Some Considerations on PSF Asymmetry and its Impact on the Measurement of Galaxy Shapes, 2002

The anisotropy of the Point Spread Function (PSF) delivered by the James Webb Space Telescope (JWST) will affect the measurement of the shape of faint objects, such as those required for the mapping of the cosmic shear via weak lensing observations described in Program 003 in the JWST Design Reference Mission (DRM). We revisit the scientific impact of the PSF anisotropy in light of new observational results and theoretical developments on the analysis of faint objects, and introduce a simple criterion to help quantify the effect of the PSF on the measured shape of galaxies observed with JWST. We suggest two versions of a modified Level 2 requirement on the PSF anisotropy that would better represent our current understanding of the scientific requirements of weak lensing observations.

1 Introduction

2 The effect of weak lensing

3 The effect of the PSF

4 Measuring asymmetries: some definitions

5 Galaxy shape and PSF—the unweighted version

6 Limitations of unweighted measurements

using weightedmoments.

7 A tentative PSF anisotropy criterion for lensing measurements

8 Some estimates on a model PSF

9 The current Level 2 Requirement on PSF anisotropy

10 Proposed requirements on PSF anisotropy

Acknowledgements

References

Acronyms