MORFEO is a post-focal adaptive optics module that forms part of the first light instrument suite for the Extreme Large Telescope (ELT). The project is now in the Final Design Phase. In this paper, we report the status of the project.
MORFEO (Multi-conjugate adaptive Optics Relay For ELT Observations), formerly known as MAORY, is the Multi-Conjugated Adaptive Optics (MCAO) module for the Extremely Large Telescope (ELT). MORFEO is designed to feed the near-infrared camera MICADO (with the capability to feed a second port dedicated to a future instrument) with both MCAO and Single-Conjugated AO (SCAO) operation modes. The optical configuration provides a one-to-one imaging of the telescope focal surface on the two ports and it is equipped with two post-focal deformable mirrors together with the 6 Laser Guide Star (LGS) and 3 Natural Guide Star (NGS) channels for wavefront sensing and tomographic reconstruction. We present here the Final Design of the MORFEO optical configuration and the expected performances.
We designed and manufactured an optical bench to characterize the EUCLID dichroic reflected wavefront at all useful incidences, all polarization states and each wavelength from 550nm to 950nm. The multilayer coating adds indeed some phase effect that vary suddenly in function of the wavelength. The testbed architecture, main functionalities and performances are presented. The preliminary results obtained on the dichroic mirror are compared to some simulations based on an up-to-date model of the multilayer coating.
MORFEO is the Multi-Conjugated Adaptive Optics (MCAO) module for the European Extremely Large Telescope (ELT). It provides a one to one imaging of the telescope focal surface and it is equipped with two post-focal deformable mirrors together with the Laser Guide Star (LGS) and Natural Guide Star (NGS) channels for wavefront sensing and tomographic reconstruction. One of the key optical element is a meter scale flat dichroic mirror, which is a sort of extended hot mirror. Indeed, it has to reflect the NIR spectral range up to the K band used for both science (primarily MICADO) and the NGS; Moreover, it has to efficiently transmit the sodium line used in the LGS. In this paper, the specifications of the dichroic are discussed reporting the manufacturing possibilities and the expected performances. In addition, the main issues related to the production and test of the optical element are discussed.
Volume-phase holographic gratings (VPHGs) are widely used in astronomical spectrographs due to their adaptability and high diffraction efficiency. Most VPHGs in operation use dichromated gelatin as a recording material, whose performance is sensitive to the coating and development process, especially in the near-UV. In this letter, we present the characterization of two UV-blue VPHG prototypes for the BlueMUSE integral field spectrograph on the VLT, based on dichromated gelatin and the Bayfol®HX photopolymer film as recording materials. Our measurements show that both prototypes meet the required diffraction efficiency and exhibit similar performance with a wavelength-average exceeding 70% in the 350-580 nm range. Deviations from theoretical models increase towards 350 nm, consistently with previous studies on similar gratings. We also report similar performances in terms spatial uniformity and grating-to-grating consistency. Likewise, no significant differences in wavefront error or scattered light are observed between the prototypes.
MICADO is a first light instrument for the Extremely Large Telescope (ELT), set to start operating later this decade. It will provide diffraction limited imaging, astrometry, high contrast imaging, and long slit spectroscopy at near-infrared wavelengths. During the initial phase operations, adaptive optics (AO) correction will be provided by its own natural guide star wavefront sensor. In its final configuration, that AO system will be retained and complemented by the laser guide star multi-conjugate adaptive optics module MORFEO (formerly known as MAORY). Among many other things, MICADO will study exoplanets, distant galaxies and stars, and investigate black holes, such as Sagittarius A* at the centre of the Milky Way. After their final design phase, most components of MICADO have moved on to the manufacturing and assembly phase. Here we summarize the final design of the instrument and provide an overview about its current manufacturing status and the timeline. Some lessons learned from the final design review process will be presented in order to help future instrumentation projects to cope with the challenges arising from the substantial differences between projects for 8-10m class telescopes (e.g. ESO’s VLT) and the next generation Extremely Large Telescopes (e.g. ESO’s ELT). Finally, MICADO's expected performance will be discussed in the context of the current landscape of astronomical observatories and instruments. For instance, MICADO will have similar sensitivity as the James Webb Space Telescope (JWST), but with six times the spatial resolution.
