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.
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.
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.
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).
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.
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).
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.
The Large Synoptic Survey Telescope (LSST) uses a novel, three-mirror, telescope design feeding a camera system that
includes a set of broad-band filters and three refractive corrector lenses to produce a flat field at the focal plane with a
wide field of view. Optical design of the camera lenses and filters is integrated in with the optical design of telescope
mirrors to optimize performance. We discuss the rationale for the LSST camera optics design, describe the methodology
for fabricating, coating, mounting and testing the lenses and filters, and present the results of detailed analyses
demonstrating that the camera optics will meet their performance goals.
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.
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.
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