  The Visible Tunable Filter (VTF) is a narrowband tunable filter system for
imaging spectroscopy and spectropolarimetry based. The instrument will be one
of the first-light instruments of the Daniel K. Inouye Solar Telescope that is
currently under construction on Maui (Hawaii). The VTF is being developed by
the Kiepenheuer Institut fuer Sonnenphysik in Freiburg as a German contribution
to the DKIST.
  We perform end-to-end simulations of spectropolarimetric observations with
the VTF to verify the science requirements of the instrument. The instrument is
simulated with two Etalons, and with a single Etalon.
  The clear aperture of the Etalons is 250 mm, corresponding to a field of view
with a diameter of 60 arcsec in the sky (42,000 km on the Sun). To model the
large-scale figure errors we employ low-order Zernike polynomials (power and
spherical aberration) with amplitudes of 2.5 nm RMS. We use an ideal
polarization modulator with equal modulation coefficients of 3-1/2 for the
polarization modulation
  We synthesize Stokes profiles of two iron lines (630.15 nm and 630.25 nm) and
for the 854.2 nm line of calcium, for a range of magnetic field values and for
several inclination angles. We estimated the photon noise on the basis of the
DKIST and VTF transmission values, the atmospheric transmission and the
spectral flux from the Sun.
  For the Fe 630.25 nm line, we obtain a sensitivity of 20 G for the
longitudinal component and for 150 G for the transverse component, in agreement
with the science requirements for the VTF.
