Astrometry and Photometry with Coronagraphs

We propose a solution to the problem of astrometric and photometric calibration of coronagraphic images with a simple optical device which, in theory, is easy to use. Our design uses the Fraunhofer approximation of Fourier optics. Placing a periodic grid of wires (we use a square grid) with known width and spacing in a pupil plane in front of the occulting coronagraphic focal plane mask produces fiducial images of the obscured star at known locations relative to the star. We also derive the intensity of these fiducial images in the coronagraphic image. These calibrator images can be used for precise relative astrometry, to establish companionship of other objects in the field of view through measurement of common proper motion or common parallax, to determine orbits, and to observe disk structure around the star quantitatively. The calibrator spots also have known brightness, selectable by the coronagraph designer, permitting accurate relative photometry in the coronagraphic image. This technique, which enables precision exoplanetary science, is relevant to future coronagraphic instruments, and is particularly useful for `extreme' adaptive optics and space-based coronagraphy.Comment: To appear in ApJ August 2006, 27 preprint style pages 4 figure

Comparison of coronagraphs for high contrast imaging in the context of Extremely Large Telescopes

We compare coronagraph concepts and investigate their behavior and suitability for planet finder projects with Extremely Large Telescopes (ELTs, 30-42 meters class telescopes). For this task, we analyze the impact of major error sources that occur in a coronagraphic telescope (central obscuration, secondary support, low-order segment aberrations, segment reflectivity variations, pointing errors) for phase, amplitude and interferometric type coronagraphs. This analysis is performed at two different levels of the detection process: under residual phase left uncorrected by an eXtreme Adaptive Optics system (XAO) for a large range of Strehl ratio and after a general and simple model of speckle calibration, assuming common phase aberrations between the XAO and the coronagraph (static phase aberrations of the instrument) and non-common phase aberrations downstream of the coronagraph (differential aberrations provided by the calibration unit). We derive critical parameters that each concept will have to cope with by order of importance. We evidence three coronagraph categories as function of the accessible angular separation and proposed optimal one in each case. Most of the time amplitude concepts appear more favorable and specifically, the Apodized Pupil Lyot Coronagraph gathers the adequate characteristics to be a baseline design for ELTs.Comment: 12 pages, 6 figures, Accepted for publication in A&

Design, analysis and test of a microdots apodizer for the Apodized Pupil Lyot Coronagraph

Coronagraphic techniques are required to detect exoplanets with future Extremely Large Telescopes. One concept, the Apodized Pupil Lyot Coronagraph (APLC), is combining an apodizer in the entrance aperture and a Lyot opaque mask in the focal plane. This paper presents the manufacturing and tests of a microdots apodizer optimized for the near IR. The intent of this work is to demonstrate the feasibility and performance of binary apodizers for the APLC. This study is also relevant for any coronagraph using amplitude pupil apodization. A binary apodizer has been designed using a halftone dot process, where the binary array of pixels with either 0% or 100% transmission is calculated to fit the required continuous transmission, i.e. local transmission control is obtained by varying the relative density of the opaque and transparent pixels. An error diffusion algorithm was used to optimize the distribution of pixels that best approximates the required field transmission. The prototype was tested with a coronagraphic setup in the near IR. The transmission profile of the prototype agrees with the theoretical shape within 3% and is achromatic. The observed apodized and coronagraphic images are consistent with theory. However, binary apodizers introduce high frequency noise that is a function of the pixel size. Numerical simulations were used to specify pixel size in order to minimize this effect, and validated by experiment. This paper demonstrates that binary apodizers are well suited for being used in high contrast imaging coronagraphs. The correct choice of pixel size is important and must be adressed considering the scientific field of view.Comment: A&A accepted, 8 page

Peculiarity of Lyot Coronagraphy for Highly Segmented Apertures

The existing high contrast imaging techniques cannot be applied directly to the telescope of the diameter 30–100 m without taking into account the specific characteristics of a segmented surface. While the increase in telescope diameter is an advantage for the high contrast range science, the segmentation sets a limit on the performance of the coronagraph. In particular, diffraction from intersegment gaps sets a floor to the achievable extinction of the starlight. Masking out the gaps in the Lyot plane although helps increasing the contrast, does not solve completely the problem: the high spatial frequency component of the diffractive light remains. In the paper I suggest using a Lyot stop which produces a phase-amplitude modulation in order to reduce the effect of segmentation

Deformable mirror fitting error by correcting the segmented wavefronts

The ob jective of the present study is to estimate the ability of conventional adaptive optics (AO) to correct for a wavefront created by static errors randomly distributed over the population of a multi-segmented primary mirror for an extremely large telescope (ELT). Among all errors the deformable mirror fitting error is the factor which contributes the most to the performance degradation. We quantify this error through a ratio between two root mean square (RMS) values of the phase before and after the correction. We compare the results of several AO models: from the high-pass hard edge filter to the optical waves end-to-end modeling

Statistics of undevelop ed speckles in partially polarized light

We obtain an expression for the probability density function (PDF) of partially developed speckles formed by light with an arbitrary degree of polarization. From the probability density we calculate the detection threshold corresponding to the 5σ confidence level of a normal distribution. We show that unpolarized light has an advantage in high contrast imaging for low ratios of the deterministic part of the point spread function (DL PSF) to the halo, typical in coronagraphy

Time-of-flight modeling of transversal ultrasonic scan of wood

Time-of-flight is a time for an ultrasonic pulse to cross a sample. It contains valuable information about the mechanical properties of a material. For the ultrasonic pulse propagating in wood perpendicular to the grain the relation between the time-of-flight and the elastic constants is rather complex due to the strong anisotropicity of wood. With the help of some assumptions this relation can be established from the elastic theory. The analytical calculation results in a function which represents a change of time-offlight when the direction of propagation shifts from the radial to the tangential direction while scanning a board crosswise. The function takes into account the location of the pith and the geometry of the growth rings. The measurement performed on a sample of European spruce confirms the theoretical prediction.Structural EngineeringCivil Engineering and Geoscience

On-sky results of the ZEUS phasing sensor, closed-loop precision in the context of multi-wavelength measurements

International audienceThe Active Phasing Experiment (APE) was designed to test four different phasing techniques and to validate wavefront control concepts for Extremely Large Telescopes. One of the sensors is the ZErnike Unit for Segment phasing (ZEUS), which was successfully tested on-sky along with the rest of the APE experiment at one of the Nasmyth platforms of the Very Large Telescope (VLT) in 2009. During the four observing campaigns, multiple results were obtained in open-loop and in closed-loop at different wavelengths. We present in this paper an analysis of the multi-wavelength data in terms of piston measurement precision at the edges of the segments and on the reconstructed wavefront, and an analysis of the evolution of these errors in successive closed-loop runs at different wavelengths. This work demonstrates how the applied multi-wavelength algorithm leads to convergence, allowing phasing of segments with piston errors of several microns