Thesis

Super-sensitivity with non-linear curvature wave-front sensors

• Call:

  IDPASC Portugal - PHD Programme 2014

• Academic Year:

  2014 /2015

• Domain:

  Astrophysics

• Supervisor:

  Carlos M Correia

• Co-Supervisor:

  Thierry Fusco

• Institution:

  Universidade do Porto

• Host Institution:

  Laboratoire d'Astrophysique de Marseille and SIM/FEUP

• Abstract:

  Adaptive-optics (AO), a technology at the crossroads of physics, mathematics, optics and electro-mechanics, partially corrects the image blurring of the Earth's atmosphere – a direct consequence of otherwise flat wave-front distortions undergone during propagation through a (weak) turbulent medium. AO recovers the angular resolution of the telescope (the ability to tell apart two objects, however close, proportional do D), i.e. overcomes the natural seeing when observing through turbulence and limits the tremendous sky-background contribution for improved sensitivity → reaching D^4. It uses a wave-front sensor (WFS) to measure distortions and a deformable mirror (DM) whose shapes are computed in real-time (RTC) to counter distortions and flatten the resulting wave-front. Modern AO systems routinely allow diffraction limited imaging — matching the limits set by optical theory — on 8–10m telescopes in the near-infrared. The next decade should see the application of this technique to 30–40m-class telescopes as well as to shorter wavelengths. The wave-front information that can be extracted from a given number of photons is known from first principles. Such information accurately defines the “ideal” performance which can be reached by Adaptive Optics systems on ground-based telescopes [Guyon 2005]. Direct comparison between this ideal limit and the sensitivity offered by current wave-front sensing schemes reveals a huge performance gap. The commonly used wave-front sensors in astronomical adaptive optics, such as Shack-Hartmann and linear Curvature, are very robust and flexible. However, they are poorly suited to high quality wave-front measurement – for low-order aberrations on 8 to 10m telescopes, they require ~100 to 1000 times more photons than more optimised wave-front sensing techniques. In one hand, they offer nearly ideal sensitivity at the spatial frequency defined by the sub-aperture spacing. On the other hand, they suffer from poor sensitivity at low spatial frequencies, which are most critical for wide-field tomographic AO systems employing lasers for the measurements of tilt and tilt anisoplanatism [Correia, JOSA-A 2013] modes and also on direct imagers of exo-planets and protoplanetary disks – projects SPHERE [Beuzit et al. 2008] and GPI [Macintosh et al. 2008]. Non-linear curvature wave-front sensing (nlCWFS) delivers outstanding sensitivity and high dynamic range by lifting the linearity constraint of standard curvature wave-front sensing. This is achieved by taking full advantage of diffraction, which encodes wave-front aberrations into patterns of diffraction-limited interference speckles. The high sensitivity and linear range of the nlCWFS comes at the price of a non-linear wave-front reconstruction step, which calls for innovative, fast and robust minimisation algorithms. This path has already been started in [Correia et al. 2013] using a modification to standard phase-retrieval techniques. The main goal of this thesis is to investigate and propose a physics/optics-driven model approximation to beat down the massively computation involved in processing the measurements and estimate the corresponding wave-fronts. This work will be done in collaboration with the Herzberg Institute of Astrophysics and the SCExAO team at the Subaru Telescope. Both have optical benches with a nlCWFS whose data will be used to compare model to reality. Also, the work will accompany the design, test and characterisation of a non-linear Curvature Wave-front Sensor at LAM – Laboratoire d'Astrophysique de Marseille and availability of potential candidates to spend time in Marseille is required. Profile: Student with strong interest in optics/propagation, mathematical modelling, statistical physics, numerical simulations, data/signal processing. The work will accompany the design, test and characterisation of a non-linear Curvature Wave-front Sensor at LAM – Laboratoire d'Astrophysique de Marseille and availability of potential candidates to spend time in Marseille is required. References: Correia et al, Wave-front reconstruction for the non-linear curvature wave-front sensor, AO4ELT, (2013) http://dx.doi.org/10.12839/AO4ELT3.13290 C. Correia et al, Increased sky coverage with optimal correction of tilt and tilt-anisoplanatism modes in laser-guide-star multiconjugate adaptive optics, http://dx.doi.org/10.1364/JOSAA.30.000604