Frédéric Cassaing, Béatrice Sorrente, Laurent Mugnier, Gérard Rousset, Vincent Michau, Isabelle Mocoeur, and Fabien Baron
BRISE: A MULTI-PURPOSE BENCH FOR COPHASING SENSORS (Poster)

BRISE: A MULTI-PURPOSE BENCH FOR COPHASING SENSORS

Frédéric Cassaing(1), Béatrice Sorrente(1), Laurent Mugnier(1), Gérard Rousset(1), Vincent Michau(1), Isabelle Mocoeur(1), and Fabien Baron(2)
ONERA/DOTA/HRA, 29 av. de la Division Leclerc, BP 72, F-92322 CHATILLON CEDEX
Cavendish Laboratory,University of Cambridge,Madingley Road,Cambridge UK

The cophasing sensor (CS), which measures the disturbances between the sub-apertures of multiple-aperture telescopes, is a key component. As multiple-aperture telescopes become more ambitious, requirements for the CS become more demanding: low flux (for stellar interferometers), sub-nanometric accuracy (for interferometric nullers), image with very small contrast (for wide-field telescopes, such as spaceborne Earth imagers), larger number of beams (for all applications).

Focal-plane sensing is a solution to cope with all these requirements, with a very simple opto-mechanical setup. Two implementations have been investigated at ONERA: phase retrieval, using the sole focal-plane image, and phase diversity, based on the joint analysis of a focal and an extra-focal images. Phase diversity can measure any mode on any source, while phase retrieval is more suited to real-time piston/tip/tilt measurements on an unresolved (or partially resolved) source.

To evaluate accurately the performance of CS or other high-resolution devices, ONERA has built a multipurpose bench called BRISE (Banc Reconfigurable d'Imagerie sur Sources Etendues). BRISE mainly includes: a source module delivering an extended scene and a reference point source; the detection module recording the focal-plane image of each object and implementing a phase diversity sensor; the perturbation module focusing the source on the detector, defining the aperture configuration and introducing calibrated aberrations. Afocal input/output ports are used to interface with our Zygo interferometer or a visitor instrument. The MASTIC software (Multiple-Aperture Software for Telescope Imaging and Cophasing) is used to process or simulate BRISE data.

BRISE has already been (or will be) used for several applications:

• validation of a phase diversity sensor from the image of Earth scenes (French Defense);
• validation of a phase retrieval/diversity sensor for nulling, for ESA/Kayser-Threde (cf companion paper "DARWIN fringe sensor: experimental results on the BRISE bench") or PEGASE (CNES);
• generation of distorded wavefronts for a fiber-coupling demonstrator (ESA/KONGSBERG)

This talk describes the bench, the main experiments carried out on BRISE, and reports main results such as nanometric accuracy. This shows that simple solutions based on focal-plane devices can cophase multiple-aperture optical telescope with challenging requirements.