August 23, 2007 JPL WL from space meeting 1

Shear measurements in simulated SNAP images with

realistic PSFs

Håkon Dahle, Stephanie Jouvel, Jean-Paul Kneib, Eric Prieto, Sebastien Vives, Bruno Milliard

Laboratoire d’Astrophysique de Marseille (LAM), France

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Motivation STEP-like simulations for SNAP, with PSF (800nm) from optical designInclude 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

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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’’)

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PSF variation with focus/position

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Note ellipticity dependence onsmoothing scale (PSF wings are more elliptical)

0.34o off-axis 0.57o off-axis 0.8o off-axis

0.34o 0.57o 0.8o

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Note that e1=e2=0 PSFs are not necessarily circularly symmetric

0.34o off-axis 0.57o off-axis 0.8o off-axis

0.34o 0.57o 0.8o

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0.34o off-axis 0.57o off-axis 0.8o off-axis

0.34o 0.57o 0.8o

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0.34o off-axis 0.57o off-axis 0.8o off-axis

0.34o 0.57o 0.8o

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Analysis

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

KSB+ analysis (measuring stellar ellipticities and polarizabilities at same scale as these quantities are measured for each galaxy)

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Bias of shear measurements

Size: 0.1”(blue), 0.2”(red), 0.4”(green)

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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(e1

2 + 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

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“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(e1

2 + 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|>

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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

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Uncertainty of the shear measurements Green: 0.34o off-axis Blue: 0.57o off-axis Red: 0.8o off-axis

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Mean value of each shear component (should in principle be zero; circle indicates ~1 sigma uncertainty).

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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).

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