  The WFI (Wide Field Imager) instrument is planned to be one of two
complementary focal plane cameras on ESA's next X-ray observatory Athena. It
combines unprecedented survey power through its large field of view of 40 amin
x 40 amin together with excellent count rate capability (larger than 1 Crab).
The energy resolution of the silicon sensor is state-of-the-art in the energy
band of interest from 0.2 keV to 15 keV, e.g. the full width at half maximum of
a line at 7 keV will be better than 170 eV until the end of the nominal mission
phase. This performance is accomplished by using DEPFET active pixel sensors
with a pixel size of 130 x 130 microns is well suited to the on-axis angular
resolution of 5 arcsec half energy width (HEW) of the mirror system. Each
DEPFET pixel is a combined sensor-amplifier structure with a MOSFET integrated
onto a fully depleted 450 micron thick silicon bulk. Two detectors are planned
for the WFI instrument: A large-area detector comprising four sensors with a
total of 1024 x 1024 pixels and a fast detector optimized for high count rate
observations. This high count rate capable detector permits for bright point
sources with an intensity of 1 Crab a throughput of more than 80% and a pile-up
of less than 1 %. The fast readout of the DEPFET pixel matrices is facilitated
by an ASIC development, called VERITAS-2. Together with the Switcher-A, a
control ASIC that allows for operation of the DEPFET in rolling shutter mode,
these elements form the key components of the WFI detectors. The detectors are
surrounded by a graded-Z shield, which has in particular the purpose to avoid
fluorescence lines that would contribute to the instrument
background...[Abridged]
