Dani Guzman

Atmospheric turbulence profiling using multiple laser star wavefront sensors

This paper describes the data pre-processing and reduction methods together with SLOpe Detection And Ranging (SLODAR) analysis and wind profiling techniques for the Gemini South Multi-Conjugate Adaptive Optics System (GeMS). The wavefront gradient measurements of the five GeMS Shack–Hartmann sensors, each pointing to a laser guide star, are combined with the deformable mirror (DM) commands sent to three DMs optically conjugated at 0, 4.5 and 9 km in order to reconstruct pseudo-open loop slopes. These pseudo-open loop slopes are then used to reconstruct atmospheric turbulence profiles, based on the SLODAR and wind-profiling methods. We introduce the SLODAR method, and how it has been adapted to work in a closed-loop, multi-laser guide star system. We show that our method allows characterizing the turbulence of up to 16 layers for altitudes spanning from 0 to 19 km. The data pre-processing and reduction methods are described, and results obtained from observations made in 2011 are presented. The wind profiling analysis is shown to be a powerful technique not only for characterizing the turbulence intensity, wind direction and speed, but also as it can provide a verification tool for SLODAR results. Finally, problems such as the fratricide effect in multiple laser systems due to Rayleigh scattering, centroid gain variations, and limitations of the method are also addressed.

Open-loop tomography with artificial neural networks on CANARY: on-sky results

We present recent results from the initial testing of an artificial neural network (ANN)-based tomographic reconstructor Complex Atmospheric Reconstructor based on Machine lEarNing (CARMEN) on CANARY, an adaptive optics demonstrator operated on the 4.2m William Herschel Telescope, La Palma. The reconstructor was compared with contemporaneous data using the Learn and Apply (L&A) tomographic reconstructor. We find that the fully optimized L&A tomographic reconstructor outperforms CARMEN by approximately 5percent in Strehl ratio or 15nm rms in wavefront error. We also present results for CANARY in Ground Layer Adaptive Optics mode to show that the reconstructors are tomographic. The results are comparable and this small deficit is attributed to limitations in the training data used to build the ANN. Laboratory bench tests show that the ANN can outperform L&A under certain conditions, e.g. if the higher layer of a model two layer atmosphere was to change in altitude by ∼300m (equivalent to a shift of approximately one tenth of a subaperture).

Modeling a MEMS deformable mirror using non-parametric estimation techniques

Using non-parametric estimation techniques, we have modeled an area of 126 actuators of a micro-electro-mechanical deformable mirror with 1024 actuators. These techniques produce models applicable to open-loop adaptive optics, where the turbulent wavefront is measured before it hits the deformable mirror. The models input is the wavefront correction to apply to the mirror and its output is the set of voltages to shape the mirror. Our experiments have achieved positioning errors of 3.1% rms of the peak-to-peak wavefront excursion.

Using artificial neural networks for open-loop tomography

Modern adaptive optics (AO) systems for large telescopes require tomographic techniques to reconstruct the phase aberrations induced by the turbulent atmosphere along a line of sight to a target which is angularly separated from the guide sources that are used to sample the atmosphere. Multi-object adaptive optics (MOAO) is one such technique. Here, we present a method which uses an artificial neural network (ANN) to reconstruct the target phase given off-axis references sources. We compare our ANN method with a standard least squares type matrix multiplication method and to the learn and apply method developed for the CANARY MOAO instrument. The ANN is trained with a large range of possible turbulent layer positions and therefore does not require any input of the optical turbulence profile. It is therefore less susceptible to changing conditions than some existing methods. We also exploit the non-linear response of the ANN to make it more robust to noisy centroid measurements than other linear techniques.

Deformable mirror model for open-loop adaptive optics using multivariate adaptive regression splines

Open-loop adaptive optics is a technique in which the turbulent wavefront is measured before it hits the deformable mirror for correction. We present a technique to model a deformable mirror working in open-loop based on multivariate adaptive regression splines (MARS), a non-parametric regression technique. The models input is the wavefront correction to apply to the mirror and its output is the set of voltages to shape the mirror. We performed experiments with an electrostrictive deformable mirror, achieving positioning errors of the order of 1.2% RMS of the peak-to-peak wavefront excursion. The technique does not depend on the physical parameters of the device; therefore it may be included in the control scheme of any type of deformable mirror.

The GMT-CfA, Carnegie, Catolica, Chicago Large Earth Finder (G-CLEF): a general purpose optical echelle spectrograph for the GMT with precision radial velocity capability

The GMT-CfA, Carnegie, Catolica, Chicago Large Earth Finder (G-CLEF) is a fiber fed, optical echelle spectrograph that has undergone conceptual design for consideration as a first light instrument at the Giant Magellan Telescope. GCLEF has been designed to be a general-purpose echelle spectrograph with precision radial velocity (PRV) capability. We have defined the performance envelope of G-CLEF to address several of the highest science priorities in the Decadal Survey1. The spectrograph optical design is an asymmetric, two-arm, white pupil design. The asymmetric white pupil design is adopted to minimize the size of the refractive camera lenses. The spectrograph beam is nominally 300 mm, reduced to 200 mm after dispersion by the R4 echelle grating. The peak efficiency of the spectrograph is >35% and the passband is 3500-9500Å. The spectrograph is primarily fed with three sets of fibers to enable three observing modes: High-Throughput, Precision-Abundance and PRV. The respective resolving powers of these modes are R~ 25,000, 40,000 and 120,000. We also anticipate having an R~40,000 Multi-object Spectroscopy mode with a multiplex of ~40 fibers. In PRV mode, each of the seven 8.4m GMT primary mirror sub-apertures feeds an individual fiber, which is scrambled after pupil-slicing. The goal radial velocity precision of G-CLEF is ∂V <10 cm/sec radial. In this paper, we provide a flowdown from fiducial science programs to design parameters. We discuss the optomechanical, electrical, structural and thermal design and present a roadmap to first light at the GMT.

