A two-band approach to nλ\lambda phase error corrections with LBTI's PHASECam

PHASECam is the Large Binocular Telescope Interferometer's (LBTI) phase sensor, a near-infrared camera which is used to measure tip/tilt and phase variations between the two AO-corrected apertures of the Large Binocular Telescope (LBT). Tip/tilt and phase sensing are currently performed in the H (1.65 $\mu$m) and K (2.2 $\mu$m) bands at 1 kHz, and the K band phase telemetry is used to send tip/tilt and Optical Path Difference (OPD) corrections to the system. However, phase variations outside the range [-$\pi$, $\pi$] are not sensed, and thus are not fully corrected during closed-loop operation. PHASECam's phase unwrapping algorithm, which attempts to mitigate this issue, still occasionally fails in the case of fast, large phase variations. This can cause a fringe jump, in which case the unwrapped phase will be incorrect by a wavelength or more. This can currently be manually corrected by the observer, but this is inefficient. A more reliable and automated solution is desired, especially as the LBTI begins to commission further modes which require robust, active phase control, including controlled multi-axial (Fizeau) interferometry and dual-aperture non-redundant aperture masking interferometry. We present a multi-wavelength method of fringe jump capture and correction which involves direct comparison between the K band and currently unused H band phase telemetry.Comment: 17 pages, 10 figure

LINC-NIRVANA Piston Control and Near-Infrared Polarization Images of the Orion Proplyds

This thesis is focussed on the development of optimized techniques to overcome limitations of astrophysical observations. The goal is an optimal signal estimation in noisy measurements by the consideration of underlying physical processes. This principle was applied to two different fields in astrophysics: intrumental design and analysis of polarimetric observations. In the observational part of this thesis near-infrared images of young stellar objects in the Orion constellation are studied. Limitations in resolution and sensitivity of current astronomical instruments prohibit the detailed analysis of interesting proto-stellar sources to improve theories of star formation. Radiation from the astronomical targets is not only characterized by its spectral energy, but also by polarization properties. The modeling of typical configurations of star-disk systems and the simulation of their polarization patterns helped to understand and interprete features, that were found in observations. For the case of a proto-stellar systems with both a disk and an envelope analysis techniques were developed, which are based on polarimetric effects of the scattering of light by dust. These techniques substantially improve the sensitivity and resolution and are reliable under different observing conditions. Although the obtained data did not allow investigations of substructures of the circumstellar material, the techniques are suitable to obtain constraints on star formation processes. With larger telescopes, such as the Large Binocular Telescope (LBT), the analysis of principles down to the scale of planet formation will be possible. LINC-NIRVANA, the near-infrared imaging system at the LBT, will provide unprecedented resolution and sensitivity performance combined with a wide field of view. The interferometric combination of light from two telescopes imposes new challenges in the instrument design. A so-called fringe tracker is mandatory for the interference of light at the detector by compensating optical path differences with an actuated mirror. The performance suffers from structural vibrations, limited sensitivity of sensors and processing latencies. The dynamics of actuators with significant amount of moving masses are limited. These effects are studied in the instrumental part of this thesis. Giant telescopes with large mirrors and high-resolution instruments are complex, expensive projects. The telescope time, i.e. the allocated time to the observations of individual astronomical topics, is very limited and hence valuable. It is of great importance to improve the performance in terms of sensitivity and resolution of observations under difficult conditions. By modeling all performance related subsystems of the interferometric instrument critical parameters were identified. For a realistic model several precision measurements of the telescope and parts of the LINC-NIRVANA instrument were necessary. Simulations of atmospheric effects completed the model. Several approaches for the optimization of the instrument performance were proposed. The determination of atmospheric and instrumental optical path difference or differential piston is improved by the detailed analysis of the statistical variation and appropriate filtering. The application of modern control theory provides stability and optimal dynamic response of the mirror actuator. Since both parts of this thesis deal with the impact of the atmosphere on the observational result of astronomical instruments, chapter 1 gives an overview on the basic principles and techniques. Chapter 2 presents the modeling and control analysis of the fringe tracker for the LINC-NIRVANA instrument. A software simulation is introduced and first results of a laboratory experiment are denoted. Chapter 3 gives a short introduction into star formation theories and unsolved problems, followed by the detailed description of polarimetric observations of proto-stellar objects in the Orion constellation in chapter 4. Some concepts and atmospheric effects as introduced in chapter 1 are discussed. Promising findings of one source of the sample led to extensive modeling and analysis of polarimetric properties of proto-stars and the development of innovative analysis techniques, presented in chapter 5. In the last chapter these methods are applied to the observational data and the obtained configuration parameters are compared to results of previous investigations in the literature

