Abstract
The direct detection and characterization of exoplanets will be a major scientific driver over the next decade, involving the development of very large telescopes that require high-contrast ...imaging close to the optical axis. Some complex techniques have been developed to improve the performance at small separations (coronagraphy, wavefront shaping, etc.). In this paper, we study some of the fundamental limitations of high contrast at the instrument design level, for cases that use a combination of a coronagraph and two deformable mirrors for wavefront shaping. In particular, we focus on small-separation point-source imaging (around 1 λ/D). First, we analytically or semi-analytically analyse the impact of several instrument design parameters: actuator number, deformable mirror locations and optic aberrations (level and frequency distribution). Second, we develop an in-depth Monte Carlo simulation to compare the performance of dark hole correction using a generic test-bed model to test the Fresnel propagation of multiple randomly generated optic static phase errors. We demonstrate that imaging at small separations requires a large setup and small dark hole size. The performance is sensitive to the amount of optic aberration and the spatial frequency distribution but shows a weak dependence on the actuator number or setup architecture when the dark hole is sufficiently small (from 1 to ≲ 5 λ/D).
ABSTRACT
Future large space- or ground-based telescopes will offer the resolution and sensitivity to probe the habitable zone of a large sample of nearby stars for exo-Earth imaging. To this end, ...such facilities are expected to be equipped with a high-contrast instrument to efficiently suppress the light from an observed star to image these close-in companions. These observatories will include features such as segmented primary mirrors, secondary mirrors, and struts, leading to diffraction effects on the star image that will limit the instrument contrast. To overcome these constraints, a promising method consists in combining coronagraphy and wavefront shaping to reduce starlight at small separations and generate a dark region within the image to enhance the exoplanet signal. We aim to study the limitations of this combination when observing short-orbit planets. Our analysis is focused on SPEED, the Nice test bed with coronagraphy, wavefront shaping with deformable mirrors (DMs), and complex telescope aperture shape to determine the main realistic parameters that limit contrast at small separations. We develop an end-to-end simulator of this bench with Fresnel propagation effects to study the impact of large phase and amplitude errors from the test-bed optical components and defects from the wavefront shaping system on the final image contrast. We numerically show that the DM finite stroke and non-functional actuators, coronagraph manufacturing errors, and near-focal-plane phase errors represent the major limitations for high-contrast imaging of exoplanets at small separations. We also show that a carefully defined optical set-up opens the path to high contrast at small separation.
Aims. The Antarctica Search for Transiting ExoPlanets (ASTEP) program was originally aimed at probing the quality of the Dome C, Antarctica for the discovery and characterization of exoplanets by ...photometry. In the first year of operation of the 40 cm ASTEP 400 telescope (austral winter 2010), we targeted the known transiting planet WASP-19b in order to try to detect its secondary transits in the visible. This is made possible by the excellent sub-millimagnitude precision of the binned data. Methods. The WASP-19 system was observed during 24 nights in May 2010. Once brought back from Antarctica, the data were processed using various methods, and the best results were with an implementation of the optimal image subtraction (OIS) algorithm. Results. The photometric variability level due to starspots is about 1.8% (peak-to-peak), in line with the SuperWASP data from 2007 (1.4%) and higher than in 2008 (0.07%). We find a rotation period of WASP-19 of 10.7 ± 0.5 days, in agreement with the SuperWASP determination of 10.5 ± 0.2 days. Theoretical models show that this can only be explained if tidal dissipation in the star is weak, i.e. the tidal dissipation factor Q'★ > 3×107. Separately, we find evidence of a secondary eclipse of depth 390 ± 190 ppm with a 2.0σ significance, a phase that is consistent with a circular orbit and a 3% false positive probability. Given the wavelength range of the observations (420 to 950 nm), the secondary transit depth translates into a day-side brightness temperature of 2690-220+150 K, in line with measurements in the z′ and K bands. The day-side emission observed in the visible could be due either to thermal emission of an extremely hot day side with very little redistribution of heat to the night side or to direct reflection of stellar light with a maximum geometrical albedo Ag = 0.27 ± 0.13. We also report a low-frequency oscillation in phase at the planet orbital period, but with a lower limit amplitude that could not be attributed to the planet phase alone and that was possibly contaminated with residual lightcurve trends. Conclusions. This first evidence of a secondary eclipse in the visible from the ground demonstrates the high potential of Dome C, Antarctica, for continuous photometric observations of stars with exoplanets. These continuous observations are required to understand star-planet interactions and the dynamical properties of exoplanetary atmospheres.
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ASTEP 400, the main instrument of the ASTEP (Antarctica Search for Transiting ExoPlanets) programme, is a 40 cm telescope, designed to withstand the harsh conditions in Antarctica, achieving a ...photometric accuracy of a fraction of millimagnitude on hourly time-scales for planet-hosting southern bright (R ∼ 12 mag) stars. We review the performances of this instrument, describe its operating conditions, and present results from the analysis of observations obtained during its first three years (2010–2012) of operation, before its repatriation in 2014. During this time, we observed a total of 22 stellar fields (1° × 1° field of view). Each field, in which we measured stars up to magnitude R = 18 mag, was observed continuously during ∼7 to ∼30 d. More than 200 000 frames were recorded and 310 000 stars processed, using an implementation of the optimal image subtraction photometry algorithm. We found 43 planetary transit candidates. 20 of these candidates were observed using spectroscopic follow-ups including four targets classified as good planet candidates. Our results demonstrate that accurate near-continuous photometric observations are achievable from the Concordia station at Dome C in Antarctica, even if we were not able to reach the nominal photometric precision of the instrument. We conducted a correlation analysis between the rms noise and a large number of external parameters and found that source of the ∼1 mmag correlated noise is not obvious and does not depend on a single parameter. However, our analysis provided some hints and guidance to increase the photometric accuracy of the instrument. These improvements should equip any future telescope operating in Antarctica.
