Simulation of deformation measurements with deflectometry

We have previously reported on a versatile simulation environment for deflectometric measurements (DGaO Proceedings 114, A1, 2013). Its capabilities have been extended and used for a preliminary study of achievable deformation-measurement uncertainties, to assess the feasibility of deflectometric characterisation of actuator performance and gravity sag for the mirror segments of the European Extremely Large Telescope (E-ELT). Carrying out deformation measurements with deflectometry circumvents most of the issues around calibration and shape measurement uncertainty in deflectometry, so that electronic noise is a main contributor to errors. In practice, drift of experimental parameters and distortion of the test set-up will enter the error budget as well. We present results for the relevant Zernike terms that show reliable detection of Zernike coefficients at the 25 nm level. Random artefacts related to noise in the phase measurements are seen to translate into bogus Zernike terms, and we discuss possible mitigation techniques.