The latest results from DICE (Detector Interferometric Calibration Experiment)

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 1 x 10-5 pixel. Current state-of-the-art methods for centroid estimation have reached a precision of about 3 x 10-5 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 1 x 10-5 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 4 x 10-4 pixel. After including the pixel position information, an astrometric accuracy of 6 x 10-5 pixel was obtained, for a PSF motion over more than 5 pixels. In the static mode (small jitter motion of less than 1 x 10-3 pixel), a photon noise limited precision of 3 x 10-5 pixel was reached.