Comparison of vibration mitigation controllers for adaptive optics systems.

Vibrations are detrimental to the performance of modern adaptive optics (AO) systems. In this paper, we describe new methods tested to mitigate the vibrations encountered in some of the instruments of the Gemini South telescope. By implementing a spectral analysis of the slope measurements from several wavefront sensors and an imager, we can determine the frequencies and magnitude of these vibrations. We found a persistent vibration at 55 Hz with others occurring occasionally at 14 and 100 Hz. Two types of AO controllers were designed and implemented, Kalman and H∞, in the multiconjugate AO tip-tilt loop. The first results show a similar performance for these advanced controllers and a clear improvement in vibration rejection and overall performance over the classical integrator scheme. It is shown that the reduction in the standard deviation of the residual slopes (as measured by wavefront sensors) is highly dependent on turbulence, wind speed, and vibration conditions, ranging--in terms of slopes RMS value--from an almost negligible reduction for high speed wind to a factor of 5 for a combination of low wind and strong vibrations.

Mitigation of vibrations in adaptive optics by minimization of closed-loop residuals

We describe a new technique to reduce tip and tilt vibrations via the design of adaptive optics controllers in a frequency framework. The method synthesizes controllers by minimizing an H2 norm of the tip and tilt residuals. In this approach, open loop slopes (pseudo-open-loop) are reconstructed from on-sky data and input into off-line simulations of the adaptive optics system. The proposed procedure executes a sequence of off-line closed-loop runs with increasing controller complexity and searches for the controller that minimizes the variance of residuals. Although the method avoids any identification of the vibration and turbulence models during the controller synthesis, the actual models are indirectly constructed as a by-product of the H2 norm minimization. The technique has been implemented on and tested with two operational instruments, namely Paranals NACO and Gemini-Souths GeMS, showing an effective rejection of the main vibrations in the loop and also improving the overall performance of the system over varying turbulence conditions. It is shown that a superior performance is obtained when compared to the standard integrator controller.

A novel systems engineering approach to the design of a precision radial velocity spectrograph: the GMT-Consortium Large EarthFinder (G-CLEF)

One of the first light instruments for the Giant Magellan Telescope (GMT) will be the GMT-Consortium Large Earth Finder (G-CLEF). It is an optical band echelle spectrograph that is fiber fed to enable high stability. One of the key capabilities of G-CLEF will be its extremely precise radial velocity (PRV) measurement capability. The RV precision goal is 10 cm/sec, which is expected to be achieved with advanced calibration methods and the use of the GMT adaptive optics system. G-CLEF, as part of the GMT suite of instruments, is being designed within GMTs automated requirements management system. This includes requirements flow down, traceability, error budgeting, and systems compliance. Error budgeting is being employed extensively to help manage G-CLEF technical requirements and ensure that the top level requirements are met efficiently. In this paper we discuss the G-CLEF error budgeting process, concentrating on the PRV precision and instrument throughput budgets. The PRV error budgeting process is covered in detail, as we are taking a detailed systems error budgeting approach to the PRV requirement. This has proven particularly challenging, as the precise measurement of radial velocity is a complex process, with error sources that are difficult to model and a complex calibration process that is integral to the RV measurement. The PRV budget combines traditional modeling and analysis techniques, where applicable, with semi-empirical techniques, as necessary. Extrapolation from existing PRV instruments is also used in the budgeting process.

Beam shaping for laser-based adaptive optics in astronomy

The availability and performance of laser-based adaptive optics (AO) systems are strongly dependent on the power and quality of the laser beam before being projected to the sky. Frequent and time-consuming alignment procedures are usually required in the laser systems with free-space optics to optimize the beam. Despite these procedures, significant distortions of the laser beam have been observed during the first two years of operation of the Gemini South multi-conjugate adaptive optics system (GeMS). A beam shaping concept with two deformable mirrors is investigated in order to provide automated optimization of the laser quality for astronomical AO. This study aims at demonstrating the correction of quasi-static aberrations of the laser, in both amplitude and phase, testing a prototype of this two-deformable mirror concept on GeMS. The paper presents the results of the preparatory study before the experimental phase. An algorithm to control amplitude and phase correction, based on phase retrieval techniques, is presented with a novel unwrapping method. Its performance is assessed via numerical simulations, using aberrations measured at GeMS as reference. The results predict effective amplitude and phase correction of the laser distortions with about 120 actuators per mirror and a separation of 1.4 m between the mirrors. The spot size is estimated to be reduced by up to 15% thanks to the correction. In terms of AO noise level, this has the same benefit as increasing the photon flux by 40%.