  The X-ray free-electron lasers that became available during the last decade,
like the European XFEL (EuXFEL), place high demands on their instrumentation.
Especially at low photon energies below $1\,\text{keV}$, detectors with high
sensitivity, and consequently low noise and high quantum efficiency, are
required to enable facility users to fully exploit the scientific potential of
the photon source. A 1-Megapixel pnCCD detector with a $1024\times 1024$ pixel
format has been installed and commissioned for imaging applications at the
Nano-Sized Quantum System (NQS) station of the Small Quantum System (SQS)
instrument at EuXFEL. The instrument is currently operating in the energy range
between $0.5$ and $3\,\text{keV}$ and the NQS station is designed for
investigations of the interaction of intense FEL pulses with clusters,
nano-particles and small bio-molecules, by combining photo-ion and
photo-electron spectroscopy with coherent diffraction imaging techniques. The
core of the imaging detector is a pn-type charge coupled device (pnCCD) with a
pixel pitch of $75\,\mu\text{m}\times 75\,\mu\text{m}$. Depending on the
experimental scenario, the pnCCD enables imaging of single photons thanks to
its very low electronic noise of $3$e$^-$ and high quantum efficiency. Here we
present an overview on the EuXFEL pnCCD detector and the results from the
commissioning and first user operation at the SQS experiment in June 2019. The
detailed descriptions of the detector design and capabilities, its
implementation at EuXFEL both mechanically and from the controls side as well
as important data correction steps aim to provide useful background for users
planning and analyzing experiments at EuXFEL and may serve as a benchmark for
comparing and planning future endstations at other FELs.
