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

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, HARMONI

In 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 bench

HARMONI 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.

The UK ELT programme has two main (non-academic) routes to impact: industrial contract return from ESO, including the instrument projects, and public engagement (PE). Both of these are dealt with through dedicated work-packages in the proposal, with further details given in the Pathways to Impact document.

1) Industrial return: The total hardware budget for the telescope construction project at ESO is more than 800MEur and most of that will be procured from industry in the ESO member states. UK companies are eligible to bid for ESO contracts and a major part of the industry engagement programme is to find suitable UK companies to put forward to receive calls for tender. There is still over 100MEur worth of ELT contracts to be let and through past efforts of the programme, UK companies are well placed for a number of specialist supplies in imaging detectors and software. The end goal of the programme is to see contract return to the UK increase so that we maximise our share of the construction budget. Our activities in support of this goal include publicising tender opportunities through email campaigns from the STFC tender opportunities service, targeted meetings with groups of companies and contract-specific events.

2) PE: Astronomy is recognised as a hugely inspiring way to engage the public with the big questions of science, and events such as Stargazing Oxford and Doors Open at Royal Observatory Edinburgh regularly draw thousands of visitors. We will leverage the existing PE programmes of the consortium (including the ROE Visitor Centre, STFC Public Engagement & Communications and Oxford University) and exploit the news value of significant ELT milestones between now and first light. The first phase of the programme will aim to engage audiences with the technology involved in building the ELT and its systems and will concentrate on `awareness raising' with social media campaigns, coordinating with ESO on press releases and embedding the ELT into wider STFC PE activities (e.g. piggybacking on the JWST launch). Subsequent phases will begin to plan activities leading up to telescope first light, and secure additional funding for
resource development and building partnerships with teachers, science centres, and planetaria in preparation for leading a series of national events.