In ultraintense laser experiments, optimizing on-target high-power focal spots to maximize peak intensity is critical for achieving optimal outcomes such as maximized accelerated ion energy. This requires minimizing laser aberrations, which remains challenging due to the lack of in situ, real-time, on-target focal diagnostics at ultrahigh intensity. In this study, we systematically investigated the far-field patterns of a second-order harmonic ($2{\omega }_L$) emitted from thin foil interaction with typical astigmatic laser. Aberrated laser wave fronts and nonuniform intensity profiles jointly shape the accelerated electron sheets together, thereby imprinting astigmatism information in their $2{\omega }_L$ emission far-field patterns. Based on this, we propose an easy-to-implement scheme that utilizes real-time $2{\omega }_L$ far-field-pattern feedback to guide astigmatism compensation via off-axis parabolic mirror (OAP) tuning. Optimized focal spots following this scheme yields higher proton cutoff energy, increased proton number, and more symmetrical beam profiles in laser-driven ion-acceleration experiments.

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• High intensity laser-plasma interactions
• Laser applications
• Laser driven ion acceleration
• Light-matter interaction
• Ions
• Protons
• Particle-in-cell methods
• Plasma diagnostic techniques