Laboratory For Adaptive Optics Research Articles

Laboratory demonstration of real-time focal plane wavefront control of residual atmospheric speckles

Current and future high contrast imaging instruments aim to detect exoplanets at closer orbital separations, lower masses, and/or older ages than their predecessors. However, continually evolving speckles in the coronagraphic science image limit contrasts of state-of-the-art ground-based exoplanet imaging instruments. For ground-based adaptive optics (AO) instruments, it remains challenging for most speckle suppression techniques to attenuate both the dynamic atmospheric as well as quasistatic instrumental speckles on-sky. We have proposed a focal plane wavefront sensing and control algorithm to address this challenge, called the fast atmospheric selfcoherent camera (SCC) technique (FAST), which in theory enables the SCC to operate down to millisecond timescales even when only a few photons are detected per speckle. Here, we present the first experimental results of FAST on the Santa Cruz Extreme AO Laboratory (SEAL) testbed. In particular, we illustrate the benefit of “second stage” AO-based focal plane wavefront control, demonstrating up to 5 × contrast improvement with FAST closed-loop compensation of evolving residual atmospheric turbulence—both for low and high order spatial modes—down to 20-ms timescales.

TESTING THE APODIZED PUPIL LYOT CORONAGRAPH ON THE LABORATORY FOR ADAPTIVE OPTICS EXTREME ADAPTIVE OPTICS TESTBED

We present testbed results of the Apodized Pupil Lyot Coronagraph (APLC) at the Laboratory for Adaptive Optics (LAO). These results are part of the validation and tests of the coronagraph and of the Extreme Adaptive Optics (ExAO) for the Gemini Planet Imager (GPI). The apodizer component is manufactured with a halftone technique using black chrome microdots on glass. Testing this APLC (like any other coronagraph) requires extremely good wavefront correction, which is obtained to the 1 nm rms level using the microelectricalmechanical systems (MEMS) technology, on the ExAO visible testbed of the LAO at the University of Santa Cruz. We used an APLC coronagraph without central obstruction, both with a reference super-polished flat mirror and with the MEMS to obtain one of the first images of a dark zone in a coronagraphic image with classical adaptive optics using a MEMS deformable mirror (without involving dark hole algorithms). This was done as a complementary test to the GPI coronagraph testbed at American Museum of Natural History, which studied the coronagraph itself without wavefront correction. Because we needed a full aperture, the coronagraph design is very different from the GPI design. We also tested a coronagraph with central obstruction similar to thatmore » of GPI. We investigated the performance of the APLC coronagraph and more particularly the effect of the apodizer profile accuracy on the contrast. Finally, we compared the resulting contrast to predictions made with a wavefront propagation model of the testbed to understand the effects of phase and amplitude errors on the final contrast.« less

Wide field adaptive optics laboratory demonstration with closed-loop tomographic control

HOMER, the new bench developed at ONERA devoted to wide field adaptive optics (WFAO) laboratory research, has allowed the first experimental validations of multi-conjugate adaptive optics (MCAO) and laser tomography adaptive optics (LTAO) concepts with a linear quadratic Gaussian (LQG) control approach. Results obtained in LTAO in closed loop show the significant gain in performance brought by LQG control, which allows tomographic reconstruction. We present a calibration and model identification strategy. Experimental results are shown to be consistent with end-to-end simulations. These results are very encouraging and demonstrate robustness of performance with respect to inevitable experimental uncertainties. They represent a first step for the study of very large telescope (VLT) and extremely large telescopes (ELT) instruments.

