The development of the optical components for METIS, which is the mid-infrared ELT imager and spectrograph, is a challenging task. On one hand, there are tight requirements regarding wavefront error, driven mostly by high contrast imaging modes. On the other hand, limited space envelopes are available. In this paper, we present the development of the largest flat aluminium mirror of derotator mechanism which is located within the Common Fore Optics (CFO) of METIS. The 240mm-diameter mirror is made of gold coated Rapidly Solidified Aluminium (RSA) 6061-T6 with the compact flexure-based mount directly embedded in the mirror body. This novel design includes mounting pads that are located at the front, inside the optical surface. This approach improves the universality of the proposed solution while introducing challenges from the manufacturing point of view, i.e. surface holes that make polishing more difficult. The mount design is a combination of leaf springs and rotational hinges and is optimised to reduce the aberrations due to gravity and assembly loads. Thanks to the in-house manufacturing process relying on 5-axes milling capabilities, it is possible to create such a complex integrated mirror mount with minimal impact of the holes on the quality of the optical surface. Numerical simulations of this mirror performed for various load cases contributing to surface aberrations show extremely low surface form error (< 15 nm RMS) of the metallic mirror.
METIS, the cryogenic mid-infrared imager and spectrograph designed for the ESO’s Extremely Large Telescope (ELT) in Chile, is designed to optimally utilize the high resolution provided by the large aperture of the ELT. To enable high contrast imaging, the wavefront errors in the Common Fore Optics (CFO) of METIS must be minimized. In this paper, we focus on the design, the development and verification of the largest, 260 mm diameter curved mirror (CM1) of the CFO. The mirror is made of Rapidly Solidified Aluminium (RSA) Al6061 T6 alloy to achieve a homogenous shrinkage of the instrument and as such facilitate room-temperature instrument alignment and verification. The strict requirement on the maximum surface form error of 30 nm RMS is challenging to meet with metallic mirrors. Thanks to the careful integral design of the mirror, its mount and its manufacturing, the sensitive degrees of freedom of the mirror surface are precisely tuned considering the various contributors. Using numerical optimization, the aberrations related to gravity, assembly loads and minute shrinkage differences are greatly reduced. Finally, by developing a manufacturing process based on inhouse milling, thermal treatments, diamond turning, in-house polishing and gold coating, the final mirror is manufactured. Interferometric tests show good agreement with the numerical predictions of the surface form error and confirm the excellent performance of the CM1 mirror. This is one of the state-of-art examples of metal mirrors designed, manufactured and assembled to such an accuracy.
The warm calibration unit (WCU) is one of the subsystems of the future METIS instrument on the Extremely Large Telescope (ELT). Operating at room temperature, the WCU is mounted above the main cryostat of METIS. It will be employed as a calibration reference for science observations, as well as for verification and alignment purposes during the AIT phase. The WCU is designed and constructed at the University of Cologne, one of the partner in the METIS consortium. WCU recently went through a successful Optics Long Lead Items Review by ESO. Now, the WCU is entering the last phase of the project, the Final Design Review (FDR). In this paper, we present the current status of the WCU design and summarize the mechanical and system engineering work. We describe the design of the hexapod formed by six manually adjustable links and its interfaces with the METIS cryostat together with the CFRP-based optical bench and Invar-based optical mounts. Lab prototyping results of one actuator under a nominal load of 5 kN confirms the achievable high linear resolution (20 µm). We present the status of the WCU laser cabinet. We discuss the lastest progress in the laboratory testing of some WCU functionalities, such as the fibre-fed monochromatic sources for the spectral calibration of the LM-Spectrograph of METIS, and the spatial calibration sources using the integrating sphere. We detail the activities foreseen until FDR together with the preparation of the sub-system MAIT work.
