Adaptive Optics for Extremely Large Telescopes

Adaptive Optics has become a key technology for the largest ground-based telescopes currently under or close to begin of construction. Adaptive optics is an indispensable component and has basically only one task, that is to operate the telescope at its maximum angular resolution, without optical degradations resulting from atmospheric seeing. Based on three decades of experience using adaptive optics usually as an add-on component, all extremely large telescopes and their instrumentation are designed for diffraction limited observations from the very beginning. This review illuminates the various approaches of the Extremely Large Telescope, the Giant Magellan Telescope, and the Thirty-Meter Telescope, to fully integrate adaptive optics in their designs. The article concludes with a brief look into the requirements that high-contrast imaging poses on adaptive optics.Comment: 29 pages, 13 figures, published in Journal of Astronomical Instrumentation, November 2, 201

AstraLux: the Calar Alto Lucky Imaging Camera

AstraLux is the Lucky Imaging camera for the Calar Alto 2.2-m telescope, based on an electron-multiplying high speed CCD. By selecting only the best 1-10% of several thousand short exposure frames, AstraLux provides nearly diffraction limited imaging capabilities in the SDSS i' and z' filters over a field of view of 24x24 arcseconds. By choosing commercially available components wherever possible, the instrument could be built in short time and at comparably low cost. We present the instrument design, the data reduction pipeline, and summarise the performance and characteristicsComment: 12 pages, 7 figures, to appear in "Ground-based and Airborne Instrumentation for Astronomy II" SPIE conference, Marseille, 23-28 June 200

The AstraLux large M dwarf survey

AstraLux is the Lucky Imaging camera for the Calar Alto 2.2-m telescope and the 3.5-m NTT at La Silla. It allows nearly diffraction limited imaging in the SDSS i' and z' bands of objects as faint as i'=15.5mag with minimum technical effort. One of the ongoing AstraLux observing programs is a binarity survey among late-type stars with spectral types K7 to M8, covering more than 1000 targets on the northern and southern hemisphere. The survey is designed to refine binarity statistics and especially the dependency of binarity fraction on spectral type. The choice of the SDSS i' and z' filters allows to obtain spectral type and mass estimates for resolved binaries. With an observing efficiency of typically 6 targets per hour we expect to complete the survey in mid-2009. Selected targets will be followed up astrometrically and photometrically, contributing to the calibration of the mass-luminosity relation at the red end of the main sequence and at visible wavelengths.Comment: 4 pages, 3 figures. To appear in proceedings of Cool Stars 15 conference, St. Andrews, 200

A comprehensive examination of the Eps Eri system -- Verification of a 4 micron narrow-band high-contrast imaging approach for planet searches

Due to its proximity, youth, and solar-like characteristics with a spectral type of K2V, Eps Eri is one of the most extensively studied systems in an extrasolar planet context. Based on radial velocity, astrometry, and studies of the structure of its circumstellar debris disk, at least two planetary companion candidates to Eps Eri have been inferred in the literature (Eps Eri b, Eps Eri c). Some of these methods also hint at additional companions residing in the system. Here we present a new adaptive optics assisted high-contrast imaging approach that takes advantage of the favourable planet spectral energy distribution at 4 microns, using narrow-band angular differential imaging to provide an improved contrast at small and intermediate separations from the star. We use this method to search for planets at orbits intermediate between Eps Eri b (3.4 AU) and Eps Eri c (40 AU). The method is described in detail, and important issues related to the detectability of planets such as the age of Eps Eri and constraints from indirect measurements are discussed. The non-detection of companion candidates provides stringent upper limits for the masses of additional planets. Using a combination of the existing dynamic and imaging data, we exclude the presence of any planetary companion more massive than 3 Mjup anywhere in the Eps Eri system. Specifically, with regards to the possible residual linear radial velocity trend, we find that it is unlikely to correspond to a real physical companion if the system is as young as 200 Myr, whereas if it is as old as 800 Myr, there is an allowed semi-major axis range between about 8.5 and 25 AU.Comment: 11 pages, 8 figures, A&A accepte

