Integration and alignment through mechanical measurements: the example of the ESPRESSO front-end units

Traditional techniques usually rely on optical feedback to align optical elements over all the degrees of freedom needed. This strongly iterative process implies the use of bulky and/or flexible adjustable mountings. Another solution under study consists in the characterization of every optomechanical elements and the integration of the parts without any optical feedback. The characterization can be performed using different 3D Coordinate Measuring Machines (like Laser Tracker, Articulated Arms and Cartesian ones) and referencing different parts like the optomechanical mounts or the optical surfaces. The alignment of the system is done adjusting the six degrees of freedom of every element with metallic shims. Those calibrated elements are used to correct the interfaces position of the semikinematic system composed by 3 screws and 3 pins. In this paper, the integration and alignment of the ESPRESSO Front End Units (FEUs) will be used as pathfinder to test different alignment methods and evaluate their performances

Alignment and integration of large optical systems based on advanced metrology

Optical alignment is a key activity in opto-mechanical system Integration. Traditional techniques require adjustable mounting, driven by optical references that allows the tuning of the optics position along all 6 Degree of Freedom. Nevertheless, the required flexibility imposes reduced stiffness and consequently less stability of the system. The Observatory of Brera (OAB) started few years ago a research activity focused onto the overcoming of this limits exploiting the high metrology performances of Coordinate Measuring Machines (CMM) with the main objectives of relax the manufacturing tolerances and maximize mounting stiffness. Through the T-REX grants, OAB acquired all the instrumentation needed for that activity furthermore considering the ESPRESSO project training and testing also oriented to large scale instrumentation like the E-ELT one. We will present in this paper the definition of the VLTs convergence point and the feasibility study of large mirrors alignment done by mechanical measurements methods

MMP, the Multi Mini Prism device for ESPRESSO APSU: prototyping and integration

The multi mini prism device is a crucial component of the Espresso Anamorphic Pupil Slicer (APSU). At the end of the slicer, is necessary to differently fold each field to correctly illuminate the echelle and this is made by cylindrical prisms glued onto a silica window. We present the integrated robotic system conceived to reach the required tolerances in term of alignment and integration. It consists in a tip/tilt stage to select the wedge angle, a rotational stage to select the right clock angle, coupled to an x-y stage to position the elements on the window and a z axis to perform the gluing

Integration, alignment, and verification of the ESPRESSO Front-End

ESPRESSO, Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, is now under the assembly, integration and verification phase and will be installed beginning next year at Paranal Observatory on ESO's Very Large Telescopes. The Front End is the modular system in the Combined Coudé Laboratory receiving the light from the four VLT Units, providing the needed connection between the input signal, i.e., object light, sky light, and calibration light, to feed the spectrograph through optical fibers. The modular concept of the FE Units drove the system design and the alignment workflow. We will show the integration method of the single FE modules adopted to guarantee the necessary repeatability between the different Units. The performances of the system in terms of image quality and encircled energy in the observed point spread function are reported. Finally, the strategy followed in the Paranal Combined Coudè Laboratory to define the convergence point of the four UTs is described, along with the procedure used to align the ground plates, the main structure, and the mode selector

Nightside condensation of iron in an ultra-hot giant exoplanet

Ultra-hot giant exoplanets receive thousands of times Earth's insolation. Their high-temperature atmospheres (>2,000 K) are ideal laboratories for studying extreme planetary climates and chemistry. Daysides are predicted to be cloud-free, dominated by atomic species and substantially hotter than nightsides. Atoms are expected to recombine into molecules over the nightside, resulting in different day-night chemistry. While metallic elements and a large temperature contrast have been observed, no chemical gradient has been measured across the surface of such an exoplanet. Different atmospheric chemistry between the day-to-night ("evening") and night-to-day ("morning") terminators could, however, be revealed as an asymmetric absorption signature during transit. Here, we report the detection of an asymmetric atmospheric signature in the ultra-hot exoplanet WASP-76b. We spectrally and temporally resolve this signature thanks to the combination of high-dispersion spectroscopy with a large photon-collecting area. The absorption signal, attributed to neutral iron, is blueshifted by -11+/-0.7 km s-1 on the trailing limb, which can be explained by a combination of planetary rotation and wind blowing from the hot dayside. In contrast, no signal arises from the nightside close to the morning terminator, showing that atomic iron is not absorbing starlight there. Iron must thus condense during its journey across the nightside.Comment: Published in Nature (Accepted on 24 January 2020.) 33 pages, 11 figures, 3 table

ESPRESSO front end: modular opto-mechanical integration for astronomical instrumentation

The opto-mechanical conceptual design for the Front-End unit and the calibration unit of the ESPRESSO Spectrograph is described in this paper. The front end system exploits a modular concept. Each FEU receive the beam directly from the relative Telescope Coudé Train and the calibration light from the calibration unit. On the other side the FEU feeds the fibers that carry the light to the spectrograph, corresponding in number and size to the scientific observing modes conceived for Espresso. The selection is made through a Toggling Unit. Purpose of the Front/End is to provide the needed connection between the input signal, i.e. Object light, Sky light, Calibration light, and the given output fiber in any of the foreseen observing modes

MMP: multi mini prism device for ESPRESSO APSU, prototyping, and integration

The multiprism device is a crucial component of the Espresso Anamorphic pupil Slicer (APSU). At the end of the slicer, is necessary to differently fold each field to correctly illuminate the echelle. The solution is made by gluing cylindrical prisms with proper bending low angle onto a support double plate silica window. We present here the integrated robotic system conceived to reach the required tolerances in term of alignment and Integration. It consist in a tip tilt stage to select the folding angle, coupled to an x-y stage to position the elements and a z axis to perform the gluing. Keywords: Extra-solar Planet Atmospheres, High Resolution Spectroscopy, Espresso, front En

BATMAN: a DMD-based multi-object spectrograph on Galileo telescope

Next-generation infrared astronomical instrumentation for ground-based and space telescopes could be based on MOEMS programmable slit masks for multi-object spectroscopy (MOS). This astronomical technique is used extensively to investigate the formation and evolution of galaxies. We are developing a 2048x1080 Digital-Micromirror-Device-based (DMD) MOS instrument to be mounted on the Galileo telescope and called BATMAN. A two-arm instrument has been designed for providing in parallel imaging and spectroscopic capabilities. The field of view (FOV) is 6.8 arcmin x 3.6 arcmin with a plate scale of 0.2 arcsec per micromirror. The wavelength range is in the visible and the spectral resolution is R=560 for 1 arcsec object (typical slit size). The two arms will have 2k x 4k CCD detectors. ROBIN, a BATMAN demonstrator, has been designed, realized and integrated. It permits to determine the instrument integration procedure, including optics and mechanics integration, alignment procedure and optical quality. First images and spectra have been obtained and measured: typical spot diameters are within 1.5 detector pixels, and spectra generated by one micro-mirror slits are displayed with this optical quality over the whole visible wavelength range. Observation strategies are studied and demonstrated for the scientific optimization strategy over the whole FOV. BATMAN on the sky is of prime importance for characterizing the actual performance of this new family of MOS instruments, as well as investigating the operational procedures on astronomical objects. This instrument will be placed on the Telescopio Nazionale Galileo mid-2015

BATMAN @ TNG: instrument integration and performance

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