3 October 2011 IRDIS filters: from design to qualification
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IRDIS (Infra Red Dual Imager and Spectrograph) is one of the scientific sub-systems for the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument, to be mounted on one of the four VLT 8-m telescopes in Paranal (Chile) in 2012. IRDIS and two other scientific sub systems will analyze the resulting high-contrast image with the aim of direct detection of extrasolar planets. IRDIS covers the near infrared bands Y, J, H and Ks (950-2300nm) and works at cryogenic temperature. The main observational mode of IRDIS is Dual Band Imaging, where the same object is observed simultaneously in two adjacent wavebands. For this mode, differential aberrations between the two channels are critical and filter optical quality is crucial. In this paper, we focus on the design, production and tests of the IRDIS filters. The deposition technique involves DIBS (Dual Ion Beam Sputtering) and leads to very compact coatings, with material properties close to those of bulk material, making these filters well suited for cryogenic applications. The use of an in-situ optical monitoring system in visible and near infrared range (up to 2500nm) permits to reach the demanding spectral filter specifications (bandwidth, rise and fall widths, peak transmission, wide band blocking) and to have a good agreement with the theoretical design. Spectral measurements at ambient and cryogenic temperatures are then presented.
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Hélène Krol, Nathalie Valette, Didier Torricini, Catherine Grèzes-Besset, Kjetil Dohlen, David Le Mignant, Michel Saïsse, Gabriel Moreaux, Maud Langlois, Claire Moutou, Arthur Vigan, Frédéric Lemarquis, "IRDIS filters: from design to qualification", Proc. SPIE 8168, Advances in Optical Thin Films IV, 81680X (3 October 2011); doi: 10.1117/12.897224; https://doi.org/10.1117/12.897224

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