Fresnel diffractive imager: instrument for space mission in the visible and UV

Abstract

We propose a new concept of diffractive optics: Fresnel arrays, for a 4 m aperture space telescope in the UV domain. Fresnel arrays focus light by diffraction through a very thin binary mask. They form images optically and deliver very high quality wavefronts, specially in the UV. Up to 8% of the incident light is focussed, providing high angular resolution and high contrast images of compact objects. Due to their focal lengths of a few kilometers in the UV, large Fresnel arrays will require two spacecraft in formation flying, but with relatively tolerant positioning. Diffraction focusing is also very chromatic; this chromatism is corrected, allowing relatively broad (30 to 100 nm) spectral channels in the 120-350 nm range. A 4 m aperture Fresnel imager providing 7 to 10 milli arc seconds resolution is very competitive for imaging compact and high contrast objects such as protoplanetary disks and young planetary systems, AGNs, and deep sky objects. We have developed prototypes to validate the optical concept and related technologies : first a laboratory setup, then a 20 cm aperture ground-based prototype, which provides high contrast and diffraction limited images of sky objects in the visible and close IR. A new laboratory prototype is also being prepared for validation in the 250 - 350 nm wavelength range.