STEP4 Konrad Kuijken Leiden

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

STEP4 Concentrate on ellipticity and PSF: –Reveal positions to within 1 pixel –Well-separated sources –Input catalogues rigorously with =0 –Brute-force PSF convolution, pixellation: sample –Different S/N simulations –Build up complexity gradually (gal, PSF) –Blind catalogues, some shear values public

S/N=15, de Vauc

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)  (e i ) #galaxies (x32) Galaxy models: Exp & deVauc fwhm 7pix Moffat PSF fwhm 3.5pix  (  )~10 -4

S/N=15, de Vauc

S/N=50, deVauc

S/N=200, deVauc

S/N=15, exp

S/N=50, exp

S/N=200, exp

Ellipticity distributions From Lambas et al 1992 (APM survey)

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

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)

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 10 7 ‘photons’ from galaxy and PSF