Celia Blain

High-Contrast Imaging and Wavefront Control with a PIAA Coronagraph: Laboratory System Validation

The Phase-Induced Amplitude Apodization (PIAA) coronagraph is a high-performance coronagraph concept able to work at small angular separation with little loss in throughput. We present results obtained with a laboratory PIAA system including active wavefront control. The system has a 94.3% throughput (excluding coating losses) and operates in air with monochromatic light. Our testbed achieved a 2.27 × 10-7 raw contrast between 1.65λ/D (inner working angle of the coronagraph configuration tested) and 4.4λ/D (outer working angle). Through careful calibration, we were able to separate this residual light into a dynamic coherent component (turbulence, vibrations) at 4.5 × 10-8 contrast and a static incoherent component (ghosts and/or polarization mismatch) at 1.6 × 10-7 contrast. Pointing errors are controlled at the 10-3λ/D level using a dedicated low-order wavefront sensor. While not sufficient for direct imaging of Earthlike planets from space, the 2.27 × 10-7 raw contrast achieved already exceeds requirements for a ground-based extreme adaptive optics system aimed at direct detection of more massive exoplanets. We show that over a 4 hr period, averaged wavefront errors have been controlled to the 3.5 × 10-9 contrast level. This result is particularly encouraging for ground-based extreme-AO systems relying on long-term stability and absence of static wavefront errors to recover planets much fainter than the fast boiling speckle halo.

Exoplanet Imaging with a Phase-induced Amplitude Apodization Coronagraph. III. Diffraction Effects and Coronagraph Design

Properly apodized pupils can deliver point-spread functions (PSFs) free of Airy rings and are suitable for high dynamical range imaging of extrasolar terrestrial planets (ETPs). To reach this goal, classical pupil apodization (CPA) unfortunately requires most of the light gathered by the telescope to be absorbed, resulting in poor throughput and low angular resolution. Phase-induced amplitude apodization (PIAA) of the telescope pupil combines the advantages of classical pupil apodization (particularly low sensitivity to low-order aberrations) with full throughput, no loss of angular resolution and little chromaticity, which makes it, theoretically, an extremely attractive coronagraph for direct imaging of ETPs. The two most challenging aspects of this technique are (1) the difficulty of polishing the required optics shapes and (2) diffraction propagation effects, which, because of their chromaticity, can decrease the spectral bandwidth of the coronagraph. We show that a properly designed hybrid system combining classical apodization with the PIAA technique can solve both problems simultaneously. For such a system, the optics shapes can be well within todays optics manufacturing capabilities, and the 10-10 PSF contrast at ≈1.5λ/D required for efficient imaging of ETPs can be maintained over the whole visible spectrum. This updated design of the PIAA coronagraph maintains the high performance of the earlier design, since only a small part of the light is lost in the classical apodizer(s).

Distributed modal command for a two-deformable-mirror adaptive optics system.

The design of future single-altitude conjugated adaptive optics (AO) systems may include at least two deformable mirrors (DMs) instead of one as in the current AO system. Each DM will have to correct for a specific spatial frequency range. A method is presented to derive a DM modal basis based on the influence functions of the DM. The modal bases are derived such that they are orthogonal to a given set of modes that restrict the DM correction to a spatial frequency domain. The modal bases have been tested on the woofer-tweeter test bench at the University of Victoria. It has been shown that the rms amplitude of the woofer DM and tweeter DM stroke can be reduced by factors of 3 and 9, respectively, when making the transition from a zonal-driven closed loop to a modal-driven closed loop with the same performance in both cases.

Speckle Control with a remapped-pupil PIAA-coronagraph

Phase-induced amplitude apodization (PIAA) is a well-demonstrated high-contrast technique that uses an intermediate remapping of the pupil for high-contrast coronagraphy (apodization), before restoring it to recover classical imaging capabilities. This paper presents the first demonstration of complete speckle control loop with one such PIAA coronagraph. We show the presence of a complete set of remapping optics (the so-called PIAA and matching inverse PIAA) is transparent to the wavefront control algorithm. Simple focal-plane-based wavefront control algorithms can thus be employed, without the need to model remapping effects. Using the Subaru Coronagraphic Extreme AO (SCExAO) instrument built for the Subaru Telescope, we show, using a calibration source, that a complete PIAA coronagraph is compatible with a simple implementation of a speckle nulling technique, and demonstrate the benefit of the PIAA for high-contrast imaging at small angular separation.

The Subaru coronagraphic extreme AO project: progress report

In 2009 our group started the integration of the SCExAO project, a highly flexible, open platform for high contrast imaging at the highest angular resolution, inserted between the coronagraphic imaging camera HiCIAO and the 188-actuator AO system of Subaru. In its first version, SCExAO combines a MEMS-based wavefront control system feeding a high performance PIAA-based coronagraph. It also includes a coronagraphic low-order wavefront sensor, a non-redundant aperture mask and a visible imaging mode, all of them designed to take full advantage of the angular resolution that an 8-meter telescope has to offer. SCExAO is currently undergoing commissioning, and this paper presents the first on-sky results acquired in August 2011, using together Subarus AO system, SCExAO and HiCIAO.

