One of the most simple and common diagnostics in laser-plasma interactions is shadowgraphy. This diagnostic technique is based in the refraction of light when it traverses the plasma, and is mainly used for qualitative diagnostics on its shape and structure. However, a shadowgram may include several optical artefacts, namely focal points, caustics or difraction, which contain quantitative information on the properties of the plasma. A wavefront curved by the plasma will propagate and create such artefacts, and an optical system can be used to further analyse and study them. In this work we present theoretical descriptions of these artefacts for plasma channels and columns, and how do they relate to the plasma properties. Our models are supported by various simulations, namely ray tracing, and experimental data. Interferometric measures were used to compare with the results for the plasma density profile given by these optical artefacts. Our results show that the analysis of these artefacts may lead to a simple and competitive optical diagnostic tool for laser-plasma interactions.

The ELT, Europe's Extremely Large Telescope, with its 39m main mirror will be the largest optical/infrared telescope in the world, able to work at the diffraction limit. METIS is one of its first light instruments with powerful imaging and spectroscopic capabilities in the thermal wavelengths. It contains several high contrast imaging (HCI) modes, which allow it to detect and characterize exoplanets amongst others. The HCI performance is highly dependent on pupil stabilization mechanisms and a closed loop compensation of non-common path aberrations degrading the wavefront error of the instrument.The Talbot effect is a near-field effect on collimated light, where spatial frequencies of the wavefront are re-imaged periodically along the optical path. The periodicity is known as the Talbot length, which is a function of the wavelength and the wavefront's spatial frequencies with the latter being a result of the wavefront errors caused by the surface form errors of optical elements. The aberrations oscillate from amplitude to phase, in the spatial scale of one Talbot length, which can have an impact on the performance of the HCI modes. We evaluate the impact of the Talbot effect with respect to the METIS phase aberration budget by assuming representative power spectral density profile for the surface form error of each optical surface. We propagate the errors to the subsequent pupil plane and finally investigate the resulting point spread function profile. Simulations are fed back into the HCI error budget and if necessary, the specifications regarding instrument surface form are adjusted.

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