Active optics for the DICOS project

Le résumé fourni par la source

DICOS is a technological demonstrator for a future space-based small optical telescope based on active optics and with high stability requirements.It belongs to a new CNES set of programs called "EXperimental Projects" (PEX).It favors fast developments with breaking technology to be demonstrated in a representative object but without applying the full set of space constraints.The current phase of DICOS is to build a ground-based prototype and test it by the end of 2024.It will then be adapted to fly under a stratospheric balloon and demonstrate its compatibility with conditions close to the space ones.The instrument uses a full aluminum 40 cm off axis telescope of the Cassegrain type with an optical bench behind the primary.The machining and the assembly tolerances are relaxed but compensated by active optics.The secondary mirror is mounted on a six degrees of freedom device that performs the coarse alignment.Then, an active closed optical loop located inside the optical bench corrects the surface error of the mirrors as well as the thermoelastic variations of the telescope.The goal is to reach a 20 nm rms wavefront error (WFE) for spatial frequency lower than 5 cycles/pupil diameter, stable to 1 nm rms and with a 0.01 Hz sampling frequency.The sizing case was taken from the field of coronography for exo-planets direct detection.This paper first presents a brief summary of the DICOS project.Then we report the excellent performances obtained experimentally on the active loop.It currently manages to reduce an initial 400-500 nm rms WFE (from Z4 to Z36) down to a few nm in 3-4 iterations of about 40 s each.We also present a detailed characterization of the DM and some recent efforts to demonstrate its compatibility with vacuum and thermal loading.

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