The third GRavitational lEnsing Accuracy Testing challenge, or GREAT3, is a blind data analysis competition held by the world-wide weak lensing community to test weak lensing measurement algorithms.

All simulated data are available, downloadable from our server on the GREAT3 Leaderboard web site, where you can also find the scores of all the challenge participants.

The challenge began in October 2013 and ended on April 30, 2014 (see the timeline page).

The GREAT3 Final Meeting took place on 27-29 May 2014 at Carnegie Mellon University, Pittsburgh PA

With several major astronomical surveys beginning to make large-scale cosmological weak lensing measurements in 2013 in order to better understand our cosmological model (including the mysterious dark matter and dark energy), we hope this challenge will prove to have played an important role in identifying promising measurement algorithms and quantifying their performance.

For more information about the challenge, see the menu items above, particularly our links page where you can see how to access the data, submit results, get the software included with the release, and read the challenge handbook.

GREAT3 leaderboard server is back up!

The server downtime that was mentioned previously is now over, and the GREAT3 leaderboards and data are now available at a new location. All links to the leaderboard on this site and our public GitHub repository should now point to this new location.

Please send a message to if you have trouble with the new site.

GREAT3 Leaderboard server downtime

Not the GREAT3 datacentre

In the week beginning 27 October 2014 the GREAT3 Leaderboard web pages will be temporarily shutdown while the server is physically moved to a new home at the Jodrell Bank Centre for Astrophysics, University of Manchester.

Please accept our apologies for the loss of access to the Leaderboard. The GREAT3 data will still be available via ftp from the US Mirror site at:

The GREAT3 challenge has ended!

GREAT3 Trophy

The ~6 month time period for the GREAT3 challenge ended 23:59:59 Pacific Daylight Time on April 30, 2014.

Congratulations are due to our winners, Team "sFIT", who ranked top in 8 individual branch leaderboards including all the (most challenging) full branches.

US Mirror downtime scheduled: 30 March 2014

The US Mirror for the GREAT3 data will be down for a period of 2-3 days beginning Sunday 30 March 2014. This is while the server at the California Institute of Technology undergoes a significant disk upgrade.

Anyone planning downloads from the US Mirror should avoid those days. We will send a reminder to all participants via the email list closer to the time. The primary download site (based in the UK) will be accessible throughout this period.

Supplementary downloads for GREAT3 deep fields released

Darren Hester

In GREAT3 we have provided a small number of deep fields: longer exposure imaging that overlaps a subset of the main dataset. This sort of deeper imaging is often a part of wide area extragalactic survey strategy. Methods will be able to train using this sort of data in real life, and we wanted to reflect this in GREAT3.

However, those who wish to use the deep fields for the variable PSF or full branches in GREAT3 will find have found that the galaxy catalog files for the deep fields in those branches are missing some important information.

Scores updated after revision of GREAT3 metrics

This morning we recalculated the Q values in all GREAT3 leaderboards following revision of the definition of the metrics, so do not be perturbed if you saw a change in your submission score!

The scaling of the metrics has been revised in light of submission data to ensure that a score of Q~1000 indicates excellent performance, with some allowance for statistical uncertainties. More than one team now appears to be exceeding bias targets in some branches: this is a fantastic achievement.

URGENT: Weekend shutdown of the GREAT3 Leaderboard server

Due to electrical testing and maintenance in the UCL server room, our GREAT3 Leaderboard Server will very likely be shut down over the weekend 14-16 February 2014, starting at 3pm GMT Friday.

We apologise sincerely for the short notice: the shutdown is due to the discovery of a significant electrical fault in the server room by UCL Estates yesterday.

All challenge data now available!


We are pleased to announce the release of the ‘variable PSF’ and ‘full’ simulations, which are the last 8 branches of the challenge dataset, as well as updated 'multiepoch’ simulations. As usual, you can download them from the challenge data download page.

A few other notable updates:

Example script for multiepoch data now available

In GREAT3 we have always planned to release example scripts that provide a basic, but far from complete, solution to key parts of the challenge.

GREAT3 press releases

We have been lucky enough to receive coverage about the GREAT3 Challenge, from our partners Carnegie Mellon University, NASA Jet Propulsion Laboratory, and University College London!


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