CfAO Fall Science
Retreat 2018

November 1st - 5th, 2018

Lake Arrowhead, CA

Plenary Speakers

Charlotte Bond, University of Hawaii - Friday 1:30-2:30 (Iris Room) - Adaptive Optics with an Infrared Pyramid Wavefront Sensor

Abstract: Wavefront sensing in the infrared is highly desirable for science with adaptive optics (AO). Systems of particular scientific interest, such as M dwarf stars, are faint in the visible but bright enough at infrared wavelengths to be used as AO guide stars, producing high resolution AO correction around the science target. Developments in wavefront sensing motivated by the future demands of astronomy have led to the emergence of the Pyramid wavefront sensor (WFS) as a prime candidate for new AO systems. The combination of infrared detector technology with the highly sensitive Pyramid WFS provides AO correction specifically tailored for the high contrast study of faint red objects.

Here we present an overview of the infrared Pyramid WFS, examining its behavior, advantages and challenges within the framework of a new IR Pyramid WFS for the Keck II telescope. This forms part of the Keck Planet Imager and Characterizer (KPIC) project, an instrument designed for the study of red objects such as exoplanets around M-type stars and planet formation in obscured star forming regions. We report on the integration and testing of this WFS, including installation on the Keck II AO bench and the first closed loop results with the internal source. A crucial step towards the realization of infrared wavefront sensing has been the development of low noise infrared detectors. Here we present the testing and characterization of the detector used in the KPIC system, the SAPHIRA avalanche photodiode array.

Gereon Huettmann, Institute for Biomedical Optics, University of Lübeck, Lübeck Germany- Friday 2:30-3:30 (Iris Room) - Functional imaging of the human retina by holographic OCT

Abstract: Optical coherence tomography is one of the most important technique for imaging the retina. Though it measures intensity and phase of the scattered light, current devices introduce the artifacts by beam scanning and ocular motion, which severely limit the usefulness of the phase information. We have developed a full-field Fourier domain OCT which measures millimeter sized volumes within 10 ms with completely preserved phase information. Using advance motion correction algorithms, also the phase difference between volumes can determined over several seconds. Local tissue motion and changes of the refractive index can be determined from the phase information. Our technique realizes to true holographic OCT with full access to amplitude and phase. This enabled three major applications with high scientific and clinical potential. First, we were able to correct the imaging errors (aberrations) of the eye and resolve single photoreceptor cells, solely by computer post-processing. Second, we measured pressure waves caused by blood pulsation, which provide unique information on hemodynamics and mechanical properties of the vascular system. And finally, we reliably measured spatially resolved the function of human cones and rods. Biphasic changes of the outer segments - first a nanometer contraction and then an elongation - were measured. Quantitativ modeling suggests that osmotic changes are responsible for the elongation. Osmotic effects are several hundred times more sensitive to ion changes than the refractive index. They act as leverage to visualize photoreceptor function by phase sensitive measurements. Holographic OCT makes optimal use of the scattered light in retinal imaging and may also allow to image the function neuronal cells in the inner retina.

Dmitry Savransky, Cornell University - Saturday 1:30-2:30 - Getting the Most Out of Your Hardware: Exploring novel algorithms for optical system control and data processing

Abstract: Just as optical hardware systems continue to benefit uniformly from advances across various disciplines and technology areas, it is frequently worthwhile to explore the space of existing algorithms and mathematical formalisms for potential applications to wavefront sensing and control. Here, I will discuss two such recent efforts. The first involves the exploration of Blind Source Separation (BSS) algorithms for weak signal detection, and is motivated by the success of principal component analysis (PCA) methods in astronomical imaging. PCA is just one method of BSS, which also includes such methods as common spatial pattern filtering and independent component analysis, which can be applied to many of the same classes of problems. Next, I will talk about the application of optimal estimation theory, and in particular dynamic state estimation, to wavefront control and focal-plane wavefront sensing. These methods, widely used in various engineering disciplines, can be combined with various well-known wavefront estimation techniques to produce dramatic improvements in performance. I will present several examples of current development, and discuss prospects for future work.

Jared Males, University of Arizona - Saturday 2:30-3:30 - Towards the Fundamental Limits of Ground-Based Exoplanet Imaging

Abstract: As we construct the next generation of ground-based giant segmented mirror telescopes (GSMTs), many are planning for the many ways these instruments will revolutionize our knowledge of exoplanets. A key open question is how well direct imaging will perform on the GSMTs. We will discuss an approach for determining the fundamental limits of ground-based direct imaging. A semi-analytic model of an AO system in the temporal frequency domain is used to estimate the post-coronagraph contrast given atmospheric turbulence under closed-loop predictive control. The output of this model can also be used to analyze speckle lifetimes, necessary to develop a noise model for imaging. These same techniques can be applied to quasi-static speckles sensed and controlled with techniques such as focal plane wavefront sensing and speckle nulling. We apply this framework to several interesting potential science cases for the GSMTs.