  We present the idea and proof of principle measurements for an
angular-selective active filter for charged particles. The motivation for the
setup arises from the need to distinguish background electrons from signal
electrons in a spectrometer of MAC-E filter type. While a large fraction of the
background electrons exhibit predominantly small angles relative to the
magnetic guiding field (corresponding to a low amount of kinetic energy in the
motion component transverse to the field lines, in the following referred to as
transverse energy) and pass the filter mostly unhindered, signal electrons from
an isotropically emitting source interact with the active filter and are
detected. The concept is demonstrated using a microchannel plate (MCP) as an
active filter element. When correctly aligned with the magnetic field,
electrons with a small transverse energy pass the channels of the MCP without
interaction while electrons with large transverse energies hit the channel
walls and trigger an avalanche of secondary electrons that is subsequently
detected. Due to several drawbacks of MCPs for an actual transverse energy
filter, an alternative detection technique using microstructured Si-PIN diodes
is proposed.
