(a target survey field of ACT telescope)
D56 (for Deep56) is a survey field that has been a target of
Atacama Cosmology Telescope (ACT) surveys such as the ACTPol Survey.
It encompasses about 700 deg², roughly a combination
of the earlier-established Deep5 (D5) and Deep6 (D6) fields
plus the intervening region of the sky.
ACT survey data from D56 has produced results compatible
with Planck regarding the Lambda-CDM model parameters.