  Spectrometers based on the magnetic adiabatic collimation followed by an
electrostatic filter (MAC-E-filter) principle combine high angular acceptance
with an excellent energy resolution. These features make MAC-E-filters very
valuable for experiments where the kinetic energy of ions or electrons from
rare processes has to be measured with utmost sensitivity and precision.
Examples are direct neutrino mass experiments like KATRIN which investigate the
energy of electrons in the endpoint region of the tritium $\beta$-spectrum.
However, the MAC-E-filter is a very sharp energy high-pass filter but not a
differential spectrometer. To determine a spectral shape of a charged particle
source, different electric retarding potentials have to be used sequentially,
reducing the statistics.
  In a previous work we have shown that the advantages of the standard
MAC-E-filter can be combined with a measurement of the time-of-flight (TOF),
allowing to determine spectral information over a certain energy range with one
retarding potential only, with the corresponding gain in statistics. This TOF
method requires to know the start time of the charged particles, which is not
always possible. Therefore, we propose a new method which does not require the
determination of the start time and which we call "time-focusing
Time-of-Flight" (tfTOF): By applying a time dependent acceleration and
deceleration potential at a subsequent MAC-E-filter, an energy dependent
post-bunching of the charged particles is achieved.
