Wenli Xu

LUCIFER : a multi-mode NIR instrument for the LBT

LUCIFER (LBT NIR-Spectroscopic Utility with Camera and Integral-Field Unit for Extragalactic Research) is a NIR spectrograph and imager for the Large Binocular Telescope (LBT) on Mt. Graham, Arizona. It is built by a consortium of five German institutes and will be one of the first light instruments for the LBT. Later, a second copy for the second mirror of the telescope will follow. Both instruments will be mounted at the bent Gregorian foci of the two individual telescope mirrors. The final design of the instrument is presently in progress. LUCIFER will work at cryogenic temperature in the wavelength range from 0.9 μm to 2.5 μm. It is equipped with three exchangeable cameras for imaging and spectroscopy: two of them are optimized for seeing-limited conditions, the third camera for the diffraction-limited case with the LBT adaptive secondary mirror working. The spectral resolution will allow for OH suppression. Up to 33 exchangeable masks will be available for longslit and multi-object spectroscopy (MOS) over the full field of view (FOV). The detector will be a Rockwell HAWAII-2 HgCdTe-array.

CARMENES: Calar Alto high-resolution search for M dwarfs with exo-earths with a near-infrared Echelle spectrograph

CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) is a next-generation instrument to be built for the 3.5m telescope at the Calar Alto Observatory by a consortium of Spanish and German institutions. Conducting a five-year exoplanet survey targeting ~ 300 M stars with the completed instrument is an integral part of the project. The CARMENES instrument consists of two separate spectrographs covering the wavelength range from 0.52 to 1.7 μm at a spectral resolution of R = 85, 000, fed by fibers from the Cassegrain focus of the telescope. The spectrographs are housed in a temperature-stabilized environment in vacuum tanks, to enable a 1m/s radial velocity precision employing a simultaneous ThAr calibration.

LINC-NIRVANA: a Fizeau beam combiner for the large binocular telescope

Fizeau interferometry at the Large Binocular Telescope (LBT) offers significant advantages over other facilities in terms of spatial resolution, field of view, and sensitivity. We provide an update of the LINC-NIRVANA project, which aims to bring a near-infrared and visible wavelength Fizeau beam combiner to the LBT by late 2005. As with any complex instrument, a number of detailed requirements drive the final design adopted.

A visible MCAO channel for NIRVANA at the LBT

In order to achieve moderate Field of View (2 arcmin in diameter) and nearly diffraction limited capabilities, at the reddest portion of the visible spectrum in the interferometric mode of LBT, two sophisticated MCAO channels are required. These are being designed to perform a detailed correction of the atmospheric turbulence through three deformable mirrors per telescope arm: the secondary adaptive mirror and two commercial piezostack mirrors, leading to an overall number of degree of freedom totaling ~ 3000. A combination of numerical and optical coaddition of light collected from natural reference stars located inside the scientific Field of View and in an annular region, partially vignetted, and extending up to ≈ 6 arcmin in diameter, allows for such a performance with individual loops characterized by a much smaller number of degree of freedom, making the real-time computation, although still challenging, to more reasonable levels. We implement in the MCAO channel the dual Field of View layer-oriented approach using natural guide stars, only allowing for limited, but significant, sky coverage.

LUCIFER: a NIR spectrograph and imager for the LBT

LUCIFER is a full cryogenic NIR spectrograph and imager to be built by a consortium of fiber institutes, Max Planck Institut fuer Astronomie in Heidelberg, Max Planck Institut fuer Extraterrestrische Physik in Garching, Astronomisches Institut der Ruhr Universitaet Bochum and Fachhochschule fuer Technik und Gestaltung in Mannheim. The instrument has been selected as one of three first-light instruments for the Large Binocular Telescope on Mt. Graham, Arizona which first mirror becomes available to the community in early 2003. The second mirror and a second more or less identical spectrograph/imager follows 18 months later. Both LUCIFER instruments will be mounted dat the bent Gregorian foci of the two individual LBT-mirrors and include six observing six observing modes: seeing and diffraction limited imaging, seeing and diffraction limited longslit spectroscopy, seeing limited multi-object spectroscopy and integral-field spectroscopy. The detector will be a Rockwell HAWAII-2 HgCdTe-array with a pixel-size of 18(mu) .

