STEP4

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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 ltei(e)gt0
• 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/N15, de Vauc
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STEP4 Images
Galaxy models Exp deVauc fwhm 7pix Moffat
PSF fwhm 3.5pix

• 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

?(?)10-4
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S/N15, de Vauc
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S/N50, deVauc
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S/N200, deVauc
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S/N15, exp
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S/N50, exp
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S/N200, 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 mlt1,
  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 1052
  images)

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Summary

• 100,000,000 simulated galaxies available
• S/N15,50,200
• Galaxies with sersic n1,4, 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