The Shear TEsting Programme STEP

STEP is a collaborative project to improve the accuracy and reliability of all weak gravitational lensing measurements in preparation for the next generation of wide-field surveys.

Gravitational lensing provides an unbiased method to study the distribution of dark matter in the Universe. Measurements of weak lensing by large scale structure, also known as cosmic shear, additionally allows one to probe the evolution of dark matter providing constraints on the properties of dark energy.

Technically, weak lensing is rather challenging to detect, with the accuracy of any analysis depending critically on the correction for instrumental distortions and atmospheric seeing. These effects can introduce distortions that are up to an order of magnitude larger than the underlying weak shear distortion that we wish to detect. The unique qualities of weak lensing as a dark matter and dark energy probe demand that all technical challenges are met and overcome, and this desire has lead to the development of some of the most innovative methods in astronomy.

STEP was launched in order to test and improve the accuracy and reliability of all these methods through the rigorous testing of shear measurement pipelines, the exchange of data and the sharing of technical and theoretical knowledge within the weak lensing community. Our current projects include

• The blind analysis of simulated images of ground-based observations of relatively simple galaxy morphologies.
• The blind analysis of simulated images of ground and space-based observations of complex galaxy morphologies.
• Co-ordinated analysis of shared survey data.

If you are interested in joining the STEP collaboration please e-mail Catherine Heymans