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Antony Lewis Institute of Astronomy, Cambridge

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1 Antony Lewis Institute of Astronomy, Cambridge

2 KK99 Try to find valid (robust) but non-optimal method Baseline for comparison that any good valid method should be able to beat. May help to identify important issues. In general need to have consistent galaxy model to get the correct answers. [All (?) existing models except KK99 do not use a consistent galaxy model]. On average galaxies have circular symmetry: easy to model. Simple priors obvious (e.g. smooth probably monotonic function) By Central Limit Theorem, measured averaged galaxy also has Gaussian distribution: know statistical model. Fit PSF-convolved sheared radially-symmetric averaged galaxy to stacked image Use splines to model average shape accurately. Well determined because lots of galaxies used to make average. For constant shear only need averaged PSF (varies due to pixels even in Great08) Sub-optimal but well-defined method for low noise that seems to work (possible small calibration error) Maybe hope gives ideas for new separate galaxy method where galaxy modelling errors cancel

3 Problems with noise With noise, centroid cannot be determined accurately: have to account for galaxy-dependent error in centroid estimate. Shear -> centroid error aligned with shear -> calibration error [sim bias from PSF, + other effects?] Can estimate centroid error and circularize it to eliminate some of the bias. But model no longer strictly valid – only approximate Currently no attempt made to tune weights, some obvious room for improvement.

4 GREAT08 feedback Web sites well set up and lots of useful information. Challenge appears to be well defined (modulo assumptions about galaxies) without being too easy. Files too large to be convenient (I had at most 200GB free on a disk, so had to use two; days spent downloading and shuffling files around + file corruptions, ssh connection problems, etc…) Unable to test well on noisy images due to not enough test images with same shear [hard to tell if differences due to random or systematic effects]. Making own similar simulations too time consuming. Properties of PSF worryingly idealized (i.e. no dipole, qualitatively different from reality, even if known and constant) How do you know even in principle how well we should be able to do?

5 Retrospective GREAT08 wish list Provide results for some known method (e.g. KSB): - many good methods will be iterative, e.g. provisional estimates useful for weighting galaxies in unbiased way. Save lots of time by providing results from known method. - can speed convergence by starting near known-good parameters - useful for sanity checks on things going wildly wrong At least one set of more galaxies to beat down noise for testing Source code (minus parameters) for the simulations for confidence in validity - when youre code doesnt work, very easy to doubt that simulations have been done correctly e.g. is high-enough sub-pixel resolution used, are stamps cut after rather than before PSF convolution, are galaxies occurring in mirror-image pairs, etc. Some feedback in leaderboard about what errors are due to (e.g. in my case probably a calibration-like error, |gamma| too large: i.e. error dominated by plates with largest shears) [Ps. Are fixed star images linked from site?]


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