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Shear Measurements in Simulated SNAP Images with Realistic PSFs

This study investigates shear measurements using simulated SNAP images, incorporating realistic point spread functions (PSFs). Various effects influenced by detection noise, charge diffusion, and jitter are analyzed through STEP-like simulations. A 34x34 grid of galaxies is examined with respect to varying ellipticity and significance. The KSB+ analysis method evaluates shear recovery performance, assessing dependency on focus and distance from the optical axis. We explore how bias and uncertainty in shear measurements are impacted by PSFs and galaxy properties across different magnitude and size bins.

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Shear Measurements in Simulated SNAP Images with Realistic PSFs

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  1. Shear measurements in simulated SNAP images with realistic PSFs Håkon Dahle, Stephanie Jouvel, Jean-Paul Kneib, Eric Prieto, Sebastien Vives, Bruno MilliardLaboratoire d’Astrophysique de Marseille (LAM), France JPL WL from space meeting

  2. Motivation STEP-like simulations for SNAP, with PSF (800nm) from optical design Include effects of : jitter, charge diffusion, sky background, detector noise (no CTE or diffraction spikes) Dithering and re-sampling KSB+ analysis: Shear recovery as function of focus, distance from optical axis JPL WL from space meeting

  3. Skymaker simulations 34x34 grid of galaxies in 4 simulated exposures; ditherered, re-sampled and combined 0.1’’ pixels --> 0.06’’ pixels; 34132 PSF from 49 stars All galaxies have the same intrinsic ellipticity (e = 0.25), but random orientations 5 magnitude bins (25.0,26.0,27.0,28.0,28.5) 3 size bins (galaxies of scale 0.1’’, 0.2’’ and 0.4’’) JPL WL from space meeting

  4. PSF variation with focus/position JPL WL from space meeting

  5. Note ellipticity dependence on smoothing scale (PSF wings are more elliptical) 0.34o0.57o0.8o 0.34o off-axis 0.57o off-axis 0.8o off-axis JPL WL from space meeting

  6. 0.34o0.57o0.8o Note that e1=e2=0 PSFs are not necessarily circularly symmetric 0.34o off-axis 0.57o off-axis 0.8o off-axis JPL WL from space meeting

  7. 0.34o0.57o0.8o 0.34o off-axis 0.57o off-axis 0.8o off-axis JPL WL from space meeting

  8. 0.34o0.57o0.8o 0.34o off-axis 0.57o off-axis 0.8o off-axis JPL WL from space meeting

  9. Analysis KSB+ analysis (measuring stellar ellipticities and polarizabilities at same scale as these quantities are measured for each galaxy) Select faint galaxies (6 < S/N < 250), which tend to carry most of the shear signal in real WL analyses Note: The magnitude values have an arbitrary zeropoint and should not be taken literally. S/N values are more meaningful JPL WL from space meeting

  10. Bias of shear measurements Size: 0.1”(blue), 0.2”(red), 0.4”(green) JPL WL from space meeting

  11. ShearKaiser Squires & Broadhurst Ap.J. 449 460-475 1995 (KSB) Polarization is characterized by a vector (e1, e2) • e1 ~ (Qxx–Qyy)/(Qxx+Qyy) • e2 ~ 2Qxy/(Qxx+Qyy) • The Qij are Gaussian weighted second moments of the intensity distribution • NOTE: In these simulations, all galaxies have the same intrinsic |e| = sqrt(e12 + e22) • The simulated galaxies of different sizes have their ellipticities diluted by the PSF to a varying extent --> concentric circles in e1-e2 space • Noisier (fainter) galaxies make “fuzzier” circles • Anisotropic PSFs make them slightly non-concentric & non-circular JPL WL from space meeting

  12. “Shear recovery” • Apply standard KSB methods (+later modifications by Luppino & Kaiser 1997 and Hoekstra et al. 1998) to recover the intrinsic values of e1 , e2 (see fig) • From these, calculate the mean value of the modulus <|e|> = <sqrt(e12 + e22)> • Define “bias” as the ratio of the output value to the input value of the ellipticity • Define “uncertainty” as the rms scatter around <|e|> JPL WL from space meeting

  13. Bias of the shear measurements (ellipticity modulus after PSF correction, relative to input ellipticity) Green: 0.34o off-axis Blue: 0.57o off-axis Red: 0.8o off-axis JPL WL from space meeting

  14. Uncertainty of the shear measurements Green: 0.34o off-axis Blue: 0.57o off-axis Red: 0.8o off-axis JPL WL from space meeting

  15. Mean value of each shear component (should in principle be zero; circle indicates ~1 sigma uncertainty). JPL WL from space meeting

  16. This is work in progress… Currently probing features/limitations of KSB+ more than SNAP ? Would be useful to compare to another method for shape measurement Simulations with finer sampling of different focus values (checking how smoothly uncertainty & bias vary as function of focus). JPL WL from space meeting

  17. JPL WL from space meeting

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