400d is an X-ray survey for galaxy clusters using archival ROSAT observations. It is sensitive to "typical", M ~ 3e14 h⁻¹ Msun, clusters out to z ~ 1 and covers 400 square degrees — essentially all suitable ROSAT pointed data. Our sensitive detection algorithm has produced a statistically complete and uncontaminated cluster sample.

Our catalog contains 242 clusters, all with spectroscopic redshfits. The survey area and volume are carefully calibrated.

A complete subsample of 41 clusters at 0.35 < z < 0.9 was observed with Chandra. These clusters are the basis of our cosmological work. They are also used in several accompanying projects.