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[astro-ph/0608643] The Shear TEsting Programme 2: Factors affecting high precision weak lensing analyses
Authors:Richard Massey, Catherine Heymans, Joel Berge, Gary Bernstein, Sarah Bridle, Douglas Clowe, Hakon Dahle, Richard Ellis, Thomas Erben, Marco Hetterscheidt, F. William High, Christopher Hirata, Henk Hoekstra, Patrick Hudelot, Mike Jarvis, Dav
Abstract:The Shear TEsting Programme (STEP) is a collaborative project to improve the accuracy and reliability of weak lensing measurement, in preparation for the next generation of wide-field surveys. We review sixteen current and emerging shear measurement methods in a common language, and assess their performance by running them (blindly) on simulated images that contain a known shear signal. We determine the common features of algorithms that most successfully recover the input parameters. We achieve previously unattained discriminatory precision in our analysis, via a combination of more extensive simulations, and pairs of galaxy images that have been rotated with respect to each other, thus removing noise from their intrinsic ellipticities. The robustness of our simulation approach is also confirmed by testing the relative calibration of methods on real data.
Weak lensing measurement has improved since the first STEP paper. Several methods now consistently achieve better than 2% precision, and are still being developed. However, the simulations can now distinguish all methods from perfect performance. Our main concern continues to be the potential for a multiplicative shear calibration bias: not least because this can not be internally calibrated with real data. We determine which galaxy populations are responsible and, by adjusting the simulated observing conditions, we also investigate the effects of instrumental and atmospheric parameters. We have isolated several previously unrecognised aspects of galaxy shape measurement, in which focussed development could provide further progress towards the sub-percent level of precision desired for future surveys.
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Adam Amara

Joined: 25 Sep 2004
Posts: 18
Affiliation: ETH

PostPosted: September 05 2006  Reply with quote

With a lot of attention recently being focused on cosmic shear I think this is very interesting paper by the STEP group (the Shear TEst Programme). 16 different shape measurements codes have been run (blindly) on set of image simulations. It looks like many of the methods do well (and have improved since STEP1) and are able to achieve 2% precision, but this is still some way off what will be needed in the future (roughly 0.1% accuracy), I’m very curios to know if STEP3 will be able to push the methods towards this level of accuracy. And are we able to reach 2% on real data today?

The paper is very technical but there is a very nice discussion in section 5.8 that discusses ‘consequences for previously published measurements’, although figure 10 is a little worrying especially regarding the most recent cosmic shear measurements. Can someone comment on this? For a few years now it looks like the value of sigma8 measured through cosmic shear has been a little unstable (and a little inconsistent with WMAP3), so I’m very reassured that different methods being blindly tested/compared and the results published in a clear and transparent way.

It sounds like there will be a dataSTEP out soon that should help a great deal. Does anyone one when this will be done?

Adam Amara
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Douglas Clowe

Joined: 05 Nov 2005
Posts: 12
Affiliation: Ohio University

PostPosted: September 28 2006  Reply with quote

Since nobody else has replied:
STEP3 is very similar to STEP2 except the simulations are for space-based observations (so PSFs which are intrinsically smaller but much more power in the wings). They were done on the same timescale as STEP2, so I doubt you'll see a dramatic improvement, it's more a test of how well the PSF correction can be applied for current HST images and the future JDEM mission (and other space based imaging telescopes).

STEP4 is in the planning stages, and will probably be a larger sequence of simluations trying isolate which particular stages in the correction process are producing the errors.
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