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August 23, 2007JPL WL from space meeting3 Skymaker simulations 34x34 grid of galaxies in 4 simulated exposures; ditherered, re-sampled and combined 0.1’’ pixels --> 0.06’’ pixels; 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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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
August 23, 2007 JPL WL from space meeting 2
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
August 23, 2007 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’’)
August 23, 2007 JPL WL from space meeting 4
PSF variation with focus/position
August 23, 2007 JPL WL from space meeting 5
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
August 23, 2007 JPL WL from space meeting 6
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
August 23, 2007 JPL WL from space meeting 7
0.34o off-axis 0.57o off-axis 0.8o off-axis
0.34o 0.57o 0.8o
August 23, 2007 JPL WL from space meeting 8
0.34o off-axis 0.57o off-axis 0.8o off-axis
0.34o 0.57o 0.8o
August 23, 2007 JPL WL from space meeting 9
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)
August 23, 2007 JPL WL from space meeting 10
Bias of shear measurements
Size: 0.1”(blue), 0.2”(red), 0.4”(green)
August 23, 2007 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(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
August 23, 2007 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(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|>
August 23, 2007 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
August 23, 2007 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
August 23, 2007 JPL WL from space meeting 15
Mean value of each shear component (should in principle be zero; circle indicates ~1 sigma uncertainty).
August 23, 2007 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).
August 23, 2007 JPL WL from space meeting 17
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