ASKI: full-sky lensing map making algorithms

Within the context of upcoming full-sky lensing surveys, the edge-preserving nonlinear algorithm Aski (All Sky κ Inversion) is presented. Using the framework of Maximum A Posteriori inversion, it aims at recovering the optimal full-sky convergence map from noisy surveys with masks. Aski contributes two steps: (i) CCD images of possibly crowded galactic fields are deblurred using automated edge-preserving deconvolution; (ii) once the reduced shear is estimated using standard techniques, the partially masked convergence map is also inverted via an edge-preserving method. The efficiency of the deblurring of the image is quantified by the relative gain in the quality factor of the reduced shear, as estimated by Sextractor. Cross validation as a function of the number of stars removed yields an automatic estimate of the optimal level of regularization for the deconvolution of the galaxies. It is found that when the observed field is crowded, this gain can be quite significant for realistic ground-based eight-metre class surveys. The most significant improvement occurs when both positivity and edge-preserving l1 − l2 penalties are imposed during the iterative deconvolution. The quality of the convergence inversion is investigated on noisy maps derived from the horizon-4π N-body simulation with SNR within the range lcut = 500−2500, with and without Galactic cuts, and quantified using one-point statistics (S3 and S4), power spectra, cluster counts, peak patches and the skeleton. It is found that (i) the reconstruction is able to interpolate and extrapolate within the Galactic cuts/nonuniform noise; (ii) its sharpness-preserving penalization avoids strong biasing near the clusters of the map (iii) it reconstructs well the shape of the PDF as traced by its skewness and kurtosis (iv) the geometry and topology of the reconstructed map is close to the initial map as traced by the peak patch distribution and the skeleton’s differential length (v) the two-points statistics of the recovered map is consistent with the corresponding smoothed version of the initial map (vi) the distribution of point sources is also consistent with the corresponding smoothing, with a significant improvement when l1− l2 prior is applied. The contamination of B-modes when realistic Galactic cuts are present is also investigated. Leakage mainly occurs on large scales. The non-linearities implemented in the model are significant on small scales near the peaks in the field.