step4 konrad kuijken leiden. back to basics? step1,2,3 –‘end-to-end’ tests for constant-shear...
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STEP4
Konrad Kuijken
Leiden
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Back to basics?
• STEP1,2,3– ‘end-to-end’ tests for constant-shear images– What causes discrepancies?
• Selection bias• PSF modelling error• Ellipticity measurement error function of (mag, size)• Noise biases• Neighbours• Simulation errors?• … …
– Controlled simulations of effects separately
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STEP4
• Concentrate on ellipticity and PSF:– Reveal positions to within 1 pixel– Well-separated sources
– Input catalogues rigorously with <ei(e)>=0
– Brute-force PSF convolution, pixellation:
sample– Different S/N simulations– Build up complexity gradually (gal, PSF)– Blind catalogues, some shear values public
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S/N=15, de Vauc
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STEP4 Images
• 3720x3720 pixel fits files• 60x60 grid of galaxies• 60-pixel separation• 240 stars around the sides• Stars 10x as bright as galaxies• 32 different shear values, ePSF per simulation set
S/N (detection) 15 50 200
(ei) 0.11 0.03 0.01
#galaxies (x32) 36000 18000 7200
Galaxy models:Exp & deVaucfwhm 7pix
Moffat PSFfwhm 3.5pix
()~10-4
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S/N=15, de Vauc
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S/N=50, deVauc
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S/N=200, deVauc
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S/N=15, exp
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S/N=50, exp
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S/N=200, exp
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Ellipticity distributions
• From Lambas et al 1992 (APM survey)
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How to analyze STEP4 data?
• First 8 shear & ePSF values are public (2%)
• If your method can reproduce these with m<1%, then send me all results to get a measurement of m,c
• Intended as a resource for method development - prerequisite to test other nuisance effects
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STEP4 - next simulations?
• PSF, galaxy sizes• Galaxy type (varying
ellipticity; spiral arms; irregulars)
• PSF type
• About 30GB per set of simulations (32 x [10+5+2] images)
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Summary
• 100,000,000 simulated galaxies available– S/N=15,50,200– Galaxies with sersic n=1,4, fwhm 7pix– PSF Moffat, FWHM 3.5pix
• Well-controlled images• No shape noise• Statistical noise ()~10-4 per gal/PSF/SNR simulation
• More simulations possible (just need a MC sample of 107 ‘photons’ from galaxy and PSF