Arising from light deflections by large-scale structures in the Universe, the weak gravitational lensing (WL) effect has been identified as one of the most important probes in cosmological studies, in particular for understanding the nature of dark matter and dark energy, and the law of gravity. WL effects result in only tiny shape distortions (namely shear) and flux magnifications on background objects. Extracting their signals accurately is therefore highly challenging observationally, methodologically, and statistically. Space-borne missions have unique advantages of being devoid of terrestrial atmospheric impacts to achieve stable high-resolution observations. For both the ESA Euclid mission and the Chinese Space Station Optical Survey (CSS-OS), WL studies are the key science driver. They target at accurate photometric measurements for over a billion galaxies to extract WL signals. Together with the 3-D galaxy distribution from spectroscopic observations, precision cosmological studies can be achievable with an order of magnitude higher statistical power compared to the current surveys. To realize such huge capabilities, however, systematic errors need to be thoroughly understood and controlled. Developing different statistical methods to fully use the data is also greatly important. To investigate these critical issues and explore the synergy and complementarities of Euclid and CSS-OS, we have formed an excellent team consisting of key WL members of the Euclid project and of the CSS-OS under the ISSI/ISSI-BJ International Team Program. Our studies will focus on the following three aspects:
1) Investigate different statistical tools and possible systematic effects. They include how to extract WL signals from magnification measurements; higher order statistics beyond two-point correlations; joint analyses of shear and magnification; joint analyses of WL effects and galaxy clustering.
2) Develop and test different shear measurement methods taking into account specific survey designs, especially CSS-OS which is less systematically studied yet than Euclid.
3) Explore the synergistic power of the two surveys, particularly the photometric redshift measurements by combining the multi-band observations in the optical from CSS-OS and that of NIR from Euclid.