Eduardo Bendek

HAWK-I: the high-acuity wide-field K-band imager for the ESO Very Large Telescope

We describe the design, development, and performance of HAWK-I, the new High-Acuity Wide-field K-band Imager for ESO’s Very Large Telescope, which is equipped with a mosaic of four 2 k × 2 k arrays and operates from 0.9−2.4 μm over 7.5 � × 7.5 � with 0.1 �� pixels. A novel feature is the use of all reflective optics that, together with filters of excellent throughput and detectors of high quantum efficiency, has yielded an extremely high throughput. Commissioning and science verification observations have already delivered a variety of excellent and deep images that demonstrate its high scientific potential for addressing important astrophysical questions of current interest.

A ground-layer adaptive optics system with multiple laser guide stars

To determine the influence of the environment on star formation, we need to study the process in the extreme conditions of massive young star clusters (∼104 solar masses) near the centre of our own Galaxy. Observations must be carried out in the near infrared because of very high extinction in visible light within the Galactic plane. We need high resolution to identify cluster members from their peculiar motions, and because most such clusters span more than 1′, efficient observation demands a wide field of view. There is at present no space-based facility that meets all these criteria. Ground-based telescopes can in principle make such observations when fitted with ground-layer adaptive optics (GLAO), which removes the optical aberration caused by atmospheric turbulence up to an altitude of ∼500 m (refs 7–10). A GLAO system that uses multiple laser guide stars has been developed at the 6.5-m MMT telescope, in Arizona. In previous tests, the system improved the resolution of the telescope by 30–50%, limited by wavefront error in the optics, but that was insufficient to allow rapid determination of cluster membership. Here we report observations of the core of the globular cluster M3 made after commissioning a sensor to monitor and remove slowly varying aberration in the optics. In natural seeing of 0.7′′, the point spread function at 2.2-μm wavelength was sharpened uniformly to 0.3′′ over a field of at least 2′. The wide-field resolution was enhanced by a factor of two to three over previous work, with better uniformity, and extends to a wavelength of 1.2 μm. Entire stellar clusters may be examined in a single pointing, and cluster membership can be determined from two such observations separated by just one year.

HIGH PRECISION ASTROMETRY WITH A DIFFRACTIVE PUPIL TELESCOPE

Astrometric detection and mass determination of Earth-mass exoplanets requires sub-µas accuracy, which is theoretically possible with an imaging space telescope using field stars as an astrometric reference. The measurement must however overcome astrometric distortions which are much larger than the photon noise limit. To address this issue, we propose to generate faint stellar diffraction spikes using a teo-dimensional grid of regularly spaced small dark spots added to the surface of the primary mirror (PM). Accurate astrometric motion of the host star is obtained by comparing the position of the spikes to the background field stars. The spikes do not contribute to scattered light in the central part of the field and therefore allow unperturbed coronagraphic observation of the star’s immediate surrounding. Because the diffraction spikes are created on the PM and imaged on the same focal plane detector as the background stars, astrometric distortions affect equally the diffraction spikes and the background stars, and are therefore calibrated. We describe the technique, detail how the data collected by the wide-field camera are used to derive astrometric motion, and identify the main sources of astrometric error using numerical simulations and analytical derivations. We find that the 1.4 m diameter telescope, 0.3 deg 2 field we adopt as a baseline design achieves 0.2 µas single measurement astrometric accuracy. The diffractive pupil concept thus enables sub-µas astrometry without relying on the accurate pointing, external metrology or high stability hardware required with previously proposed high precision astrometry concepts. Subject headings: astrometry — telescopes — techniques: high angular resolution — planets and satellites: detection

First light of the ESO laser guide star facility

Two teams of scientists and engineers at Max Planck Institut fuer Extraterrestrische Physik and at the European Southern Observatory have joined forces to design, build and install the Laser Guide Star Facility for the VLT. The Laser Guide Star Facility has now been completed and installed on the VLT Yepun telescope at Cerro Paranal. In this paper we report on the first light and first results from the Commissioning of the LGSF.