Euclid is the second M-class mission of ESA’s Cosmic Vision Program. It implements a space telescope to be launched at L2. The objective is to characterize the dynamics of the early Universe by using two instruments: the high definition camera VIS (visible instrument) and the spectrophotometer NISP (Near Infrared Spectrometer and Photometer). Light entering Euclid is either reflected toward VIS in the visible band, or transmitted to NISP in the infrared band by a dichroic mirror. In order to guarantee the quality of scientific data delivered by the mission, the knowledge of any chromatic dependence of the optical payload’s Point-Spread function (PSF) is critical. However, previous works showed that complex coatings, such as high-performance dichroic coating, are likely to induce high chromatic variations in reflection, either as a chromatic “Wave-Front-error” (WFE) and/or as inhomogeneous reflectance profile (R), both affecting PSF morphology. In-depth knowledge of the reflected wavefront by the Euclid Dichroic is then necessary in order to calibrate the in-flight Euclid Observations. This work focuses on two aspects. On the one hand, we present an experimental campaign to measure the dichroic WFE and R at any wavelength, incidence, and polarization state, with an extreme precision. This metrology work implements a bench funded by ESA, designed by Imagine Optic Company, and commissioned at LMA. On the other hand we build a numerical model of the dichroic based on these on-ground measurements. By reproducing the experimental optical properties of the dichroic mirror, we ensure the subjacent thinfilms physics at play is well understood, ultimately providing adequate inputs for the in-flight calibration of Euclid with a suitable level of accuracy.
Direct bonding has been a major key technology in many fields nowadays. From microelectronic to optoelectronic technologies, it became a technique used for mass production technology in many different applications. Direct bonding of silicon or silicon dioxide is now a well known process. In this article, we explore this technology through the transfer of a crystalline multilayer made of III-V materials (AlGaAs/GaAs) from its native GaAs substrate upon a fused silica substrate (SiO2). The goal of this work is to explore the conception of crystalline Bragg mirrors with low mechanical loss and high optical quality for precision measurement applications. We present the main results obtained for each step of the transfer process. Various experiment such as AFM characterization have been performed on the wafers to probe surface quality. Chemical wet etching with different experimental conditions have been tested to remove the GaAs substrate. We discuss the main challenges of the process, especially the bonding of two rather different materials from the thermo-mechanic point of view. Focus is also made on the chemistry used in the wet etching part to have a selective etching between GaAs and AlGaAs.
Due to the increasing number of astronomical projects and of their associated instruments, the request of large optics with higher optical performances does not stop growing. An important step in the manufacture of these optics is the deposition of high-precision optical coatings. To answer to this request, we developed dichroic coatings, with sharp transition for large optics, working at different angles of incidence and spectral ranges. The work done for the dichroic optics of the HRS and LRS instruments for the 4MOST project will be presented.
High precision measurements of the filters bandpass used on wide-field imagers mounted on large telescopes is critical for type Ia supernovae studies. A dedicated spectrophotometric bench is used to re-measure the now decommissioned ugriz filters used for the SNLS on CFHT-MegaCam. A full characterization of the optical response with respect to the location on the surface and the angle of incidence was performed for each filter. Strong variation over the filter surface is observed. The impact of the actual response on the observation is evaluated and we demonstrate an improvement with respect to the previous published results (SNLS1 and 2).
Due to the increase of astronomical projects and of their instruments, the request of large optics with higher optical performances does not stop growing. An important step in the manufacture of these optics is the deposition of high-precision optical coatings.
To answer to this request we developed coatings working at different angles of incidence and spectral ranges on large surface:
- anti-reflective coatings for large lenses with strong curvatures,
- dichroic coatings with sharp transition for large optics.
Main results will be presented on the basis of several examples of realization.
The first large Fabry-Perot etalon (Ø35 cm) of the VTF instrument was coated successfully using IBS technique. The High Reflective (HR) coatings need to meet the reflectivity specifications (95 +/- 1%) over the entire wavelength range 520-870 nm and the entire aperture (Ø25 cm) and also preserve the plate's flatness and airgap uniformity between the two platesto be better than 3 nm RMS. The change of the figure error of the individual faces after HR coating was exceptionally small: For plate 1 (upper) it changed from 1.7nm RMS before coating to 2.12 nm after coating, no change at all for plate 2 (lower).