Modeling and parameter estimation for point-actuated continuous-facesheet deformable mirrors

We present a variant of the model introduced by Vogel and Yang [J. Opt. Soc. Am. A23, 1074 (2006)] for point-actuated deformable mirrors (DMs) with continuous facesheets, and we describe a robust efficient regularized- output least-squares computational scheme to estimate the parameters in the model, given noisy discrete observations of the DM response to known actuation. We demonstrate the effectiveness of this approach with experimental data obtained from a pair of DMs--a piezo-actuated prototype DM built by CILAS for the Thirty Meter Telescope Project and an electrostatically actuated commercial micro-electro-mechanical systems (MEMS) DM produced by Boston Micromachines Corporation.

Multi-object adaptive optics on-sky results with Raven

Raven is a Multi-Object Adaptive Optics (MOAO) technical and science demonstrator which had its first light at the Subaru telescope on May 13-14, 2014. Raven was built and tested at the University of Victoria AO Lab before shipping to Hawai`i. Raven includes three open loop wavefront sensors (WFSs), a central laser guide star WFS, and two independent science channels feeding light to the Subaru IRCS spectrograph. Raven supports different kinds of AO correction: SCAO, open-loop GLAO and MOAO. The MOAO mode can use different tomographic reconstructors, such as Learn-and-Apply or a model-based reconstructor. This paper presents the latest results obtained in the lab, which are consistent with simulated performance, as well as preliminary on-sky results, including echelle spectra from IRCS. Ensquared energy obtained on sky in 140mas slit is 17%, 30% and 41% for GLAO, MOAO and SCAO respectively. This result confirms that MOAO can provide a level of correction in between GLAO and SCAO, in any direction of the field of regard, regardless of the science target brightness.

Status of the Raven MOAO science demonstrator

Raven is a Multi-Object Adaptive Optics (MOAO) scientific demonstrator which will be used on-sky at the Subaru observatory. Raven is currently being built at the University of Victoria AO Lab. In this paper, we present an overview of the final Raven design and then describe lab tests involving prototypes of Raven subsystems. The final design includes three open loop wavefront sensors (WFSs), a laser guide star WFS and two figure/truth WFSs. Two science channels, each containing a deformable mirror (DM), feed light to the Subaru IRCS spectrograph. Central to the Raven MOAO system is a Calibration Unit (CU) which contains multiple sources, a telescope simulator including two rotating phase screens and a ground layer DM that can be used to calibrate and test Raven. We are working with the Raven CU and open loop WFSs to test and validate our open loop calibration and alignment techniques.

Raven: a harbinger of multi-object adaptive optics-based instruments at the Subaru Telescope

In the context of instrumentation for Extremely Large Telescopes (ELTs), an Integral Field Spectrographs (IFSs), fed with a Multi-Object Adaptive Optics (MOAO) system, has many scientific and technical advantages. Integrated with an ELT, a MOAO system will allow the simultaneous observation of up to 20 targets in a several arc-minute field-of-view, each target being viewed with unprecedented sensitivity and resolution. However, before building a MOAO instrument for an ELT, several critical issues, such as open-loop control and calibration, must be solved. The Adaptive Optics Laboratory of the University of Victoria, in collaboration with the Herzberg Institute of Astrophysics, the Subaru telescope and two industrial partners, is starting the construction of a MOAO pathfinder, called Raven. The goal of Raven is two-fold: first, Raven has to demonstrate that MOAO technical challenges can be solved and implemented reliably for routine on-sky observations. Secondly, Raven must demonstrate that reliable science can be delivered with multiplexed AO systems. In order to achieve these goals, the Raven science channels will be coupled to the Subarus spectrograph (IRCS) on the infrared Nasmyth platform. This paper will present the status of the project, including the conceptual instrument design and a discussion of the science program.

Open-loop control demonstration of Micro-Electro-Mechanical-System MEMS Deformable Mirror

New astronomical challenges revolve around the observation of faint galaxies, nearby star-forming regions and the direct imaging of exoplanets. The technologies required to progress in these fields of research rely on the development of custom Adaptive Optics (AO) instruments such as Multi-Object AO (MOAO) or Extreme AO (ExAO). Many obstacles remain in the development of these new technologies. A major barrier to the implementation of MOAO is the utilisation of deformable mirrors (DMs) in an open-loop control system. Micro-Electro-Mechanical-System (MEMS) DMs show promise for application in both MOAO and ExAO. Despite recent encouraging laboratory results, it remains an immature technology which has yet to be demonstrated on a fully operational on-sky AO system. Much of the research in this area focuses on the development of an accurate model of the MEMS DMs. In this paper, a thorough characterization process of a MEMS DM is performed, with the goal of developing an open-loop control strategy free of computationally heavy modelling (such as the use of plate equations). Instead, a simpler approach, based on the additivity of the influence functions, is chosen. The actuator stroke-voltage relationship and the actuator influence functions are carefully calibrated. For 100 initial phase screens with a mean rms of 97 nm (computer generated following a Von Karman statistic), the resulting mean residual open-loop rms error is 16.5 nm, the mean fitting error rms is 13.3 nm and the mean DM error rms is 10.8 nm (error reflecting the performances of the model under test in this paper). This corresponds to 11% of residual DM error.