Measuring and calibration non common path aberrations in adaptive optics assisted image-slicer based spectrographs

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The DPhil Project is part of the instrument development for ESO's Extrmely Large Telescope (ELT), namely the development of the first light, adaptive optics assisted, integral field spectrograph, HARMONIIn the context of HARMONI, the integral-field spectrograph for the E-ELT, this DPhil project deals with the calibration of a type of aberrations known as non-common path aberrations (NCPA). NCPA are caused by the difference in optical path between the science light and the light used for wavefront sensing within the Adaptive Optics system (AO). One of the most widespread state-of-the-art techniques for NCPA calibration is Phase Diversity (PD), which uses a pair of in-focus and defocused images to estimate the wavefront aberrations using a maximum likelihood approach and a model of light propagation through the optical system.HARMONI contains an optical component known as an image slicer whose role is to slice the input 2D field of view into long slits that can be fed to the spectrograph. This poses a challenge when using techniques such as PD because the behaviour of image slicers in the presence of a defocus has not been characterised in depth. The effects of image slicers on light propagation, which traditional techniques do not account for, could potentially have an impact on NCPA calibration.In this context, the general goals for this research project can be summarised as:(1) Improve the understanding of the effects of image slicers on light propagation(2) Characterize the behaviour of state-of-the-art NCPA calibration techniques in the presence of image slicers. This would help us understand their limitations and applicability to the HARMONI instrument. Potentially modify those techniques to incorporate image slicer effects(3) Develop alternative techniques for NCPA calibration that could circumvent the drawbacks of state-of-the-art algorithms(4) Experimental work: demonstrate and validate these findings with an experimental benchHARMONI is a facility instrument, and will tackle a wide variety of questions in observational astrophysics. These have direct bearing on two of STFC's three science challenges: (B) How do stars and planetary systems develop and how do they support the existence of life? and (A) How did the Universe begin and how is it evolving?. This research falls with the PPAN remit, it is part of the "Astronomy and Space Sciences" research area.