  Theia is an astrometric mission proposed to ESA in 2014 for which one of the
scientific objectives is detecting Earth-like exoplanets in the habitable zone
of nearby solar-type stars. This objective requires the capability to measure
stellar centroids at the precision of 1e-5 pixel. Current state-of-the-art
methods for centroid estimation have reached a precision of about 3e-5 pixel at
two times Nyquist sampling, this was shown at the JPL by the VESTA experiment.
A metrology system was used to calibrate intra and inter pixel quantum
efficiency variations in order to correct pixelation errors. The Theia
consortium is operating a testbed in vacuum in order to achieve 1e-5 pixel
precision for the centroid estimation. The goal is to provide a proof of
concept for the precision requirement of the Theia spacecraft.
  The testbed consists of two main sub-systems. The first one produces pseudo
stars: a blackbody source is fed into a large core fiber and lights-up a
pinhole mask in the object plane, which is imaged by a mirror on the CCD. The
second sub-system is the metrology, it projects young fringes on the CCD. The
fringes are created by two single mode fibers facing the CCD and fixed on the
mirror. In this paper we present the latest experiments conducted and the
results obtained after a series of upgrades on the testbed was completed. The
calibration system yielded the pixel positions to an accuracy estimated at 4e-4
pixel. After including the pixel position information, an astrometric accuracy
of 6e-5 pixel was obtained, for a PSF motion over more than 5 pixels. In the
static mode (small jitter motion of less than 1e-3 pixel), a photon noise
limited precision of 3e-5 pixel was reached.
