Near-infrared wavefront sensing for the VLT interferometer

The very large telescope (VLT) interferometer (VLTI) in its current operating state is equipped with high-order adaptive optics (MACAO) working in the visible spectrum. A low-order near-infrared wavefront sensor (IRIS) is available to measure non-common path tilt aberrations downstream the high-order deformable mirror. For the next generation of VLTI instrumentation, in particular for the designated GRAVITY instrument, we have examined various designs of a four channel high-order near-infrared wavefront sensor. Particular objectives of our study were the specification of the near-infrared detector in combination with a standard wavefront sensing system. In this paper we present the preliminary design of a Shack-Hartmann wavefront sensor operating in the near-infrared wavelength range, which is capable of measuring the wavefronts of four telescopes simultaneously. We further present results of our design study, which aimed at providing a first instrumental concept for GRAVITY.Comment: 10 pages, 7 figures, to appear in "Ground-based and Airborne Instrumentation for Astronomy II" SPIE conference, Marseille, 23-28 June 200

The GRAVITY Coud\'e Infrared Adaptive Optics (CIAO) system for the VLT Interferometer

GRAVITY is a second generation instrument for the VLT Interferometer, designed to enhance the near-infrared astrometric and spectro-imaging capabilities of VLTI. Combining beams from four telescopes, GRAVITY will provide an astrometric precision of order 10 micro-arcseconds, imaging resolution of 4 milli-arcseconds, and low and medium resolution spectro-interferometry, pushing its performance far beyond current infrared interfero- metric capabilities. To maximise the performance of GRAVITY, adaptive optics correction will be implemented at each of the VLT Unit Telescopes to correct for the effects of atmospheric turbulence. To achieve this, the GRAVITY project includes a development programme for four new wavefront sensors (WFS) and NIR-optimized real time control system. These devices will enable closed-loop adaptive correction at the four Unit Telescopes in the range 1.4-2.4 {\mu}m. This is crucially important for an efficient adaptive optics implementation in regions where optically bright references sources are scarce, such as the Galactic Centre. We present here the design of the GRAVITY wavefront sensors and give an overview of the expected adaptive optics performance under typical observing conditions. Benefiting from newly developed SELEX/ESO SAPHIRA electron avalanche photodiode (eAPD) detectors providing fast readout with low noise in the near-infrared, the AO systems are expected to achieve residual wavefront errors of \leq400 nm at an operating frequency of 500 Hz.Comment: to be published in Proc. SPIE vol. 8446 (2012

A novel athermal approach for high-performance cryogenic metal optics

This paper describes a new athermal approach for high performance metal optics, particularly with regard to extreme environmental conditions as they usually may occur in terrestrial as well as in space applications. Whereas for mid infrared applications diamond turned aluminium is the preferred mirror substrate, it is insufficient for the visual range. For applications at near infrared wavelengths (0.8 μm - 2.4 μm) as well as at on cryogenic temperatures (-200°C) requirements exist, which are only partially met for diamond turned substrates. In this context athermal concepts such as optical surfaces with high shape accuracy and small surface micro-roughness without diffraction effect and marginal loss of stray light, are of enormous interest. The novel, patented material combination matches the Coefficient of Thermal Expansion (CTE) of an aluminium alloy with high silicon content (AlSi, Si >= 40 %) as mirror substrate with the CTE of the electroless nickel plating (NiP). Besides the harmonization of the CTE (~ 13 * 10-6 K-1), considerable advantages are achieved due to the high specific stiffness of these materials. Hence, this alloy also fulfils an additional requirement: it is ideal for the manufacturing of very stable light weight metal mirrors. To achieve minimal form deviations occurring due to the bimetallic effect, a detailed knowledge of the thermal expansion behavior of both, the substrate and the NiP layer is essential. The paper describes the reduction of the bimetallic bending by the use of expansion controlled aluminium-silicon alloys and NiP as a polishing layer. The acquisition of CTE-measurement data, the finite elements simulations of light weight mirrors as well as planned interferometrical experiments under cryogenic conditions are pointed out. The use of the new athermal approach is described exemplary

A cryogenic dithering stage for moving SPHERE-IRDIS' detector

This paper describes the development of the detector motion stage for the instrument SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch). The detector movement is necessary because the instrument SPHERE has exceptional requirements on the flatfield accuracy: In order not to limit planetary detections, the photon response of every pixel with respect to the detector's mean response must be known to an accuracy of 10-4. As only 10-3 can be reached by calibration procedures, detector dithering is essential to apply ~100 pixels at a single spatial detection area and time-average the result to reduce the residual flatfield noise. We will explain the design of the unit including the detector package and report on extensive cold and warm tests of individual actuators. The novel, patented NEXLINE® drive actuator design combines long travel ranges (hundreds of millimeters) with high stiffness and high resolution (better than 0.1 nm). Coordinated motion of shear and longitudinal piezo elements is what allows NEXLINE® to break away from the limitations of conventional nanopositioning actuators. Motion is possible in two different modes: a high resolution, high dynamics analogue mode, and a step mode with theoretically unlimited travel range. The drive can always be brought to a condition with zero-voltage on the individual piezo elements and with the full holding force available to provide nanometer stability, no matter where it is along its travel range. The NEXLINE® stage is equipped with capacitive sensors for the closed loop mode. The piezo modules are specially designed for cryogenic application

Design Of And Progress Towards The Gravity Wave-front Sensors

International audienceThe GRAVITY instrument is a beam-combining interferometer for the four telescopes of the VLT, and relies upon four near-infrared (1.4-2.4 micron) Shack-Hartmann style wave-front sensors to determine the atmospheric distortion due to atmospheric turbulence. The GRAVITY AO system will then drive the VLT's MACAO deformable mirrors to correct the wavefront, permiting 10 micro-arcsecond astrometry. We present the current design and status of the wave front sensor system, as well as future plans for integration and test