Presentation + Paper
26 August 2024 Manufacturing requirements and challenges of the fully integrated aluminium tip-tilt chopper mirror for the METIS instrument
Elfi Dijkstra, Niels Tromp, Marijn Versteeg, Menno de Haan, Menno Schuil, Mirka Lewińska-Maresca, Ivan Lloro, Jeff Lynn, Rik ter Horst, Maurice Teuwen, Ties van Loon, Remco de Bruijn, Sander Paalvast
Author Affiliations +
Abstract
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.
Conference Presentation
© (2024) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Elfi Dijkstra, Niels Tromp, Marijn Versteeg, Menno de Haan, Menno Schuil, Mirka Lewińska-Maresca, Ivan Lloro, Jeff Lynn, Rik ter Horst, Maurice Teuwen, Ties van Loon, Remco de Bruijn, and Sander Paalvast "Manufacturing requirements and challenges of the fully integrated aluminium tip-tilt chopper mirror for the METIS instrument", Proc. SPIE 13100, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation VI, 131000A (26 August 2024); https://doi.org/10.1117/12.3018400
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KEYWORDS
Mirrors

Tolerancing

Manufacturing

Sensors

Aluminum

Aluminum mirrors

Optics manufacturing

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