The ELT, Europe’s Extremely Large Telescope, with its 39m main mirror will be the largest optical/infrared telescope in the world, able to work at the diffraction limit. METIS is one of its first light instruments with powerful imaging and spectroscopic capabilities in the thermal wavelengths. It contains several high contrast imaging (HCI) modes, which allow it to detect and characterize exoplanets amongst others. The HCI performance is highly dependent on pupil stabilization mechanisms and a closed loop compensation of non-common path aberrations degrading the wavefront error of the instrument. The Talbot effect is a near-field effect on collimated light, where spatial frequencies of the wavefront are re-imaged periodically along the optical path. The periodicity is known as the Talbot length, which is a function of the wavelength and the wavefront’s spatial frequencies with the latter being a result of the wavefront errors caused by the surface form errors of optical elements. The aberrations oscillate from amplitude to phase, in the spatial scale of one Talbot length, which can have an impact on the performance of the HCI modes. We evaluate the impact of the Talbot effect with respect to the METIS phase aberration budget by assuming representative power spectral density profile for the surface form error of each optical surface. We propagate the errors to the subsequent pupil plane and finally investigate the resulting point spread function profile. Simulations are fed back into the HCI error budget and if necessary, the specifications regarding instrument surface form are adjusted.
We present the manufacturing and optical verification of the germanium immersed grating for the L/M band high resolution spectrograph (LMS). The LMS is one of the science subsystems of the Mid-infrared ELT Imager and Spectrograph, METIS. The immersed grating has very demanding requirement specifications: <100 nm RMS transmitted Wave Front Error (WFE) after double pass, and >70% peak throughput in all orders within the 2.9-5.3 μm wavelength range over the pupil. The grating has a period of 18.2 μm, a sawtooth groove profile with 89.6 degrees apex angle and a grating area of 150mm x 60mm. The germanium immersed grating was produced by Canon’s high precision mechanical cutting technology. We present the interferometric tests that were performed in order to verify WFE and two different measurements (based on cascade laser and Fourier Transform Spectrometer, respectively) for throughput verification.
The Mid-infrared Imager and Spectrograph (METIS) is one of the first light instruments of the European Extremely Large Telescope (ELT). METIS optical design contains a first stage called common fore optics, which operates at cryogenic temperatures. Because of the azimuthal motions of the telescope, it integrates a field derotator, which has to run under vacuum and at 67 K. The French Alternative Energies and Atomic Energy Commission (CEA) is in charge of developing the cryogenic rotation stage, which actuates this field derotator. This is a kind of technological breakthrough as all the existing derotator systems operate outside the cryogenic vessels. This paper gives an overview of the derotator actuator design with the different trades that have been studied and some preliminary tests results.
We present the preliminary design of the calibration unit of the future E-ELT instrument METIS. This independent subunit is mounted externally to the main cryostat of METIS and will function both as calibration reference for science observations, as well as verification and alignment tool during the AIT phase. In this paper, we focus on describing its preliminary layout and foreseen functionalities, based on the performance requirements defined at system level and the constraints imposed by warm IR background. We discuss the advantage of employing an integrating sphere as common radiation emitter, leading to a novel and versatile design, where the source’s spatio-spectral properties can be varied with high fidelity and repeatability. By combining only few tuneable sources and mechanisms we show how a large instrument such as METIS can be calibrated and tested, without the need of a complex cold calibration unit.
METIS, a mid-infrared imager and spectrograph for the wavelength range 2.9–19μm (astronomical L-, M-, N- and Q-band), will be one of the first three science instruments at the European Extremely Large Telescope (E-ELT). It will provide diffraction limited imaging, coronagraphy, high resolution integral field spectroscopy and low and medium resolution slit spectroscopy. Within the international METIS consortium, the 1st Institute of Physics of the University of Cologne in Germany is responsible for the design, manufacturing, integration and qualification of the Warm Calibration Unit (WCU) of the instrument. The WCU will be a self-contained unit operating at ambient temperature outside of the voluminous METIS dewar, feeding a variety of optical calibration and alignment signals into the optical path of METIS. The functionalities of the WCU will be used for routine daily daytime calibrations after astronomical observing nights and verification of the internal alignment of METIS during assembly, integration and verification (AIV). In this contribution we present the preliminary optical design and principle of operation of the WCU in its current state of the preliminary design phase of METIS.
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