Amplitude variations on a MEMS-based extreme adaptive optics coronagraph testbed

High-contrast imaging techniques such as coronagraphy are expected to play an important role in the imaging of extrasolar planets. Instruments like the Gemini Planet Imager (GPI) or the Spectro-Polar-Imetric High-Contrast Exoplanet Research (SPHERE) require high-dynamic range, achieved using coronagraphs to block light coming from the parent star. An extremely good adaptive optics (AO) system is required to reduce dynamic atmospheric wavefront errors to 50-100 nm rms. Systematic wavefront errors must also be controlled at the nanometer-equivalent level to remove persistent speckle artifacts. While precision AO systems can control wavefront phase errors at this level, systematic amplitude or intensity errors can also produce speckle artifacts and are uncontrolled by traditional AO phase conjugation. On the Laboratory for Adaptive Optics (LAO) extreme AO testbed, we observed a discrepancy between the coronagraphic image profile and the profile predicted by simple simulations using the measured optical phase, which could potentially be explained by amplitude variations. Measurements showed up to 7% rms intensity changes across the microelectrical mechanical (MEM) plane of the system. We identified potential sources of amplitude variation and compared them to a Fresnel model of the system. One potential concern was the surface structure of the MEM system's (MEMS) deformable mirror, but analysis shows that it induces at most 2% rms variation. The bulk of the observed intensity variation is due to nonuniform illumination of the system by the input single-mode fiber and phase errors mixing into amplitude at the nonpupil-plane due to the Talbot effect, coupled with residual astigmatism in the pupil imager.

The effect of a small heat source on PSF stability for high-contrast imaging

High-contrast adaptive optics systems, such as those needed to image extrasolar planets, are known to require excellent wavefront control and diffraction suppression. The Laboratory for Adaptive Optics at UC Santa Cruz is investigating limits to high-contrast imaging in support of the Gemini Planet Imager (GPI). In this paper we examine the effect of heat sources in the testbed on point-spread-function (PSF) stability. Introducing a heat source primarily introduces image motion. The GPI error budget requires image motion to be less than 0.1 lambda /D. Systematic motion of the PSF core is typically 0.01 lambda /D rms and with a 20 watt heat source introduced near the pupil plane image motion is increased to 0.02 lambda /D rms. Therefore, even a heat source as large as 20 watts near the pupil plane causes errors below the GPI requirement, but the combination of the heat source and additional air turbulence on the system introduced by changes to the enclosure or the fan of other components can produce significantly more motion. Heat also can affect the speckle pattern in the high-contrast region, but in the final instrument other sources of error should be more significant.

Experimental assessment of the matched filter for laser guide star wavefront sensing

Laser guide star wavefront sensing comes with several limitations. When imaged with a Shack-Hartmann wavefront sensor, the laser guide star is seen as extended sources elongated in the directions given by the lenslet locations and the laser axis. A test bed has been built in the Adaptive Optics Laboratory of the University of Victoria that reproduces this effect as seen on extremely large telescopes. A new wavefront sensing algorithm, the matched filter, has been implemented and its performance assessed with the test bed. Its ability to mitigate laser guide star aberrations by tracking the sodium layer fluctuations in a closed loop adaptive optics system is shown.

Multiconjugate adaptive optics results from the laboratory for adaptive optics MCAO/MOAO testbed

We report on the development of wavefront reconstruction and control algorithms for multiconjugate adaptive optics (MCAO) and the results of testing them in the laboratory under conditions that simulate an 8 meter class telescope. The University of California Observatories (UCO) Lick Observatory Laboratory for Adaptive Optics multiconjugate testbed allows us to test wide-field-of-view adaptive optics systems as they might be instantiated in the near future on giant telescopes. In particular, we have been investigating the performance of MCAO using five laser beacons for wavefront sensing and a minimum-variance algorithm for control of two conjugate deformable mirrors. We have demonstrated improved Strehl ratio and enlarged field-of-view performance when compared to conventional AO techniques. We have demonstrated improved MCAO performance with the implementation of a routine that minimizes the generalized isoplanatism when turbulent layers do not correspond to deformable mirror conjugate altitudes. Finally, we have demonstrated suitability of the system for closed loop operation when configured to feed back conditional mean estimates of wavefront residuals rather than the directly measured residuals. This technique has recently been referred to as the "pseudo-open-loop" control law in the literature.