ARGOS the Advanced Rayleigh guided Ground layer adaptive Optics System for the LBT (Large Binocular Telescope)
is built by a German-Italian-American consortium. It will be a seeing reducer correcting the turbulence in the lower
atmosphere over a field of 2' radius. In such way we expect to improve the spatial resolution over the seeing of about a
factor of two and more and to increase the throughput for spectroscopy accordingly. In its initial implementation,
ARGOS will feed the two near-infrared spectrograph and imager - LUCI I and LUCI II.
The system consist of six Rayleigh lasers - three per eye of the LBT. The lasers are launched from the back of the
adaptive secondary mirror of the LBT. ARGOS has one wavefront sensor unit per primary mirror of the LBT, each of the
units with three Shack-Hartmann sensors, which are imaged on one detector.
In 2010 and 2011, we already mounted parts of the instrument at the telescope to provide an environment for the main
sub-systems. The commissioning of the instrument will start in 2012 in a staged approach. We will give an overview of
ARGOS and its goals and report about the status and new challenges we encountered during the building phase. Finally
we will give an outlook of the upcoming work, how we will operate it and further possibilities the system enables by
ARGOS, the laser-guided adaptive optics system for the Large Binocular Telescope (LBT), is now under construction at
the telescope. By correcting atmospheric turbulence close to the telescope, the system is designed to deliver high
resolution near infrared images over a field of 4 arc minute diameter. Each side of the LBT is being equipped with three
Rayleigh laser guide stars derived from six 18 W pulsed green lasers and projected into two triangular constellations
matching the size of the corrected field. The returning light is to be detected by wavefront sensors that are range gated
within the seeing-limited depth of focus of the telescope. Wavefront correction will be introduced by the telescope's
deformable secondary mirrors driven on the basis of the average wavefront errors computed from the respective guide
star constellation. Measured atmospheric turbulence profiles from the site lead us to expect that by compensating the
ground-layer turbulence, ARGOS will deliver median image quality of about 0.2 arc sec across the JHK bands. This will
be exploited by a pair of multi-object near-IR spectrographs, LUCIFER1 and LUCIFER2, with 4 arc minute field already
operating on the telescope. In future, ARGOS will also feed two interferometric imaging instruments, the LBT
Interferometer operating in the thermal infrared, and LINC-NIRVANA, operating at visible and near infrared
wavelengths. Together, these instruments will offer very broad spectral coverage at the diffraction limit of the LBT's
combined aperture, 23 m in size.
ARGOS is the Laser Guide Star adaptive optics system for the Large Binocular Telescope. Aiming for a wide field
adaptive optics correction, ARGOS will equip both sides of LBT with a multi laser beacon system and corresponding
wavefront sensors, driving LBT's adaptive secondary mirrors. Utilizing high power pulsed green lasers the artificial
beacons are generated via Rayleigh scattering in earth's atmosphere. ARGOS will project a set of three guide stars above
each of LBT's mirrors in a wide constellation. The returning scattered light, sensitive particular to the turbulence close to
ground, is detected in a gated wavefront sensor system. Measuring and correcting the ground layers of the optical
distortions enables ARGOS to achieve a correction over a very wide field of view. Taking advantage of this wide field
correction, the science that can be done with the multi object spectrographs LUCIFER will be boosted by higher spatial
resolution and strongly enhanced flux for spectroscopy. Apart from the wide field correction ARGOS delivers in its
ground layer mode, we foresee a diffraction limited operation with a hybrid Sodium laser Rayleigh beacon combination.
The Mid-Infrared Instrument (MIRI) of the James Webb Space Telescope, scheduled for launch in 2013, will provide a
variety of observing modes such as broad/narrow-band imaging, coronagraphy and low/medium resolution
spectroscopy. One filter wheel and two dichroic-grating wheel mechanisms allow to configure the instrument between
the different observing modes and wavelength ranges. The main requirements for the three mechanisms with up to 18
positions on the wheel include: (1) reliable operation at T ~ 7 K, (2) optical precision, (3) low power dissipation, (4)
high vibration capability, (5) functionality at 6 K < T < 300 K and (6) long lifetime (5-10 years). To meet these stringent
requirement, a space-proven mechanism design based on the European ISO mission and consisting of a central bearing
carrying the optical wheels, a central torque motor for wheel actuation, a ratchet system for precise and powerless
positioning and a magnetoresistive position sensor has been selected. We present here the detailed design of the flight
models and report results from the extensive component qualification.
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<sup>-4</sup>. As only 10<sup>-3</sup> 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<sup>®</sup> 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<sup>®</sup> 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<sup>®</sup> stage is equipped with capacitive sensors for the closed loop mode. The piezo modules are specially
designed for cryogenic application.
SUNRISE is a balloon-borne instrument for spectro-polarimetric high-resolution observation of the solar atmosphere. It has a lightweight UV-VIS telescope of Gregory type with an aperture of 1 m, designed to be close to the VIS diffraction limit. The paper will first present the basic prescriptions of the optical design and the achievable performance. The re-quirements for the mechanisms in order to maintain the alignment over the range of environmental conditions will be derived. Secondly, the structural and thermal requirements will be discussed. Here, structural deflections due to gravity and residual thermal imbalances have to be taken into account. Preliminary structural and thermal designs will be out-lined.
Four different designs for a polarising beamsplitter (BS) are compared under the aspect of their suitability for high resolution solar spectropolarimetry. The four designs are: A solution based on a Savart-plate, two air-spaced Wollaston prisms, and a glass beamsplitter cube with polarisation sensitive dielectric coating, and a single Wollaston prism inside a focal reducer. Using ray-tracing algorithms these beamsplitters are characterised with the help of spot diagrams for the two light paths of orthogonal polarisation.
It is shown that with the current spectrographs employed in solar research, the differential optical aberrations introduced by the beamsplitter are negligible, thanks to the slow F/#-ratio of existing solar telescopes and the limited field of view of currently used array detectors. It will, however, be demonstrated that with the new generation of large solar telescopes care must be exercised on the design of the beamsplitter. This will be shown using an example spectrograph, as could be used in a new 1.5 m class solar telescope.