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Enhancing the sensitivity of gravitational-wave detectors represents a major challenge that requires stable operation at increasingly high optical power. Optical absorption within Fabry–Pérot cavities induces thermal aberrations that, if not properly compensated, can significantly degrade the interferometer performance. The active correction of these effects in present-day and next-generation detectors requires an accurate local measurement of the aberration budget in mirrors.
In Advanced Virgo, direct wavefront sensing is currently performed using a Hartmann Wavefront Sensor (HWS), a differential measurement device developed in collaboration with the LIGO group at the University of Adelaide.
To address the requirements of both upcoming observing runs and future detector configurations, a new CMOS-based HWS has been developed. Experimental tests demonstrate a wavefront reconstruction accuracy better than 0.4 nm RMS. However, the sensor performance was found to be significantly affected by a spurious thermal defocus caused by temperature fluctuations. To mitigate this effect, a dedicated temperature monitoring and stabilization system has been developed and tested. Furthermore, based on thermal and structural simulations, a new sensor housing has been designed and built using a composite structure of Invar and aluminium to enhance thermal stability.