Shear measurements in simulated SNAP images with realistic PSFs
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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. Motivation. STEP-like simulations for SNAP, with PSF (800nm) from optical design

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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


Motivation

Motivation PSFs

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


Skymaker simulations

Skymaker simulations PSFs

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
PSF variation with focus/position PSFs

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Note ellipticity dependence on PSFs

smoothing scale

(PSF wings are more elliptical)

0.34o0.57o0.8o

0.34o off-axis

0.57o off-axis

0.8o off-axis

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0.34 PSFs o0.57o0.8o

Note that e1=e2=0 PSFs are not

necessarily circularly symmetric

0.34o off-axis

0.57o off-axis

0.8o off-axis

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0.34 PSFs o0.57o0.8o

0.34o off-axis

0.57o off-axis

0.8o off-axis

JPL WL from space meeting


0.34 PSFs o0.57o0.8o

0.34o off-axis

0.57o off-axis

0.8o off-axis

JPL WL from space meeting


Analysis
Analysis PSFs

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

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Bias of shear measurements PSFs

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

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Shear kaiser squires broadhurst ap j 449 460 475 1995 ksb
Shear PSFs Kaiser 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

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Shear recovery
“Shear recovery” PSFs

  • 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|>

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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 PSF

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 PSF

be zero; circle indicates ~1 sigma uncertainty).

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This is work in progress
This is work in progress… PSF

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



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