The Palomar Adaptive Optics System actively corrects for changing aberrations in light due to atmospheric turbulence. However, the underlying internal static error is unknown and uncorrected by this process. The dedicated wavefront sensor device necessarily lies along a different path than the science camera, and, therefore, doesn’t measure the true errors along the path leading to the final detected imagery. This is a standard problem in adaptive optics (AO) called “non-common path error.”
The previous method of calibrating this error consisted of manually applying different polynomial shapes (via actuator voltages) at different magnitudes onto the deformable mirror and noting if the final image quality had improved or deteriorated, before moving onto the next polynomial mode. This is a limited, time-consuming, and subjective process, and structural and environmental changes over time necessitate a new calibration over a period of months.
The Autonomous Phase Retrieval Calibration (APRC) software suite performs automated sensing and correction iterations to calibrate the Palomar AO system to levels that were previously unreachable. APRC controls several movable components inside the AO system to collect the required data, automatically processes data using an adaptive phase retrieval algorithm, and automatically calculates new sets of actuator voltage commands for the deformable mirror. APRC manages and preserves all essential data during this process.
The APRC software calculates the true wavefront error of the full optical system, then uses the existing AO system deformable mirror (DM) to correct the detected error. This provides a significant leap in performance by precisely correcting what were once “un-calibratable” errors. Furthermore, the corrective pattern found by this process serves as the underlying nominal shape of the DM, upon which the adaptive corrections for atmospheric turbulence are based.
This work was done by Siddarayappa A. Bikkannavar, Catherine M. O’Hara, and Mitchell Troy of Caltech for NASA’s Jet Propulsion Laboratory.
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