Context. Dome C in Antarctica is a promising site for photometric observations thanks to the continuous night during the Antarctic winter and favorable weather conditions. Aims.We developed ...instruments to assess the quality of this site for photometry in the visible and to detect and characterize variable objects through the Antarctic Search for Transiting ExoPlanets (ASTEP) project. Methods. Here, we present the full analysis of four winters of data collected with ASTEP South, a 10 cm refractor pointing continuously toward the celestial south pole. We improved the instrument over the years and developed specific data reduction methods. Results. We achieved nearly continuous observations over the winters. We measure an average sky background of 20 mag arcsec−2 in the 579–642 nm bandpass. We built the lightcurves of 6000 stars and developed a model to infer the photometric quality of Dome C from the lightcurves themselves. The weather is photometric 67.1 ± 4.2% of the time and veiled 21.8 ± 2.0% of the time. The remaining time corresponds to poor quality data or winter storms. We analyzed the lightcurves of σ Oct and HD 184465 and find that the amplitude of their main frequency varies by a factor of 3.5 and 6.7 over the four years, respectively. We also identify 34 new variable stars and eight new eclipsing binaries with periods ranging from 0.17 to 81 days. Conclusion. The phase coverage that we achieved with ASTEP South is exceptional for a ground-based instrument and the data quality enables the detection and study of variable objects. These results demonstrate the high quality of Dome C for photometry in the visible and for time series observations in general.
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Context.
High-contrast imaging of exoplanets around nearby stars with future large-segmented apertures requires starlight suppression systems optimized for complex aperture geometries. Future ...extremely large telescopes (ELTs) equipped with high-contrast instruments operating as close as possible to the diffraction limit will open a bulk of targets in the habitable zone around M-stars. In this context, the phase-induced amplitude apodization complex mask coronagraph (PIAACMC) is a promising concept for high-efficiency coronagraphic imaging at small angular separations with segmented telescopes.
Aims.
The complex focal plane mask of the PIAACMC is a multi-zone, phase-shifting mask comprised of tiled hexagons that vary in depth. The mask requires micro-fabrication techniques because it is generally made of hundreds micron-scale hexagonal zones with depths ranging over a few microns. We aim to demonstrate that the complex focal plane mask of a PIAACMC with a small inner working angle can be designed and manufactured for segmented apertures.
Methods.
We report on the numerical design, specifications, manufacturing, and characterization of a PIAACMC complex focal plane mask for the segmented pupil experiment for exoplanet detection facility.
Results.
Our PIAACMC design offers an inner working angle of 1.3
λ
/
D
and is optimized for a 30% telescope-central-obscuration ratio including six secondary support structures (ESO/ELT design). The fabricated reflective focal plane mask is made of 499 hexagons, and the characteristic size of the mask features is 25
μ
m, with depths ranging over ±0.4
μ
m. The mask sag local deviation is measured to an average error of 3 nm and standard deviation of 6 nm rms. The metrological analysis of the mask using interferential microscopy gives access to an in-depth understanding of the component’s optical quality, including a complete mapping of the zone depth distribution zone-depth distribution. The amplitude of the errors in the fabricated mask are within the wavefront control dynamic range.
Conclusions.
We demonstrate the feasibility of fabricating and characterizing high-quality PIAA complex focal plane masks.
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Context. The Concordia base in Dome C, Antarctica, is an extremely promising site for photometric astronomy due to the 3-month long night during the Antarctic winter, favorable weather conditions, ...and low scintillation. Aims. The ASTEP project (Antarctic Search for Transiting ExoPlanets) is a pilot project to discover transiting planets and understand the limits of visible photometry from the Concordia site. Methods. ASTEP South is the first phase of the ASTEP project. The instrument is a fixed 10 cm refractor with a 4k $\times$ 4k CCD camera in a thermalized box, pointing continuously a 3.88 $\times$ 3.88°2 field of view centered on the celestial south pole. We describe the project and report results of a preliminary data analysis. Results. ASTEP South became fully functional in June 2008 and obtained 1592 hours of data during the 2008 Antarctic winter. The data are of good quality but the analysis has to account for changes in the PSF (point spread function) due to rapid ground seeing variations and instrumental effects. The pointing direction is stable within 10 arcsec on a daily timescale and drifts by only 34 arcsec in 50 days. A truly continuous photometry of bright stars is possible in June (the noon sky background peaks at a magnitude R ≈ 15 arcsec-2 on June 22), but becomes challenging in July (the noon sky background magnitude is R ≈ 12.5 arcsec-2 on July 20). The weather conditions are estimated from the number of stars detected in the field. For the 2008 winter, the statistics are between 56.3% and 68.4% of excellent weather, 17.9% to 30% of veiled weather (when the probable presence of thin clouds implies a lower number of detected stars) and 13.7% of bad weather. Using these results in a probabilistic analysis of transit detection, we show that the detection efficiency of transiting exoplanets in one given field is improved at Dome C compared to a temperate site such as La Silla. For example we estimate that a year-long campaign of 10 cm refractor could reach an efficiency of 69% at Dome C versus 45% at La Silla for detecting 2-day period giant planets around target stars from magnitude 10 to 15. The detection efficiency decreases for planets with longer orbital periods, but in relative sense it is even more favorable to Dome C. Conclusions. This shows the high potential of Dome C for photometry and future planet discoveries.
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