The Mid-infrared ELT Imager and Spectrograph (METIS) is one of the scientific instruments for the ELT. The Common Fore Optics (CFO) is the first set of optics and mechanisms within the cryogenic instrument. A key element of the CFO is the Pupil Stabilization Mechanism (PSM), a tip/tilt mirror used for periodic adjustments of the pupil alignment during observations. The design challenges for the PSM involved achieving the required stability, resolution, and a sufficient stroke range/hold power while operating within the stringent requirements of the CFO. The design development lead to a monolithic design and integrated flexures driven by actuators with an integrated closed loop optical encoder.
The chopper is a cryogenic tip-tilt mechanism in the Common Fore Optics (CFO) of METIS, the mid-infrared imager and spectrograph for ESO’s Extremely Large Telescope (ELT). In this paper we delve into the milling process of the mirror of the chopper mechanism, denominated as CM5. This highly accurate mirror is fully integrated into a tip-tilt mechanism and operates to reduce the background noise by shifting between the target object and the background, this happens with a frequency of 5Hz. We will primarily focus on the challenges encountered during the mirror-mechanism fabrication. Notably, the tip/tilt mechanism in the chopper requires precision levels that surpass the accuracy of the 5-axis milling machine that is used. The general accuracy of the used machine on a multi sided part is typically 20 to 30 microns, and is dominated by thermal drift of the machine itself. This significantly increases the manufacturing complexity, therefore a specialized production methodology was devised to ensure the achievement of the micron-level tolerances. In the next paragraphs we will describe how the critical tolerances of down to 5 microns in the mechanism features have been achieved. The use of a so-called Golden Sample provides a known and stable reference during manufacturing, allowing to bypass the drift and static errors of the machine. The usage of CMM measurements proves that the developed production method delivers excellent results overcoming the limitations of the machine accuracy. This monolithic aluminium cryogenic mirror has been produced through in-house CNC programming, milling, and thermal ageing followed by thermal cycling, Single point diamond turning (SPDT) and Wire Electrical Discharging Machining (Wire EDM) by external suppliers. After SPDT of the optical surface, the surface is polished to optical quality and finally a gold coating layer is applied to increase the reflectivity in the desired wavelength.
This paper describes the work performed by the CEA Paris-Saclay team to build and qualify a cryogenic derotator mechanism for the ELT-METIS instrument. As the Extremely Large Telescope is an azimuthal telescope, a field derotator device is mandatory to keep the sky image perfectly aligned on the detector frame during the observations. The METIS consortium has chosen to place the derotator mechanism inside the cryogenic vessel, operating at 70 K under vacuum to reduce the background noise at the lowest level. This article gives the main results of the achieved performances, including a positioning accuracy in the arc second range.
The Mid-infrared ELT Imager and Spectrograph (METIS) is one of the first-light scientific instruments for the ELT. The common fore optics (CFO) sits at the heart of METIS, conditioning the beam and distributing the light across the wavefront sensor and science channels. To ensure it can reach its science goals, rigorous analysis of manufacturing and alignment tolerances is essential. For the CFO, an end-to-end tolerancing system was developed, integrating component level analyses, FEA results, as-built data to continuously optimize system performance during the design phase, as well as manufacturing and assembly phase.
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