Binaries among low-mass stars in nearby young moving groups

The solar galactic neighbourhood contains a number of young co-moving associations of stars (so-called `young moving groups') with ages of ~10--150 Myr, which are prime targets for a range of scientific studies, including direct imaging planet searches. The late-type stellar population of such groups still remain in their pre-main sequence phase, and are thus well suited for purposes such as isochronal dating. Close binaries are particularly useful in this regard, since they allow for a model-independent dynamical mass determination. Here we present a dedicated effort to identify new close binaries in nearby young moving groups, through high-resolution imaging with the AstraLux Sur Lucky Imaging camera. We surveyed 181 targets, resulting in the detection of 61 companions or candidates, of which 38 are new discoveries. An interesting example of such a case is 2MASS J00302572-6236015 AB, which is a high-probability member of the Tucana-Horologium moving group, and has an estimated orbital period of less than 10 years. Among the previously known objects is a serendipitous detection of the deuterium burning boundary circumbinary companion 2MASS J01033563-5515561 (AB)b in the z'-band, thereby extending the spectral coverage for this object down to near-visible wavelengths.Comment: 12 pages, 3 figures, accepted for publication in A&

The discrepancy between dynamical and theoretical mass in the triplet-system 2MASS J10364483+1521394

We combine new Lucky Imaging astrometry from NTT/AstraLux Sur with already published astrometry from the AstraLux Large M-dwarf Multiplicity Survey to compute orbital elements and individual masses of the 2MASS J10364483+1521394 triple system belonging to the Ursa-Major moving group. The system consists of one primary low-mass M-dwarf orbited by two less massive companions, for which we determine a combined dynamical mass of $M_{\rm{B}+\rm{C}}= 0.48 \pm 0.14\ M_\odot$. We show from the companions relative motions that they are of equal mass (with a mass ratio of$1.00 \pm 0.03$), thus$0.24 \pm 0.07\ M_\odot$individually, with a separation of$3.2 \pm 0.3\ $AU and conclude that these masses are significantly higher ($30\%$) than what is predicted by theoretical stellar evolutionary models. The biggest uncertainty remains the distance to the system, here adopted as$20.1 \pm 2.0$pc based on trigonometric parallax, whose ambiguity has a major impact on the result. With the new observational data we are able to conclude that the orbital period of the BC pair is$8.41^{+0.04}_{-0.02}\ $years.Comment: 9 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

Single conjugate adaptive optics for the ELT instrument METIS

The ELT is a 39m large, ground-based optical and near- to mid-infrared telescope under construction in the Chilean Atacama desert. Operation is planned to start around the middle of the next decade. All first light instruments will come with wavefront sensing devices that allow control of the ELT's intrinsic M4 and M5 wavefront correction units, thus building an adaptive optics (AO) system. To take advantage of the ELT's optical performance, full diffraction-limited operation is required and only a high performance AO system can deliver this. Further technically challenging requirements for the AO come from the exoplanet research field, where the task to resolve the very small angular separations between host star and planet, has also to take into account the high-contrast ratio between the two objects. We present in detail the results of our simulations and their impact on high-contrast imaging in order to find the optimal wavefront sensing device for the METIS instrument. METIS is the mid-infrared imager and spectrograph for the ELT with specialised high-contrast, coronagraphic imaging capabilities, whose performance strongly depends on the AO residual wavefront errors. We examined the sky and target sample coverage of a generic wavefront sensor in two spectral regimes, visible and near-infrared, to pre-select the spectral range for the more detailed wavefront sensor type analysis. We find that the near-infrared regime is the most suitable for METIS. We then analysed the performance of Shack-Hartmann and pyramid wavefront sensors under realistic conditions at the ELT, did a balancing with our scientific requirements, and concluded that a pyramid wavefront sensor is the best choice for METIS. For this choice we additionally examined the impact of non-common path aberrations, of vibrations, and the long-term stability of the SCAO system including high-contrast imaging performance.Comment: 37 pages, 27 figures, accepted for publication in Experimental Astronom

Atmosphere-like turbulence generation with surface-etched phase-screens

We built and characterized an optical system that emulates the optical characteristics of an 8m-class telescope like the Very Large Telescope. The system contains rotating glass phase-screens to generate realistic atmosphere-like optical turbulence, as needed for testing multi-conjugate adaptive optics systems. In this paper we present an investigation of the statistical properties of two phase-screens etched on glass-plate surfaces, obtained from Silios Technologies. Those etched screens are highly transmissive (above 85%) from 0.45 to 2.5 microns. From direct imaging, their Fried parameter r0 values (0.43+-0.04 mm and 0.81+-0.03 mm, respectively, at 0.633 microns) agree with the expectation to within 10%. This is also confirmed by a comparison of measured and expected Zernike coefficient variances. Overall, we find that those screens are quite reproducible, allowing sub-millimetre r0 values, which were difficult to achieve in the past. We conclude that the telescope emulator and phase-screens form a powerful atmospheric turbulence generator allowing systematic testing of different kinds of AO instrumentation.Comment: 10 pages, 8 figures, 3 mpeg movies. Submitted to Optics Expres