  The imaging spectro-polarimeter VTF (Visible Tunable Filter) will be operated
at the Daniel K. Inouye Solar Telescope (DKIST). Due to its capability of
resolving dynamic fine structure of smaller than 0.05'', the finite acquisition
time of typically 11 s affects the measurement process and potentially causes
errors in deduced physical parameters. We estimate those errors and investigate
ways of minimising them.
  We mimic the solar surface using a magneto-hydrodynamic simulation with a
spatially averaged vertical field strength of 200 G. We simulate the
measurement process scanning through successive wavelength points with a
temporal cadence of 1 s. We synthesise FeI 617.3 nm. Besides the classical
composition of the line profile, we introduce a novel method in which the
intensity in each wavelength point is normalised using the simultaneous
continuum intensity. Milne-Eddington inversions are used to infer the
line-of-sight velocity, v(los), and the vertical (longitudinal) component of
the magnetic field, B(los).
  We quantify systematic errors, defining the temporal average of the
simulation during the measurement as the truth. We find that with the classical
composition of the line profiles, errors exceed the sensitivity for v(los) and
in filigree regions also for B(los). The novel method that includes
normalisation reduces the measurement errors in all cases. Spatial binning
without reducing the acquisition time decreases the measurement error slightly.
  The evolutionary time-scale in inter-granular lanes, in particular in areas
with magnetic features (filigree), is shorter than the time-scale within
granules. Hence less accumulations could be used for strong magnetic field in
inter-granular lanes and more accumulations could be used for the weak granular
magnetic fields. As a key result, we suggest to include the novel method of
normalisation in corresponding data pipelines.

Solar and Stellar Astrophysics;Instrumentation and Methods for Astrophysics