Adaptive optics system on large ground-based telescope is widely used in resident space object (RSO) observation. As a common wavefront sensor for adaptive optics, the pyramid wavefront sensor (PyWFS) possesses advantages such as high sensitivity in closed-loop corrections and adjustable sampling in real-time operation, which makes it suitable for high-resolution imaging. However, daytime application of PyWFS is greatly challenged by bright fluctuating sky background, especially in the visible.

In a recent study published in Optics Express, Prof. WANG Jianli, Prof. YAO Kainan and Dr. HUANG Linshu from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences proposed a daytime-Py approach, which provides an alternative for daylight natural guide star adaptive optics in the visible.

During the day, bject light and rather strong background light are mixed as incident light. Firstly, the disturbance of extended background light was deduced by a field stop. The daytime-PyWFS combined with a PyWFS and a lenslet array was then employed to separate the object signal from the background signal and improve the signal-to-noise ratio (SNR). Finally, a background elimination algorithm was applied to extract clear object information for subsequent output signal calculation.

Moreover, a daylight adaptive optics system was established to determine the feasibility of the daytime-Py approach in real-time daylight adaptive optics correction.

Experimental results indicate that the daytime-Py approach is effective when the SNR is greater than 1.9, while the usual PyWFS is effective when the SNR is greater than 16.9. It can als be proved that the daytime-Py approach allows real-time adjustable sampling to achieve the optimum system performance for targets with different brightness.

All in all, the daytime-Py approach is more suitable for real-time daylight adaptive optics closed-loop corrections in low SNR compared with the usual PyWFS. And it presents higher sensitivity and adjustable pupil sampling in real time for daylight adaptive optics compared with the commonly-used Shack-Hartmann wavefront sensor.

This study enables high-resolution observation of RSOs during both daytime and nighttime, and it realizes daylight natural guide star adaptive optics tracking and correction of solar-illuminated object.