Comparing Shack-Hartmann Wavefront Sensor and Donut Method for Low-Order Aberration Analysis at the 182 cm Copernico Telescope

This Master’s thesis investigates the low-order optical aberrations of the 182 cm Copernico Telescope at the Asiago Observatory by comparing two diagnostic techniques: the Shack-Hartmann Wavefront Sensor (SHWFS) and the Donut method, which retrieves aberrations from defocused point spread functions. Speckle images were also analyzed to study the aberrations induced by the atmosphere. The Donut technique offers a promising way to measure non-common path aberrations using scientific images alone, which is relevant for MORFEO, the Multiconjugate Adaptive Optics module for the ELT, where periodic low-order realignment is required. Observations of bright stars at multiple elevations and at both Nasmyth and Coudé focal planes allowed characterization of aberrations caused by atmospheric turbulence and gravity-induced telescope flexures. WFS data were fitted with Zernike polynomials, while speckle analysis provided turbulence parameters and confirmed the trend of decreasing aberration with elevation. The Donut method was applied to out-of-focus PSFs and, when good-quality donuts were available, reproduced low-order aberrations in reasonable agreement with the WFS. However, its reliability under a wider range of on-sky conditions requires further testing and calibration. The code also proved computationally efficient, producing fits within seconds, which aligns with MORFEO’s operational requirements. Although the current implementation still needs refinement, these results suggest that, with further calibration and validation, the Donut method could become a useful complement to conventional wavefront sensors in adaptive optics systems.