The Visible Tunable Filter (VTF) is a diffraction-limited narrowband tunable instrument for imaging spectropolarimetry in the wavelength range between 520 and 860 nm. It is based on large-format Fabry Perot. The instrument will be one of the first-light instruments of the 4m aperture Daniel K. Inoue Solar Telescope (DKIST). To provide a field of view of 1 arcmin and a spectral resolution λ/Δλ of about 100.000, the required free aperture of the Fabry Perot is 250mm. The high reflectivity coatings for the Etalon plates need to meet the specifications for the reflectivity over the entire wavelength range and preserve the plate figure specifications of better λ/300, and a micro roughness of < 0.4 nm rms. Coated surfaces with similar specifications have successfully been made for reflecting mirrors on thick substrates but not for larger format Fabry-Perot systems. Ion Beam Sputtering (IBS) based coatings provide stable, homogeneous, and smooth coatings. But IBS coatings also introduce stresses to the substrate that influence the plate figure in our case at the nm level. In a joint effort with an industry partner and a French CNRS research laboratory, we developed and tested processes on small and full size substrates, to provide coated Etalon plates to the required specifications. Zygo Extreme Precision Optics, Richmond, CA, USA, is polishing and figuring the substrates, doing the metrology and FE analysis. LMA (Laboratoire Matériaux Avancés, Lyon, France) is designing and making the IBS coatings and investigating the detailed behavior of the coatings and related processes. Both partners provide experience from manufacturing coated plane optics for gravitational wave detection experiments and EUV optics. The Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany is designing and building the VTF instrument and is leading the coating development. We present the characteristics of the coatings and the substrate processing concept, as well as results from tests on sample size and from full size substrate processing. We demonstrate that the tight specifications for a single Etalon can be reached.
In order to enhance the final performances of complex optical systems it is required to limit the overall wasted reflected light coming from all the different surfaces involved. Ultra-low-reflectance coating becomes even a crucial point for high sensitivity experiments such as gravitational wave detectors where surfaces must have a reflectance lower than 100 ppm. Some tenths of percent is a common value for Anti-Reflective (AR) coating but reflectance below 100 ppm is trickier to achieve. The coating design sensitivity with respect to thickness errors or refractive index error can lead quickly to noncompliant reflectance.
When an AR coating has failed it is very difficult to recover the low reflectance. In theory adding one or two layers could correct the reflectance but it requires knowing exactly the actual coated stack. For large optics (diameter up to 500 mm), we developed a new technique based on reflectance measurements with different polarizations and incidence angles at one wavelength. The measurements were performed in s-polarized and p-polarized light to discriminate between several solutions. Then a correction based on one or two layers is computed in order to decrease the reflectance. The efficiency of this method is demonstrated in the case of a four-layer AR coating designed for zero reflectance at 1064nm and coated onto a 350mm diameter and 200mm thick substrate. The reflectance has decreased from 500ppm to 26ppm thanks to a correcting bilayer.
The LSST design foresees the use of six wide-band large optical filters that can alternatively be moved in front
of the CCD camera. Each of the six filters has a different band-pass covering all the wavelengths from 300 nm to 1200
nm. The way to achieve this is to coat an optimized optical thin films stack on a filter substrate. Each filter requires a
specific design using specific appropriate materials. The main characteristics of these filters, that constitute a real
technological challenge, are: their relatively large size - their radii of curvature (about 5.6 m) that represent a sagitta of
12,5 mm that increases the uniformity complexity, the large rejection band requirements with transmission lower than
0.01 % out of the band and a transmission of 95 % over the band-pass. This paper proposes to show the problematic and
the results obtained at LMA (Laboratoire des Matériaux Avancés-FRANCE) to the purpose of realizing these filters
using the IBS (Ion Beam Sputtering) deposition technique. The results obtained with High-Pass/Low-Pass structures will
be presented. Experimental results will be shown concerning the R-band filter (552-691 nm). An overview of the work to
be done to realize transmittance map over large filters will be given.
The second generation of gravitational wave detectors will aim at improving by an order of magnitude
their sensitivity versus the present ones (LIGO and VIRGO). These detectors are based on long-baseline Michelson
interferometer with high finesse Fabry-Perot cavity in the arms and have strong requirements on the mirrors quality.
These large low-loss mirrors (340 mm in diameter, 200 mm thick) must have a near perfect flatness. The coating process
shall not add surface figure Zernike terms higher than second order with amplitude <0.5 nm over the central 160 mm
diameter. The limits for absorption and scattering losses are respectively 0.5 and 5 ppm. For each cavity the maximum
loss budget due to the surface figure error should be smaller than 50 ppm. Moreover the transmission matching between
the two inputs mirrors must be better than 99%.
We describe the different configurations that were explored in order to respect all these requirements. Coatings are done
using IBS.
