Gaia on-board metrology: basic angle and best focus
  The Gaia payload ensures maximum passive stability using a single material, SiC, for most of its elements. Dedicated metrology instruments are, however, required to carry out two functions: monitoring the basic angle and refocusing the telescope. Two interferometers fed by the same laser are used to measure the basic angle changes at the level of $\mu$as (prad, micropixel), which is the highest level ever achieved in space. Two Shack-Hartmann wavefront sensors, combined with an ad-hoc analysis of the scientific data are used to define and reach the overall best-focus. In this contribution, the systems, data analysis, procedures and performance achieved during commissioning are presented 
A. Mora, M. Biermann, A.G.A. Brown, D. Busonero, L. Carminati, J.M.   Carrasco, F. Chassat, M. Erdmann, W.L.M. Gielesen, C. ...
astro-ph.IM