  We describe a focal plane array (FPA) system, called Apertif, that is being
developed for the Westerbork Synthesis Radio Telescope (WSRT). The aim of
Apertif is to increase the instantaneous field of view of the WSRT by a factor
of 37 and its observing bandwidth to 300 MHz with high spectral resolution.
This system will turn the WSRT into an effective survey telescope with
scientific applications such as deep imaging surveys of the northern sky of HI
and OH emission, of the polarised continuum and efficient searches for pulsars
and transients. Such surveys will detect the HI in more than 100,000 galaxies
out to z = 0.4, will allow to determine the detailed structure of the magnetic
field of the Galaxy, and will discover more than 1,000 pulsars. We present
experimental results obtained with a prototype FPA installed in one of the WSRT
dishes. These results demonstrate that FPAs do have the performance that is
required to make all these surveys possible.
