lensfit

galaxy shape measurement for weak-lensing surveys

Introduction

Studies of weak gravitational lensing in astronomy need measurements of the shapes of large numbers of galaxies from faint imaging surveys. In Miller, Kitching, Heymans, Heavens & van Waerbeke (2007) and Kitching, Miller, Heymans, van Waerbeke & Heavens (2008) we describe a Bayesian model-fitting approach to galaxy shape measurement that is fast enough to be used for large weak-lensing surveys.

The method provides optimum (maximum signal-to-noise) shape measurement for each galaxy in a survey, with unbiased correction for the point-spread function (PSF). The output for each galaxy is a Bayesian "posterior probability surface" of the two ellipticity parameters, marginalised over the galaxy size, brightness and position. The code currently takes around 1-2 secs per galaxy using a standard single-processor desktop machine. There are several steps in the shape and shear measurement process:

  1. Basic image reduction - standard methods are used to bias-correct and flat-field images. If fringing is present it is important to defringe. It is best also to determine and remove the sky background, especially if this varies over an image. Any camera image distortion should be removed if appropriate (although care is needed in this step if images are poorly sampled).
  2. Galaxy and star detection and classification - this can be achieved using standard analysis code such as SExtractor. Accurate star-galaxy discrimination is important as stars are used to define the PSF.
  3. PSF creation, by combining many images of stars into a position-dependent PSF.
  4. Shape measurement as described by Miller et al. (2007) and Kitching et al. (2008). If multiple images exist for a galaxy, the method allows optimum combination of information.
Once the ellipticity probability surface has been obtained for each galaxy, that information may be used to create either maps or statistical measures of the shear caused by weak lensing. New code has been developed to carry out steps (3) and (4) above. The algorithm and code have so far been tested on the Shear TEsting Program (STEP) simulated galaxies.

If you are interested in using this code, please register on the accompanying page.

 

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