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 <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

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) 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

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 107 ‘photons’ from galaxy and PSF