Manufacturing and verification of ZnS and Ge prisms for the JWST MIRI imager

Abstract

The JWST Mid-Infrared Instrument (MIRI) is designed to meet the JWST science requirements for mid-IR capabilities and includes an Imager MIRIM provided by CEA (France). A double-prism assembly (DPA) allows MIRIM to perform low-resolution spectroscopy. The MIRIM DPA shall meet a number of challenging requirements in terms of optical and mechanical constraints, especially severe optical tolerances, limited envelope and very high vibration loads.The University of Cologne (Germany) and the Centre Spatial de Liege (Belgium) are responsible for design, manufacturing, integration, and testing of the prism assembly. A companion paper (Fischer et al. 2008) is presenting the science drivers and mechanical design of the DPA, while this paper is focusing on optical manufacturing and overall verification processes.The first part of this paper describes the manufacturing of Zinc-sulphide and Germanium prisms and techniques to ensure an accurate positioning of the prisms in their holder. (1) The delicate manufacturing of Ge and ZnS materials and (2) the severe specifications on the bearing and optical surfaces flatness and the tolerance on the prism optical angles make this process innovating. The specifications verification is carried out using mechanical and optical measurements; the implemented techniques are described in this paper.The second part concerns the qualification program of the double-prism assembly, including the prisms, the holder and the prisms anti-reflective coatings qualification. Both predictions and actual test results are shown.

Design and development of MIRI, the mid-IR instrument for JWST

“…The design and testing of the MIRI mechanisms is presented in [4] and [5], the prism for low resolution spectroscopy in [10] and [11], optical blacks in [12] and the detector performances and focal plane system in [6], [7] and [9].…”

The JWST MIRI double-prism: design and science drivers

“…The mechanical design is developed by the University of Cologne (UoC), in collaboration with the Centre Spatial de Liège (CSL). Manufacturing of the two prisms is provided by AMOS [1], all other high-precision manufacturing is performed by the fine mechanical workshop of UoC's 1st Institute of Physics.…”

“…In order to cope with the extreme conditions of the prisms' surroundings, the low resolution double prism assembly (LRSDPA) design makes high demands on manufacturing accuracy. The design and the manufacturing of the mechanical parts are presented here, while 'Manufacturing and verification of ZnS and Ge prisms for the JWST MIRI imager' are described in a second paper [1]. We also give insights on the astronomical possibilities of a sensitive MIR spectrometer.…”