Modern optical astronomy: technology and impact of interferometry

The present `state of the art' and the path to future progress in high spatial resolution imaging interferometry is reviewed. The review begins with a treatment of the fundamentals of stellar optical interferometry, the origin, properties, optical effects of turbulence in the Earth's atmosphere, the passive methods that are applied on a single telescope to overcome atmospheric image degradation such as speckle interferometry, and various other techniques. These topics include differential speckle interferometry, speckle spectroscopy and polarimetry, phase diversity, wavefront shearing interferometry, phase-closure methods, dark speckle imaging, as well as the limitations imposed by the detectors on the performance of speckle imaging. A brief account is given of the technological innovation of adaptive-optics (AO) to compensate such atmospheric effects on the image in real time. A major advancement involves the transition from single-aperture to the dilute-aperture interferometry using multiple telescopes. Therefore, the review deals with recent developments involving ground-based, and space-based optical arrays. Emphasis is placed on the problems specific to delay-lines, beam recombination, polarization, dispersion, fringe-tracking, bootstrapping, coherencing and cophasing, and recovery of the visibility functions. The role of AO in enhancing visibilities is also discussed. The applications of interferometry, such as imaging, astrometry, and nulling are described. The mathematical intricacies of the various `post-detection' image-processing techniques are examined critically. The review concludes with a discussion of the astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics, 2002, to appear in April issu

A Calibration System for a Rayleigh Laser Guide Star Constellation & a Planet around a Giant Star

My thesis consists of two parts. In Part I, I describe my contributions to the laser guide star adaptive optics facility at the Large Binocular Telescope, the Advanced Rayleigh guided Ground layer adaptive Optics System , ARGOS. I investigate the effect of scattered light from propagating the laser light across the aperture, and find that contamination is not a concern for the science instruments. I present a study of reflective laser launch telescopes based on commercial optics, and compare this to the refractive design chosen as baseline for ARGOS. The discussed options present an interesting alternative with only small additional light loss at substantially reduced cost and procurement risk. I develop a calibration scheme for the full adaptive optics system, based on artificial light sources that illuminate the deformable mirror, imitating the laser guide star beacons. This enables the interaction between deformable mirror and wavefront sensor to be calibrated at any time, greatly enhancing the possibilities and time available for engineering on the installed system. The light source has to be placed in the prime fcous, as ARGOS uses the adaptive secondary mirrors of LBT. The optical design of the calibration light source is not trivial as the guide star constellation comprises of three beacons forming an equilateral triangle on a circle with a radius of 2 arcminutes. The images of the beacons that have to be reproduced suffer from strong aberrations caused by the large off-axis distance. To match the wavefront of the beacons' images to the desired precision of 50 nanometres rms, I designed a custom objective, incorporating a computer generated hologram to shape the wavefronts of three optical fibres forming the light sources. The elliptical front surface of the objective is used in reflection to generate a central, diffraction limited spot serving as an alignment aid and truth sensor for the measurements. A thorough tolerance analysis including the assembly and the alignment at the telescope ensures that the design specifications can be met during operation. The second part of the thesis concerns the search for planets around a sample of nearby giant stars with the Doppler technique carried out at Lick Observatory's CAT telescope. I analyse new data from the previous three years, which together with the existing data from our survey form a database covering 11 years. The radial velocity measurements reveal the presence of a planetary companion around one of our target stars, which was previously unknown. This is the lightest planet found around a giant star to date with a minimum mass of 1.92 Jupiter masses in a 785 day orbit

Review of small-angle coronagraphic techniques in the wake of ground-based second-generation adaptive optics systems

Small-angle coronagraphy is technically and scientifically appealing because it enables the use of smaller telescopes, allows covering wider wavelength ranges, and potentially increases the yield and completeness of circumstellar environment – exoplanets and disks – detection and characterization campaigns. However, opening up this new parameter space is challenging. Here we will review the four posts of high contrast imaging and their intricate interactions at very small angles (within the first 4 resolution elements from the star). The four posts are: choice of coronagraph, optimized wavefront control, observing strategy, and post-processing methods. After detailing each of the four foundations, we will present the lessons learned from the 10+ years of operations of zeroth and first-generation adaptive optics systems. We will then tentatively show how informative the current integration of second-generation adaptive optics system is, and which lessons can already be drawn from this fresh experience. Then, we will review the current state of the art, by presenting world record contrasts obtained in the framework of technological demonstrations for space-based exoplanet imaging and characterization mission concepts. Finally, we will conclude by emphasizing the importance of the cross-breeding between techniques developed for both ground-based and space-based projects, which is relevant for future high contrast imaging instruments and facilities in space or on the ground