The fresnel interferometric imager

Abstract

The Fresnel Interferometric Imager has been proposed to the European Space Agency (ESA) Cosmic Vision plan as a class L mission. This mission addresses several themes of the CV Plan: Exoplanet study, Matter in extreme conditions, and The Universe taking shape. This paper is an abridged version of the original ESA proposal. We have removed most of the technical and financial issues, to concentrate on the instrumental design and astrophysical missions. The instrument proposed is an ultra-lightweight telescope, featuring a novel optical concept based on diffraction focussing. It yields high dynamic range images, while releasing constraints on positioning and manufacturing of the main optical elements. This concept should open the way to very large apertures in space. In this two spacecraft formation-flying instrument, one spacecraft holds the focussing element: the Fresnel interferometric array; the other spacecraft holds the field optics, focal instrumentation, and detectors. The Fresnel array proposed here is a 3.6 × 3.6 m square opaque foil punched with 10 5 to 10 6 void "subapertures". Focusing is achieved with no other optical element: the shape and positioning of the subapertures (holes in the foil) is responsible for beam combining by diffraction, and 5% to 10% of the total incident light ends up into a sharp focus. The consequence of this high number of subapertures is high dynamic range images. In addition, as it uses only a combination of vacuum and opaque material, this focussing method is potentially efficient over a very broad wavelength domain. The focal length of such diffractive focussing devices is wavelength dependent. However, this can be corrected. We have tested optically the efficiency of the chromatism correction on artificial sources (500 < λ < 750 nm): the images are diffraction limited, and the dynamic range measured on an artificial double source reaches 6.2 10 −6 . We have also validated numerical simulation algorithms for larger Fresnel interferometric arrays. These simulations yield a dynamic range (rejection factor) close to 10 −8 for arrays such as the 3.6 m one we propose. A dynamic range of 10 −8 allows detection of objects at contrasts as high as than 10 −9 in most of the field. The astrophysical applications cover many objects in the IR, visible an UV domains. Examples are presented, taking advantage of the high angular resolution and dynamic range capabilities of this concept.

The Fresnel imager: instrument numerical model

“…Each time a phase is added to that of a wavefront, the algorithm must make sure that there are at least two pixels of the matrix each time a phase shift of 2π is added when applying (1) and (2).…”

“…The "focal module" holds a pupil optical element, an optical device which will correct the chromatism, and the focal instrument. Detailed presentations of the Fresnel Imager concept can be found in Koechlin et al [1,2].…”

Exoplanet status report: Observation, characterization and evolution of exoplanets and their host stars

“…This first generation is expected to be in the 1.5-2 m coronagraph class which could study atmospheres of giant exoplanets, Neptune class and even super Earth atmospheres (e.g., Schneider et al, 2009). After 2020 these missions may be followed by a generation of an interferometer (Cockel et al, 2009, Lawson et al, 2009, an external occulter (Glassman et al, 2009), a large 8 m class coronagraph (visible), a Fresnel Inter ferometric Imager (Koechlin et al, 2009) or a 20 m segmented coronagraph which would resample a super JWST for the NIR (Lillie et al, 2001). …”

Astrophysical targets of the Fresnel diffractive imager

“…In 2007, we have submitted a proposal for a 3.6 m aperture array mission to ESA in the frame of the "Cosmic Vision" plan [10]. This proposal has not been accepted, but we have made progress since.…”