Calibration and alignment of the demonstrator of the PROBA-3/ASPIICS formation flying coronagraph

This article describes the calibration and alignment procedures of a demonstrator for the ASPIICS coronagraph proposed for the ESA technology mission PROBA-3 aimed at demonstrating the feasibility of a Formation Flying coronagraph. ASPIICS is distributed on two spacecrafts separated by 150 m, one hosting the external occulting disk and the other the optical part of the coronagraph. The purpose of the demonstrator is to reproduce on ground the metrology systems that will equip the coronagraph in order to realize the alignment of the two spacecrafts and the absolute pointing to the center of the Sun. The demonstrator is composed of a device that reproduces the solar umbra/penumbra created by the solar occulter[1] and of a Three Mirror Anastigmatic (TMA) telescope mounted on a hexapod, a new-generation platform that allows 6 degrees of freedom. A large plane folding mirror is used on ground to obtain a distance between the occulter and the TMA up to 30 m. Photo sensors located around the entrance pupil of the TMA determine the absolute positioning of the instrument by sensing the penumbra behind the occulting disk. Light sources (LEDs) located on the rear-side of the occulting disk allow verifying the alignment of the formation. The paper describes the whole demonstrator, its integration, its calibration, and the performance of the metrology systems of the coronagraph. This study has been conducted in the framework of an ESA "STARTIGER" Initiative, a novel approach aimed at demonstrating the feasibility of a new and promising technology on a very short time scale (six months).