COSMOGRAIL is the COSmological MOnitoring of GRAvItational Lenses. Our project is aimed at measuring time delays for most known lensed quasars, from optical light curves obtained with small but (almost) dedicated telescopes in the northern and southern hemispheres. COSMOGRAIL involves people from different countries, spanning a broad range of expertises. The goal is to measure individual time delays with an accuracy below 3%, in order to determine the Hubble constant H0. A standalone measurement of H0 is highly complementary to other cosmological probes, such as the observations of the CMB made by the ESA Planck mission. This complementarity will help us to understand better the so called “dark energy” which is driving the accelerated expansion of our Universe.

Measuring time delays is difficult, but not as difficult as it first appeared in the late 80s when the first monitoring programs were started. Obtaining regular observing time on telescopes in good sites was (and is still) not easy and the small angular separations between the quasar images require to perform accurate photometry of blended objects, sometimes with several quasar images plus the lensing galaxy within the seeing disk. The COSMOGRAIL project, started in April 2004 and lead by EPFL, addresses both issues of carrying out photometry of faint blended sources and of obtaining well sampled light curves. In order to use all the available data, even with bad seeing, the data are processed using the MCS deconvolution algorithm.

A presentation of COSMOGRAIL and its scientific motivation can be found in our recent ESO Messenger article.