How to directly image a habitable planet around Alpha Centauri with a ~30-45cm space telescope

Several mission concepts are being studied to directly image planets around nearby stars. It is commonly thought that directly imaging a potentially habitable exoplanet around a Sun-like star requires space telescopes with apertures of at least 1m. A notable exception to this is Alpha Centauri (A and B), which is an extreme outlier among FGKM stars in terms of apparent habitable zone size: the habitable zones are ~3x wider in apparent size than around any other FGKM star. This enables a ~30-45cm visible light space telescope equipped with a modern high performance coronagraph or starshade to resolve the habitable zone at high contrast and directly image any potentially habitable planet that may exist in the system. We presents a brief analysis of the astrophysical and technical challenges involved with direct imaging of Alpha Centauri with a small telescope and describe two new technologies that address some of the key technical challenges. In particular, the raw contrast requirements for such an instrument can be relaxed to 1e-8 if the mission spends 2 years collecting tens of thousands of images on the same target, enabling a factor of 500-1000 speckle suppression in post processing using a new technique called Orbital Difference Imaging (ODI). The raw light leak from both stars is controllable with a special wavefront control algorithm known as Multi-Star Wavefront Control (MSWC), which independently suppresses diffraction and aberrations from both stars using independent modes on the deformable mirror. We also show an example of a small coronagraphic mission concept to take advantage of this opportunity.

SIMULTANEOUS EXOPLANET CHARACTERIZATION AND DEEP WIDE-FIELD IMAGING WITH A DIFFRACTIVE PUPIL TELESCOPE

High-precision astrometry can identify exoplanets and measure their orbits and masses while coronagraphic imaging enables detailed characterization of their physical properties and atmospheric compositions through spectroscopy. In a previous paper, we showed that a diffractive pupil telescope (DPT) in space can enable sub-μas accuracy astrometric measurements from wide-field images by creating faint but sharp diffraction spikes around the bright target star. The DPT allows simultaneous astrometric measurement and coronagraphic imaging, and we discuss and quantify in this paper the scientific benefits of this combination for exoplanet science investigations: identification of exoplanets with increased sensitivity and robustness, and ability to measure planetary masses to high accuracy. We show how using both measurements to identify planets and measure their masses offers greater sensitivity and provides more reliable measurements than possible with separate missions, and therefore results in a large gain in mission efficiency. The combined measurements reliably identify potentially habitable planets in multiple systems with a few observations, while astrometry or imaging alone would require many measurements over a long time baseline. In addition, the combined measurement allows direct determination of stellar masses to percent-level accuracy, using planets as test particles. We also show that the DPT maintains the full sensitivity of the telescope for deep wide-field imaging, and is therefore compatible with simultaneous scientific observations unrelated to exoplanets. We conclude that astrometry, coronagraphy, and deep wide-field imaging can be performed simultaneously on a single telescope without significant negative impact on the performance of any of the three techniques.

Compatibility of a diffractive pupil and coronagraphic imaging

Detection and characterization of exo-Earths require direct imaging techniques that can deliver contrast ratios of 1010 at 100 mas or smaller angular separation. At the same time, astrometric data is required to measure planet masses and to help detect planets and constrain their orbital parameters. To minimize costs, a single space mission can be designed using a high-efficiency coronagraph to perform direct imaging and a diffractive pupil to calibrate wide field distortions to enable high-precision astrometric measurements. This article reports the testing of a diffractive pupil on the high-contrast test bed at the NASA Ames Research Center to assess the compatibility of using a diffractive pupil with coronagraphic imaging systems. No diffractive contamination was found within our detectability limit of 2 × 10-7 contrast outside a region of 12 λ/D and 2.5 × 10-6 within a region spanning from 2 to 12 λ/D. Morphology of the image features suggests that no contamination exists even beyond the detectability limit specified or at smaller working angles. In the case that diffractive contamination is found beyond these stated levels, active wavefront control would be able to mitigate its intensity to 10-7 or better contrast.

Techniques for High-contrast Imaging in Multi-star Systems. II. Multi-star Wavefront Control

Direct imaging of exoplanets represents a challenge for astronomical instrumentation due to the high-contrast ratio and small angular separation between the host star and the faint planet. Multi-star systems pose additional challenges for coronagraphic instruments due to the diffraction and aberration leakage caused by companion stars. Consequently, many scientifically valuable multi-star systems are excluded from direct imaging target lists for exoplanet surveys and characterization missions. Multi-star wavefront control (MSWC) is a technique that uses a coronagraphic instruments deformable mirror (DM) to create high-contrast regions in the focal plane in the presence of multiple stars. Our previous paper introduced the Super-Nyquist Wavefront Control (SNWC) technique that uses a diffraction grating to enable the DM to generate high-contrast regions beyond the nominal region correctable by the DM. These two techniques can be combined to generate high-contrast regions for multi-star systems at any angular separation. As a case study, a high-contrast wavefront control (WC) simulation that applies these techniques shows that the habitable region of the Alpha Centauri system can be imaged reaching at least

Demonstration of broadband contrast at 1.2λ/D and greater for the EXCEDE starlight suppression system

8 \times 10^{-9}

Status of the 6.5m MMT Telescope laser adaptive optics system

mean contrast in 10\% broadband light in one-sided dark holes from 1.6-5.5