The two first configurations based on a single rotation motion combined or not with uniformity masks allow to obtain
coating thickness uniformity around 0.2 % rms on 160 mm diameter. But this is not sufficient to meet all the
specifications.
A planetary motion completed by masking technique has been studied. With simulated values the loss cavity is below 20
ppm, better than the requirements. First experimental results obtained with the planetary system will be presented.
Optical coatings are one of the key elements of the VLT’s second generation instrument MUSE. The Multi Unit
Spectroscopic Explorer is developed for the European Southern Observatory (ESO) and will be installed in 2013 at the
VLT (Very Large Telescope). MUSE is a panoramic integral field spectrograph (1x1arcmin² Field of View) operating in
the visible wavelength range (465 nm - 930 nm). The throughput, which strongly depends on the optical coatings, is one
of the most important parameters of the MUSE instrument, which aims at observing very faint objects.
This article focuses on the different refractions and reflections required by the optical design of MUSE. Between the
output of the VLT and the final detectors of MUSE, photons are typically reflected 7 times by mirrors and transmitted 26
times through antireflective coatings. A comparison between metallic and multi-dielectric coatings is presented here in
order to explain the best compromise that has been chosen for MUSE purpose. High reflective multi-dielectric coatings
of large bandwidth are rather thick and induce significant stress on the substrate which may bend the substrate. This
deformation of mirrors is simulated and compared to measurements on MUSE optics. Finally, systematic optical coating
tests have been conducted, so as to check the durability under severe conditions such as humidity, temperature change,
abrasion.
In the end, the choice of high quality optical coatings should allow MUSE to reach a global throughput higher than 40%.
Gravitational wave detectors such as Virgo and LIGO use long-baseline Michelson interferometers with high
finesse Fabry-Perrot cavity in the arms. The symmetry of these cavities is essential to prevent the interferometer
from sensitivity to laser fluctuations. For this purpose the difference between the transmissions of the two input
mirrors has to be minimized. Advanced LIGO, the upgrade of LIGO, plans a transmission matching between the
two input mirrors as high as 99%. A small deviation in the process fabrication from run to run might induce
transmission mismatch larger than 1%. Consequently, the two input mirrors have to be coated during the same
coating run. That requires ability to deposit the reflective coating, based on a stack of titanium doped tantala
(Ti:Ta2O5) layers and silica layers, uniformly over a 800 mm diameter aperture. This paper presents the study to
improve the thickness uniformity of a reflective coating and the preliminary results achieved on two Ø350mm
substrates coated in the run.
Volume Phase Holographic Gratings (VPHG) are key elements for the second generation instrument MUSE (Multi Unit
Spectroscopic Explorer) developed for the VLT (Very Large Telescope) for ESO (European Southern Observatory).
MUSE operates in the visible wavelength range (465-930nm) and is composed of 24 spectrographs including one VPHG
each. This article briefly describes the design of the grating manufactured by Kaiser Optical Systems, to reach the MUSE
spectral resolution and efficiency. On the other hand the set up developed in CRAL (Centre de Recherche Astrophysique
de Lyon) to test the VPHG final performance is deeply discussed. This set up uses a broadband source coupled to a
monochromator, and a compensation arm to remove the source intensity fluctuations. The source is amplitude modulated
by a chopper, and a lock-in amplifier extracts the modulated signal from the photodiodes.
The measurement arm scans the 0, 1st and 2nd diffraction orders of the grating and allows tests of different areas over its
whole surface of 120mm*60mm. The accuracy reached is below one percent in efficiency, allows us to validate the
performance and its uniformity over the surface of the gratings.
Through its participation to European programs, SAGEM has worked on the design and manufacturing of normal
incidence collectors for EUV sources. By opposition to grazing incidence, normal incidence collectors are expected to
collect more light with a simpler and cheaper design. Designs are presented for the two current types of existing sources:
Discharge Produced Plasma (DPP) and Laser Produced Plasma (LPP). Collection efficiency is calculated in both cases. It
is shown that these collectors can achieve about 10 % efficiency for DPP sources and 40 % for LPP sources. SAGEM
works on the collectors manufacturability are also presented, including polishing, coating and cooling. The feasibility of
polishing has been demonstrated with a roughness better than 2 angstroms obtained on several materials (glass, silicon,
Silicon Carbide, metals...). SAGEM is currently working with the Institut d'Optique and the Laboratoire des Materiaux
Avancés on the design and the process of EUV coatings for large mirrors. Lastly, SAGEM has studied the design and
feasibility of an efficient thermal control, based on a liquid cooling through slim channels machined close to the optical
surface.