Commissioning of the FORS instruments at the ESO VLT

FORS is an all dioptric focal reducer designed for direct imaging, low-dispersion multi-object spectroscopy, imaging polarimetry and spectropolarimetry of faint objects. Two almost identical copies of the instrument were built by a consortium of three astronomical institutes under contract and in cooperation with ESO. FORS1 was installed in September 1998 and FORS2 in October 1999 at the Cassegrain foci of the ESO VLT unit telescope nos. 1 and 2. FORS1 is in regular operation since April 1999. Regular observation with FORS2 are scheduled to begin in April 2000.

Broadband, static wave-front generation: Na-Ag ion-exchange phase screens and telescope emulation.

We test the statistical properties of static, atmospherelike wave fronts in glass that allow repeatable testing of astronomical adaptive optics instrumentation. The technology is mask-structured ion exchange (MSI) in glass and has significant advantages over other transmissive technologies. The screens are easy to clean, are insensitive to ambient temperature changes, and have high optical-to-near-infrared transmission. However, the effective coherence length (r0) on each of the fabricated screens is systematically too large or, equivalently, the fabricated aberrations are too weak. Despite this strong caveat, the screens appear to be quite useful: Long-exposure point-spread functions have realistic shapes, and power spectrum indices closely match those of the computer-generated wave fronts. Most significant, stacking screens with similar r0 values reduced r0 by the amount predicted by turbulence theory. The refractivity of MSI screens remains unmeasured. Finally, we present our design of an optical system that emulates the key characteristics of the Very Large Telescope, made to contain glass phase screens and to mimic an array of stars for multiconjugate adaptive optics system testing.

LINC-NIRVANA: Optical design of an interferometric imaging camera

Combining the two 8.4 m telescopes of the Large Binocular Telescope 1(LBT) offers the unique possibility to achieve diffraction limited images with 23 m spatial resolution. This requires an interferometric superposition of the two telescope beams in a Fizeau-type interferometer. LINC-NIRVANA delivers a 10 arcsec x 10 arcsec panoramic field of view with 5 mas pixel size. In addition to delivering diffraction limited, single-telescope images, the optics have several additional constraints imposed by interferometric operation. In this paper, we describe the evolution of the optical design and how the individual optical subsystems were developed in parallel to provide optimal combined performance. We also present an alignment strategy to setup the optics and to achieve zero optical path difference.

LINC-NIRVANA: the single arm MCAO experiment

LINC-NIRVANA is an imaging interferometer for the Large Binocular Telescope (LBT) and will make use of multi-conjugated adaptive optics (MCAO) with two 349 actuators deformable mirrors (DM), two 672 actuator deformable secondary mirrors and a total of 4 wavefront sensors (WFS) by using 8 or 12 natural guide stars each. The goal of the MCAO is to increase sky coverage and achieve a medium Strehl-ratio over the 2 arcmin field of view. To test the concepts and prototypes, a laboratory setup of one MCAO arm is being built. We present the layout of the MCAO prototype, planned and accomplished tests, especially for the used Xinetics DMs, and a possible setup for a test on sky with an existing 8m class telescope.

Phase screens for astronomical multiconjugate adaptive optics: application to MAPS

In this paper, we review the salient facts for a range of available atmosphere emulation technologies, and in the framework of the ESO Multi-Conjugate-AO demonstrator project, aptly called MAD, we present our phase screen test results for silver-sodium ion-exchange, transmissive phase screens. We find (a) that the measured power spectrum of phase fluctuations is consistent with the input Von Karman spectrum and (b) that by tracking the best focus of ten spots formed by a silver-sodium ion-exchange micro-lens array, it was found that the wavelength dependence of 1.266μm of phase-shift is 1.5±2.5% relative to air in the wavelength range 550nm to 800μm. Additionally, we present our optical design and specifications for MAPS, the Multi-Atmospheric Phase screens and Stars instrument that will be used to test MAD before shipment to the VLT. It includes glass screens conjugate to the 0.25km, 3.0km, and 9.0km atmospheric layers above the telescope. We explain the reasoning behind the choice of pupil size and implications for phase screen proximity, footprint sizes, and wind speed gradients. Our design mimics the VLT Nasmyth F/15 focal plane in terms of plate scale, field of view, high Strehl, and field curvature.