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 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.
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.
EUV pellicle membranes are being pursued to protect scanner images from repeating defects caused by reticle fall-on particle defects. Because most materials highly absorb EUV, pellicle membranes must be ultrathin. In an attempt to increase the strength of the ultrathin membranes, grid-supported pellicle membranes have been proposed. In this study we compare grid-supported pellicles (GSP) over free-standing pellicles (FSP). We considered imaging, thermal, mechanical, and thermo-mechanical characteristics. Finite Element Methods (FEM) was used to investigate the thermal, and (thermo-)mechanical behavior of pellicles. The maximum temperature reached under operational conditions by the pellicle film was determined. Using a thermo-mechanical analysis wrinkling behavior was quantified. The mechanical analysis considered the influence of grid structures on the sagging behavior, on crack propagation, on the pellicle film resistance to collision with solid particles, and on the resistance to shocks on the pellicle frame. The analysis shows that GSP that meets imaging requirements will not bring any advantages over FSP.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.