Fausto Acernese, Paolo Amico, N. Arnaud, Saverio Avino, D. Babusci, Regis Barille, Fabrizio Barone, L. Barsotti, M. Barsuglia, F. Beauville, M. Bizouard, C. Boccara, Francois Bondu, L. Bosi, C. Bradaschia, S. Braccini, Alain Brillet, V. Brisson, L. Brocco, Damir Buskulic, G. Calamai, Enrico Calloni, E. Campagna, F. Cavalier, G. Cella, Eric Chassande-Mottin, Frederic Cleva, T. Cokelaer, J.-P. Coulon, Elena Cuoco, Vincenzino Dattilo, M. Davier, Rosario De Rosa, Luciano Di Fiore, A. Di Virgilio, B. Dujardin, Antonio Eleuteri, Daniel Enard, I. Ferrante, F. Fidecaro, I. Fiori, Raffaele Flaminio, J.-D. Fournier, S. Frasca, Franco Frasconi, Andreas Freise, Luca Gammaitoni, Alberto Gennai, Adalberto Giazotto, Gianfranco Giordano, Lara Giordano, G. Guidi, H. Heitmann, P. Hello, P. Heusse, L. Holloway, S. Kreckelbergh, Paolo La Penna, Vincent Loriette, Magali Loupias, G. Losurdo, Jean-Marie Mackowski, E. Majorana, Catherine Man, E. Marchetti, Frederique Marion, Fabrizio Martelli, Alain Masserot, Louis Massonnet, Massimo Mazzoni, Leopoldo Milano, J. Moreau, F. Moreau, Nazario Morgado, F. Mornet, Benoit Mours, J. Pacheco, A. Pai, C. Palomba, Federico Paoletti, Silvio Pardi, R. Passaquieti, D. Passuello, B. Perniola, Laurent Pinard, R. Poggiani, Michele Punturo, P. Puppo, Ketevan Qipiani, J. Ramonet, P. Rapagnani, V. Reita, Alban Remillieux, F. Ricci, Iolanda Ricciardi, Guido Russo, Salvatore Solimeno, Ruagero Stanga, E. Tournefier, F. Travasso, Herve Trinquet, Didier Verkindt, Flavio Vetrano, Oliver Veziant, A. Vicere, J.-Y. Vinet, H. Vocca, Michel Yvert
The French-Italian interferometric gravitational wave detector VIRGO is currently being commissioned. Its principal instrument is a Michelson interferometer with 3 km long optical cavities in the arms and a power-recycling mirror. This paper gives an overview of the present status of the system. We report on the presently attained sensitivity and the system’s performance during the recent commissioning runs.
Bernard Cimma, Danielle Forest, Patrick Ganau, Bernard Lagrange, Jean-Marie Mackowski, Christophe Michel, Jean-Luc Montorio, Nazario Morgado, Renee Pignard, Laurent Pinard, Alban Remillieux
The coating deposition on large optical components (diameter 350 mm) has required the development of new metrology tools at 1064 nm. To give realistic values of the optical performances, the whole surface of the component needs to be scanned. Our scatterometer (commercial system) has been upgraded to support large and heavy samples. The other metrology tools are prototypes we have developed. We can mention the absorption (photothermal effect) and birefringence bench, a control interferometer equipped with an original stitching option, the optical profilometer (RMS roughness and small defect measurements). A detailed description of these metrology benches will be exposed. Their sensitivity, accuracy and capability to map the optical properties of substrates or mirrors will be discussed. We will
describe the recent developments: the stitching option adapted to the Micromap profilometer to measure the RMS roughness on larger area (exploration of a new spatial frequency domain), the accurate bulk absorption calibration.
