Optical design, fabrication, and testing a prototype of the NIRSpec IFU

Abstract

A group of European Research Institutes (Centre de Recherche Astronomique de Lyon (CRAL), Laboratoire d'Astrophysique de Marseille (LAM), University of Durham) and company (CYBERNETIX) have proposed to implement an Integral Field Unit (IFU) in NIRSpec instrument for the James Webb Space Telescope (JWST). After a brief presentation of the optical design of NIRSpec IFU, we will focus on the prototype of this module built by CRAL. This prototype is composed of two optical elements: a stack of eleven spherical tilted slices associated with a row of ten spherical tilted pupil mirrors. All the optical elements were manufactured by CYBERNETIX. We will introduce the fabrication procedures and an original method of assembling by molecular adhesion in order to comply with environment specifications. Afterwards, the image slicer is tested on the optical bench at CRAL. In fact, the first measure consists in placing a CCD camera in the pupil mirrors plane and determining the characteristics of the eleven images of the telescope pupil such as sizes, positions, photometric quality, diffraction effects and angular errors on slices and comparing these results with specifications. Then, the row of pupil mirrors is installed on the optical bench. In the slit mirrors plane, we observe a pseudo slit (ten images of the slices). We establish the same characteristics as in the first measure. Moreover, in the same plane, some Point Spread Function (PSF) measurements are made and we analyse the PSF in comparison with the simulation. The main results of the tests of the first image slicer prototype are presented. With the exploitation of the results, we validate and improve the image slicer systems for others instruments such as Multi Unit Spectroscopic Explorer (MUSE, study of a second generation instrument for the European Very Large Telescope (VLT)) and the European Space Agency (ESA) Integral Field Spectrograph (IFS) prototype.