  The demand for higher resolution telescopes leads to segmented primary
mirrors which need to be phased for operation. A phasing sensor applying a
wavelength sweep technique provides a large capture range without modulating
the position of individual mirror segments. This technique offers the potential
to monitor the phasing state of a segmented telescope in parallel to the
science observations. We evaluate the performance of the wavelength sweep
technique using a Zernike phase contrast sensor for coarse phasing. Tests
results on a dedicated bench show 112 nm rms precision. With the help of a
simulation, we explain a known error of the method and we suggest ways for
improvements.
