  The KATRIN experiment is going to search for the average mass of the electron
antineutrino with a sensitivity of 0.2 eV/c2. It uses a retardation
spectrometer of MAC-E filter type to accurately measure the shape of the
electron spectrum at the endpoint of tritium beta decay. In order to achieve
the planned sensitivity the transmission properties of the spectrometer have to
be understood with high precision for all initial conditions. For this purpose
an electron source has been developed that emits single electrons at adjustable
total energy and adjustable emission angle. The emission is pointlike and can
be moved across the full flux tube that is imaged onto the detector. Here, we
demonstrate that this novel type of electron source can be used to investigate
the transmission properties of a MAC-E filter in detail.