F. Beauville, D. Buskulic, R. Flaminio, F. Marion, A. Masserot, L. Massonnet, B. Mours, F. Moreau, J. Ramonet, E. Tournefier, D. Verkindt, O. Veziant, M. Yvert, Regis Barille, Vincenzino Dattilo, Daniel Enard, Franco Frasconi, Alberto Gennai, Paolo La Penna, Magali Loupias, Federico Paoletti, L. Bracci, Giovanni Calamai, E. Campagna, G. Conforto, E. Cuoco, I. Fiori, G. Guidi, G. Losurdo, Fabrizio Martelli, Massimo Mazzoni, B. Perniola, Ruagero Stanga, Flavio Vetrano, A. Vicere, D. Babusci, Gianfranco Giordano, Jean-Marie Mackowski, Nazario Morgado, Laurent Pinard, Alban Remillieux, Fausto Acernese, Fabrizio Barone, Enrico Calloni, Rosario De Rosa, Luciano Di Fiore, Antonio Eleuteri, Leopoldo Milano, Ketevan Qipiani, Iolanda Ricciardi, G. Russo, Salvatore Solimeno, M. Varvella, Francois Bondu, Alain Brillet, Eric Chassande-Mottin, Frederic Cleva, T. Cokelaer, J.-P. Coulon, B. Dujardin, J.-D. Fournier, H. Heitmann, Catherine Man, F. Mornet, J. Pacheco, A. Pai, Herve Trinquet, J.-Y. Vinet, N. Arnaud, M. Barsuglia, M. Bizouard, V. Brisson, F. Cavalier, M. Davier, P. Hello, P. Heusse, S. Kreckelberg, A. Claude Boccara, Vincent Loriette, J. Moreau, V. Reita, P. Amico, L. Bosi, Luca Gammaitoni, M. Punturo, F. Travasso, H. Vocca, L. Barsotti, S. Braccini, C. Bradaschia, G. Cella, C. Corda, A. Di Virgilio, I. Ferrante, F. Fidecaro, Adalberto Giazotto, E. Majorana, L. Holloway, R. Passaquieti, D. Passuello, R. Poggiani, A. Toncelli, M. Tonelli, L. Brocco, S. Frasca, C. Palomba, P. Puppo, P. Rapagnani, F. Ricci
The goal of the VIRGO program is to build a giant Michelson type interferometer (3 kilometer long arms) to detect gravitational waves. Large optical components (350 mm in diameter), having extremely low loss at 1064 nm, are needed. Today, the Ion beam Sputtering is the only deposition technique able to produce optical components with such performances.
Consequently, a large ion beam sputtering deposition system was built to coat large optics up to 700 mm in diameter. The performances of this coater are described in term of layer uniformity on large scale and optical losses (absorption and scattering characterization).
The VIRGO interferometer needs six main mirrors. The first set was ready in June 2002 and its installation is in progress on the VIRGO site (Italy). The optical performances of this first set are discussed. The requirements at 1064 nm are all satisfied. Indeed, the absorption level is close to 1 ppm (part per million), the scattering is lower than 5 ppm and the R.M.S. wavefront of these optics is lower than 8 nm on 150 mm in diameter. Finally, some solutions are proposed to further improve these performance, especially the absorption level (lower than 0.1 ppm) and the mechanical quality factor Q of the mirrors (thermal noise reduction).
Several French research laboratories set up goniometers allowing BRDF measurements at different laser wavelengths in the infrared. On the effor of the Delegation Generale de l'Armement (DGA/STTC), a round robin set of painted targets BRDF measurements was undertaken, under the ONERA expertise. The laboratories participating in this round robin were the Aerospatiale Matra CCR Suresnes, The IPN SMA-Virgo Lyon, the Institut Fresnel Marseille, and the CEA DAM CESTA Le Barp. The goniometers of the four laboratories are firstly described. The targets studied are seven 5cm diameter painted disks of aluminum or steel, a spectralon reference sample, and a sandpaper sample. We have first demonstrated that the pollution of painted targets with dust has a very weak influence on the BRDF. Before and after each measurement series, the directional-hemispherical reflectance of the samples was measured at ONERA. The measurements have been achieved according to a protocol specifying the sample position and laser probe size. Chosen wavelengths for the inter-comparison are 1.064 micrometers . For both wavelengths, the characteristics of the different goniometers are compared in term of noise and repeatability. The difference between the painted targets BRDF measured with the various devices are relatively limited at 1.06 micrometers , and mainly induced by speckle. More important differences are obtained at 10.6 micrometers , particularly for a BRDF measurement device using an absolute calibration method. In order to explain these differences, few hypotheses are advanced. Information on the absolute accuracy is obtained by the comparison of the measured directional-hemispherical reflectance and the one computed from BRDF measurements.
Characterization of high reflectivity multilayers coatings at 1.064 micrometers are reported. Measurements of reflectivity were performed by a Fabry-Perot cavity decay time method and a 10-6 precision was obtained. Measurements of absorption was done in the 10-6 range by collinear mirage detection with a 10-8 precision. Diffusion was investigated with a commercial scatterometer and total integrated scattering in the sub-ppm range on home made Rmax 0 degree(s) mirrors was measured.
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