer for observations of Mercury’s sodium exosphere. Further systematic observations and improvements are planned for the image stabilizer system, as well as experimental observations with a low-order adaptive optics system incorporating a commercially available 37-actuator deformable mirror.

Observational Evidence of Magnetic Flux Submergence in Flux Cancellation Sites

Using high-resolution vector magnetograms of NOAA Active Region 10043, observed on 2002 July 26 with the Advanced Stokes Polarimeter and low-order adaptive optics system, we studied the magnetic field topology and line-of-sight velocities in two flux cancellation sites. We found that the magnetic field is near horizontal at the place where two opposite polarities cancel each other. In addition, we observed significant downflows of about 1 km s-1 near the polarity reversal line, where the field is horizontal. We interpret these observations as the direct evidence of the magnetic flux submergence of concave-down (Ω-shaped) magnetic loop at the flux cancellation sites.

Adaptive Optics as a Spatial Coherence Modifier

The possibility of adaptive optics as a spatial coherence modifier is discussed. The initial field to be corrected is produced in such a way that a quasi-monochromatic plane wave of light is incident on a moving diffuser plate, so that it is spatially partially coherent. It is shown that adaptive optics serves to enhance the spatial coherence of the resultant field and that the magnitude of the enhancement depends on the spatial coherence of the initial field to be corrected. The results are illustrated by numerical examples based on an idealized low-order adaptive optics system.

Bisector Analysis of Stokes Profiles: Effects Due to Gradients in the Physical Parameters

The asymmetry of Stokes V profiles of the spectral lines λ6301.5 and λ6302.5 was utilized to systematically study a sunspot observed close to the disk center. The Stokes spectra were taken with the National Solar Observatory (NSO)/High Altitude Observatory Advanced Stokes Polarimeter (ASP). The NSO low-order adaptive optics system was used to record a data set of consistently high resolution. We find the following results from this analysis: (1) a strong correlation between the center-of-gravity (COG) velocity derived from the intensity profiles with the V-profile asymmetry; (2) the amplitude asymmetry is much more sensitive to changes in the COG velocity than the area asymmetry; and (3) plotting area versus amplitude asymmetry for the entire active region results in a bimodal distribution. Different areas within the active region, such as penumbra, umbra, light bridge, and small-scale fields outside the sunspot, are clearly separated in this plot. The light bridges and the small-scale magnetic fields surrounding the observed sunspot show larger amplitude asymmetry compared to the area asymmetry, whereas the penumbra shows larger area asymmetry. In comparison, the Stokes V spectra measured in the umbra show little area and amplitude asymmetry. In this paper, we use bisector analysis of the V intensity profile as a new tool to determine the gradients in the physical parameters in a more direct way. The gradients derived from the bisector method provide further and more direct evidence for the physical picture derived from the study of the asymmetries. For light bridges we find that the data is consistent with a picture of small convective cells confined to lower layers of the atmosphere. Asymmetries in the penumbra are caused by steep vertical gradients in the Evershed velocities in combination with the gradient in the line-of-sight inclination angle, confirming the earlier observations. For small-scale fields, the picture is consistent with the canopy effect. We also compare the average velocity and the magnetic field strength derived from this bisector analysis with the velocity and magnetic field strength derived from the ASP inversion and find excellent agreement between these independent methods. Apart from these asymmetries, we also observe extremely asymmetric V profiles, such as one-lobed profiles and multiple reversals, mostly at the edges of the limb-side penumbra. In these regions, we also find differences in the V profiles of λ6301.5 and λ6302.5 that suggest steep gradients in the physical parameters. The asymmetries derived from a single scan match well with the ones derived from the time-averaged properties obtained by averaging 14 such scans separated by 7.5 minutes. This suggests that, in a statistical sense, the Stokes V asymmetries do not vary with time and describe a global/general property of magnetic features found in regions such as light bridge, umbra, penumbra, and small-scale fields.

Modal compensation and the atmospheric-turbulence outer scale: computer simulations

The influence of the turbulence outer scale on the Strehl ratio obtained with low-order adaptive optics systems is examined by numerical simulation. The Karhunen-Loeve approach is used to generate wave-front samples. A method that allows construction of the outer-scale-dependent Karhunen-Loeve functions is described. It is shown that the Strehl ratio produced by a second-order adaptive optics correction (tip-tilt, defocus, and astigmatism) is affected quite strongly by the finite outer scale. For the higher-order correction, the effect under study is weak and appears only when the outer-scale magnitude becomes less than the aperture diameter. It is also shown that the finite outer scale has a positive effect on the Strehl ratio of the uncorrected long-exposure image.

