Gaia in-orbit realignment. Overview and data analysis

TL;DR

This paper discusses the in-orbit realignment of the Gaia spacecraft using Shack-Hartmann wavefront sensors, focusing on the algorithms for centroiding and wavefront reconstruction to optimize telescope focus.

Contribution

It provides an overview of the WFS system and introduces advanced maximum likelihood algorithms for wavefront reconstruction under partial sampling conditions.

Findings

Algorithms achieve performance close to the Cramér-Rao lower bound.

Wavefront reconstruction handles partial pupil sampling effectively.

Enhanced centroiding precision improves in-orbit refocusing accuracy.

Abstract

The ESA Gaia spacecraft has two Shack-Hartmann wavefront sensors (WFS) on its focal plane. They are required to refocus the telescope in-orbit due to launch settings and gravity release. They require bright stars to provide good signal to noise patterns. The centroiding precision achievable poses a limit on the minimum stellar brightness required and, ultimately, on the observing time required to reconstruct the wavefront. Maximum likelihood algorithms have been developed at the Gaia SOC. They provide optimum performance according to the Cr\'amer-Rao lower bound. Detailed wavefront reconstruction procedures, dealing with partial telescope pupil sampling and partial microlens illumination have also been developed. In this work, a brief overview of the WFS and an in depth description of the centroiding and wavefront reconstruction algorithms is provided.