Flat-sky pseudo-cls analysis for weak gravitational lensing

Marika Asgari*, Andy Taylor, Benjamin Joachimi, Thomas D. Kitching

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


We investigate the use of estimators of weak lensing power spectra based on a flat-sky implementation of the 'Pseudo-Cl' (PCl) technique, where the masked shear field is transformed without regard for masked regions of sky. This masking mixes power and 'E'-convergence and 'B'-modes. To study the accuracy of forward-modelling and full-sky power spectrum recovery, we consider both large-area survey geometries and small-scale masking due to stars and a checkerboard model for field-of-view gaps. The power spectrum for the large-area survey geometry is sparsely sampled and highly oscillatory, which makes modelling problematic. Instead, we derive an overall calibration for large-area mask bias using simulated fields. The effects of small-area star masks can be accurately corrected for, while the checkerboard mask has oscillatory and spiky behaviour that leads to per cent biases. Apodization of the masked fields leads to increased biases and a loss of information. We find that we can construct an unbiased forward model of the raw PCls, and recover the full-sky convergence power to within a few per cent accuracy for both Gaussian and lognormal-distributed shear fields. Propagating this through to cosmological parameters using a Fisher-Matrix formalism, we find that we can make unbiased estimates of parameters for surveys up to 1200 deg2 with 30 galaxies per arcmin2, beyond which the per cent biases become larger than the statistical accuracy. This implies that a flat-sky PCl analysis is accurate for current surveys but a Euclid-like survey will require higher accuracy.

Original languageEnglish
Pages (from-to)454-477
Number of pages24
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date30 May 2018
Publication statusPublished - 1 Sep 2018


  • Cosmology: observations
  • Gravitational lensing: weak
  • Methods: numerical


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