Jean-Pierre Véran

The Gemini Planet Imager

The next major frontier in the study of extrasolar planets is direct imaging detection of the planets themselves. With high-order adaptive optics, careful system design, and advanced coronagraphy, it is possible for an AO system on a 8-m class telescope to achieve contrast levels of 10-7 to 10-8, sufficient to detect warm self-luminous Jovian planets in the solar neighborhood. Such direct detection is sensitive to planets inaccessible to current radial-velocity surveys and allows spectral characterization of the planets, shedding light on planet formation and the structure of other solar systems. We have begun the construction of such a system for the Gemini Observatory. Dubbed the Gemini Planet Imager (GPI), this instrument should be deployed in 2010 on the Gemini South telescope. It combines a 2000-actuator MEMS-based AO system, an apodized-pupil Lyot coronagraph, a precision infrared interferometer for real-time wavefront calibration at the nanometer level, and a infrared integral field spectrograph for detection and characterization of the target planets. GPI will be able to achieve Strehl ratios > 0.9 at 1.65 microns and to observe a broad sample of science targets with I band magnitudes less than 8. In addition to planet detection, GPI will also be capable of polarimetric imaging of circumstellar dust disks, studies of evolved stars, and high-Strehl imaging spectroscopy of bright targets. We present here an overview of the GPI instrument design, an error budget highlighting key technological challenges, and models of the system performance.

Analytical modeling of adaptive optics: foundations of the phase spatial power spectrum approach

End-to-end simulation of adaptive optics (AO) systems allows high-fidelity modeling of system performance, but at the cost of long computation time. Analytical modeling, on the other hand, can provide much faster first-order performance estimates for a rapid exploration of the AO parameter space. In this paper, we present the foundations of a modeling method for the AO optical transfer function, based on an analytical description of the residual phase spatial power spectrum. The method has been implemented in an IDL-based code, PAOLA, and comparison with end-to-end simulations demonstrates the validity of the analytical approach.

Optimal modal fourier-transform wavefront control.

Optimal modal Fourier-transform wavefront control combines the speed of Fourier-transform reconstruction (FTR) with real-time optimization of modal gains to form a fast, adaptive wavefront control scheme. Our modal basis is the real Fourier basis, which allows direct control of specific regions of the point-spread function. We formulate FTR as modal control and show how to measure custom filters. Because the Fourier basis is a tight frame, we can use it on a circular aperture for modal control even though it is not an orthonormal basis. The modal coefficients are available during reconstruction, greatly reducing computational overhead for gain optimization. Simulation results show significant improvements in performance in low-signal-to-noise-ratio situations compared with nonadaptive control. This scheme is computationally efficient enough to be implemented with off-the-shelf technology for a 2.5 kHz, 64 x 64 adaptive optics system.

Fourier transform wavefront control with adaptive prediction of the atmosphere

Predictive Fourier control is a temporal power spectral density-based adaptive method for adaptive optics that predicts the atmosphere under the assumption of frozen flow. The predictive controller is based on Kalman filtering and a Fourier decomposition of atmospheric turbulence using the Fourier transform reconstructor. It provides a stable way to compensate for arbitrary numbers of atmospheric layers. For each Fourier mode, efficient and accurate algorithms estimate the necessary atmospheric parameters from closed-loop telemetry and determine the predictive filter, adjusting as conditions change. This prediction improves atmospheric rejection, leading to significant improvements in system performance. For a 48x48 actuator system operating at 2 kHz, five-layer prediction for all modes is achievable in under 2x10(9) floating-point operations/s.

Exoplanet imaging with LOCI processing: photometry and astrometry with the new SOSIE pipeline

The Angular, Simultaneous Spectral and Reference Star Differential Imaging techniques (ADI, SSDI and RSDI) are currently the main observing approaches that are being used to pursue large-scale direct exoplanet imaging surveys and will be a key component of next-generation high-contrast imaging instrument science. To allow detection of faint planets, images from these observing techniques are combined in a way to retain the planet flux while subtracting as much as possible the residual speckle noise. The LOCI algorithm is a very efficient way of combining a set of reference images to subtract the noise of a given image. Although high contrast performances have been achieved with ADI/SSDI/RSDI & LOCI, achieving high accuracy photometry and astrometry can be a challenge, due to various biases coming mainly from the inevitable partial point source self-subtraction for ADI/SSDI and how LOCI is designed to suppress the noise. We present here several biases that we hare uncovered while analyzing data on the HR8799 planetary system and how we have modified our analysis pipeline to calibrate or remove these effects so that high accuracy astrometry and photometry is achievable. In addition, several new upgrades are presented in a new archive-based (i.e. performing ADI, SSDI and RSDI with LOCI as a single PSF subtraction step) multi-instrument reduction and analysis pipeline called SOSIE.

Experimental verification of the frozen flow atmospheric turbulence assumption with use of astronomical adaptive optics telemetry

We use closed-loop deformable mirror telemetry from Altair and Keck adaptive optics (AO) to determine whether atmospheric turbulence follows the frozen flow hypothesis. Using telemetry from AO systems, our algorithms (based on the predictive Fourier control framework) detect frozen flow >94% of the time. Usually one to three layers are detected. Between 20% and 40% of the total controllable phase power is due to frozen flow. Velocity vector RMS variability is less than 0.5 m/s (per axis) on 10-s intervals, indicating that the atmosphere is stable enough for predictive control to measure and adapt to prevailing atmospheric conditions before they change.

Predictive wavefront control for adaptive optics with arbitrary control loop delays

We present a modification of the closed-loop state space model for adaptive optics control that allows delays that are a noninteger multiple of the system frame rate. We derive the new forms of the predictive Fourier control Kalman filters for arbitrary delays and show that they are linear combinations of the whole-frame delay terms. This structure of the controller is independent of the delay. System stability margins and residual error variance both transition gracefully between integer-frame delays.

Progress on Altair: The Gemini North Adaptive Optics System

The Gemini Adaptive Optics System, (Altair), under construction at the National Research Council of Canadas Herzberg Institute of Astrophysics is unique among AO systems. Altair is designed with its deformable mirror (DM) conjugate to high altitude. We summarize construction progress. We then describe Altair in more detail. Both the Wavefront sensor foreoptics and control system are unconventional, because the guide star footprint on an altitude-conjugated DM moves as the guide star position varies. During a typical nodding sequence, where the telescope moves 10 arcseconds between exposures, this footprint moves by half an actuator and/or WFS lenslet. The advantages of altitude conjugation include increased isoplanatic patch size, which improves sky coverage, and improved uniformity of the corrected field. Altitude conjugation also reduces focal anisoplanatism with laser beacons. Although the initial installation of Altair will use natural guide stars, it will be fully ready to use a laser guide star (LGS). The infrastructure of Gemini observatory provides a variety of wavefront sensors and nested control loops that together permit some unique design concepts for Altair.

Focus errors from tracking sodium layer altitude variations with laser guide star adaptive optics for the Thirty Meter Telescope

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.

Data reduction pipeline for the Gemini Planet Imager

The Gemini Planet Imager (GPI) high-contrast adaptive optics system, which is currently under construction for Gemini South, has an IFS as its science instrument. This paper describes the data reduction pipeline of the GPI science instrument. Written in IDL, with a modular architecture, this pipeline reduces an ensemble of highcontrast spectroscopic or polarimetric raw science images and calibration data into a final dataset ready for scientific analysis. It includes speckle suppression techniques such as angular and spectral differential imaging that are necessary to achieve extreme contrast performances for which the instrument is designed. This paper presents also raw GPI IFS simulated data developed to test the pipeline.