9.5. The “double nucleus” of M31 in J, H, and K

Hubble Space Telescope images of the nucleus of M31 show a double-peaked structure with the primary peak being offset from the center by approximately 0.5″. We observed the central 13″ of M31 in the J, H, and Ks passbands to determine the nuclear structure in the near-infrared. Observations were taken at the MMT Observatory, using a low-order adaptive optics system, FASTTRAC II (Gray et. al. 1995). The diffraction limit for the system is 0.25″ in K band. PSF images showed correction to 0.5″ FWHM. Uncorrected images showed the seeing to be about 1″. The images were deconvolved using several methods to check for consistency. We used Iterative-Blind Deconvolution, Richardson-Lucy, and Wiener filter algorithms, getting similar results for each. Measurements suggest the PSF in the deconvolved images is approximately 0.35″ FWHM.

Angular correlation of Zernike polynomials for a laser guide star in adaptive optics

The residual wave-front error after correction by an adaptive optics system (AOS) with a laser guide star (LGS) as reference for the wave-front sensing is computed by using the wave-front expansion on the Zernike polynomials. The polynomial angular correlations are calculated for a spherical wave front coming from the LGS used to compensate for a plane wave front coming from a natural star and also for two arbitrary spherical wave fronts. The plot of decorrelation versus angular distance between the natural star and the LGS is much steeper for the sodium LGS than for the Rayleigh LGS. A D/n dependence is found for the 50% correlation angle, where D is the telescope diameter and n is the polynomial radial degree. For focal anisoplanatism a modal d0 parameter [d0′(J)] is computed versus J, the number of corrected polynomials. The Rayleigh LGS has the capacity for large-field-of-view observations but with poor image quality and a low-order AOS, while the sodium LGS is suitable for high-image-quality observations with a high-order AOS but at the price of a small field of view. The isoplanatic angle is computed for different D and observation wavelengths. For D=8 m and a sodium LGS, the wavelength must be larger than 1.6 μm in order to produce a λ/5 wave-front error.

Performance simulations of a daylight low-order adaptive optics system with speckle postprocessing for observation of low-earth orbit satellites

Computer simulations are used to demonstrate the feasibility of operating a low-order adaptive optics system for observation of bright low-earth orbiting satellites during daylight hours. The system considered includes speckle postprocessing of partially compensated focal plane images to produce reconstructions with resolution near the diffraction limit of the telescope.

Simulation Study of a Low-Light-Level Wavefront Sensor Driving a Low-Order, Near-IR Adaptive Optics System

Two high-fidelity computer simulations are used to study low-order adaptive optics systems operating in the near-infrared. We study obtainable system performance using very dim reference sources at three IR wavelengths.

PARTIAL ADAPTIVE COMPENSATION AND PASSIVE INTERFEROMETRY WITH LARGE GROUND-BASED TELESCOPES

ABSTRACT The possibilities for diffraction-limited imaging with large ground-basd optical telescopes have been investigated assuming the availability of a modest adaptive optics system. Using numerical simulations, we have examined the degree of partial wavefront compensation required for useful diffratcion-limited direct imaging as well as the performance improvement expected for both filled-aperture speckle imaging and non-redundant mask interferometric imaging in the presence of partial adaptive correction. A partially compensated point spread function will have a significant diffraction-limited central core if the residual rms wavefront error is less than two radians. In this regime (the "sharp core regime") deconvolution is likely to permit diffraction-limited direct imaging. However, if the residual rms wavefront error is more than two radians (the "speckle regime"), conventional passive interferometric methods will be required to attain diffraction-limited resolution. To study the improvement in passive interferometry we have investigated the effect of a low order adaptive optics system, correcting the first 21 terms in the Zernike expansion of the wavefront perturbations, for the case of D/r0=25. By using the attenuation of the monochromatic speckle transfer function to define an effective measure of r0 we find that for Kolmogorov atmospheric turbulence characterized by a value of r0 of 0.2 m, the 21 term Zernike corrected value of r0 is 0.29 m. The quantitative improvement in the power spectrum transfer function at finite bandwidth can primarily be accounted for by this increase in effective r0 which reduces the redundancy of baselines caused by both the finite bandwidth and the spatial extent of the pupil. For optical aperture synthesis imaging, the higher value of r0 permits larger subapertures to be utilized which in turn permits wider bandwidths to be employed without geometric attenuation of the power spectrum. The derived values of r0 can be used to estimate the improvement in the coherence time, pi0 and the overall signal-to-noise ratio of the imaging procedure. These results predict that the signal-to-noise ratios of the power spectrum and bispectrum are increased by factors of ~3 and ~5 respectively for both filled-aperture speckle imaging and optical aperture synthesis imaging.

A SIMPLE LOW-ORDER ADAPTIVE OPTICS SYSTEM FOR NEAR-INFRARED APPLICATIONS

ABSTRACT It is shown that low-order wavefront compensation can significantly improve astronomical images over most of the sky. A novel approach to wavefront sensing and compensation is described. It is optimized for low-order correction and high efficiency. Computer-simulation results show it can achieve the desired performance, and preliminary laboratory tests demonstrate its feasibility.